STEP 3 - High Performance Construction
An energy rating, or even a high performance design, is only a piece of paper. How it performs on the ground comes down to thermal build quality. This section covers what you need to know to make sure the house you build performs on the ground, as it does on paper.
As we move to offering Net Zero homes to the market, risk for builders goes up. Owner expectations about performance rises. And as expectations rise, builder responsibility goes up with it.
What if the house doesn’t perform?
Unfortunately, anyone who has been around the building game knows that current standard practice, though on the improve, is still full of non-compliant thermal installation.
Gaps in insulation, and membranes are common.
Taping and caulking is often missed.
And this can have BIG implications for thermal performance.
The CSIRO calculated that if you have 5% of holes in your insulation, the value of it drops by 50%. This can be made even worse if those holes allow wind washing. (ie, breezes to blow through your structure, by passing insulation.)
Added to the difficulty, standard stick framed construction is one of the hardest construction types to get right for good thermal performance.
The insulation cavity is also the services cavity, often making it difficult to avoid gaps and compression, as batts fight for space with wires and pipes.
The internal plaster lining is part of the tightness of the structure, but is also penetrated by lights, vents, and fixtures, making is harder to achieve a tight fabric.
And we have come out of a history in Australia over the last 50 years where energy has traditionally been cheap, and insulation, wraps, and sealing have been somewhat of an afterthought, and very much secondary in importance to finish.
Despite all this, the contractual onus remains on the builder to build to meet the rating assumptions. If we don't the house will not perform to its rating.
Further, if we want to move to a Net Zero carbon future, and cover our risk as customers increasingly demand more energy efficient homes over the next decade, it is vital that we build to meet rating assumptions as closely as possible.
That of course begs the question,
“What are the Energy Rating Assumptions we need to build to?”
IN AUSTRALIA THE NATIONWIDE HOUSE ENERGY RATING SCHEME IS THE PREVALENT RATING SYSTEM. BUT HAS ANYONE EVER TOLD YOU WHAT THE NatHERS BUILD ASSUMPTIONS ARE?
Most likely the answer is no, as it’s not part of normal builder education.
Let’s go over them in a quick review.
HOUSE FORM & MATERIALS ARE AS PER STAMPED PLANS
You must build the plans the rater assessed. Sounds easy. But if the rater's stamp is not on the plans you are building, you can bet something has changed since they rated it. If there’s no stamp, confirm with the Designer in writing that nothing has changed.
INSULATION IS CORRECT R, NON-COMPRESSED, & COMPLETE
NatHERS assumes perfect insulation, so we need to meet this as closely as possible. Check the insulation that turns up is as per the stamped plans, then be meticulous in installation. For insulation to work effectively it must be correct R value, maintain full loft, overlap at junctions, and be complete all the way around the conditioned zone of the building. Ie. NO HOLES
If you don't install right, the house won't perform. Nuff' said.
THERE IS NO THERMAL BRIDGING
Highly conductive elements like Structural Steel, Aluminium, Copper and Masonry which pass through the thermal envelope, or butt up against both the internal and external linings, create a conductive super-highway for unwanted heat loss or gain; circumventing the insulation. Avoid, or otherwise create a thermal break between internal and external surfaces to close this pathway.
BUILDINGS ARE REASONABLY TIGHT (<10ACH@50PA)
NatHERS airtightness is a bit nuanced as the actual tightness depends on factors entered by the assessor. While the newest software update does actually show the figure assumed, if you are under 10 air-changes/hr @ 50 Pascals of pressure you are in the right ballpark. Not sure what I'm talking about? See STEP 1, Design for Tightness. And the items regarding Blower Door Tests at the end of the Builders Thermal Performance Checklist below.
THERE IS NO INSULATION WASHING
Batt insulation requires a still airspace to be effective:
If airflow pathways are able to move through the structure it will go around batt edges, and circumvent the insulation. This is known as wind-washing.
Air needs an entry and exit point to flow. In standard construction tightness is controlled by a well taped external membrane and a well caulked internal lining (usually plaster).
Keeping insulation hard to the back of internal linings, combined with good wrap taping & plaster caulking, go a long way to negating this issue.
WINDOWS ARE AS PER THE RATING REPORT
Check U-values & SHGC on the Rating report are what is supplied or are within the substitution tolerances. Windows have a huge effect on the rating, & are expensive to replace if you put in the wrong thing...
LIGHTS, EXTRACTION FANS, & CEILING FAN No.s ARE AS PER STAMPED PLANS.
These items effect ventilation & infiltration. So to comply, what gets built needs to match the stamped plans.
And that's it!
Of course, the devil is in the detail, and there are challenges to maintaining thermal integrity at all stages of construction. That's where a check list and a good understanding of where buildings can fail thermally is vital.
See next section...
BUILDING A HOUSE IS DIFFICULT, WITH MANY TRADES, JOBS, AND SUPPLIERS THAT NEED TO BE COORDINATED AND WORK TOGETHER. THIS POSES CHALLENGES IN ALL AREAS, BUT PARTICULARLY TO THERMAL PERFORMANCE.
Gaps and cracks, missing insulation, penetrations un-caulked, and other little items missed in one stage, get covered in later stages and become inaccessible and costly to rectify later. And those 'little' misses can easily add up to significant underperformance.
And now, as builders promise more, client expectations go up, and so to therefore does builder risk. If the home doesn't perform to expectations, the finger will be pointed firstly at the builder and then at the designer.
To cover yourselves, builders need a tool...
...and it just so happens we have one for you!
The SBA Thermal Performance Checklist is a tool you can use you to help manage builder's risk for standard construction, in the area of thermal performance.
It does this in 3 ways:
Firstly, as with all quality assurance checklists, it helps to maintain a consistent product standard.
Secondly, it provides evidence a builder can point to in order to show that they have built to the required standard (along with photos, thermal images, and blower door testing - see last section of the checklist).
Thirdly, it helps differentiate a quality builder, from greenwash claims, and is a process you can point to and market to show your company's commitment to the home's thermal integrity. Such differentiation, and the reputation it builds, is a useful asset when times are lean.
The items on the checklist are not onerous and are simply memory prompts at each stage of construction to make sure that things aren't forgotten in one stage that won't be accessible, or will be more costly to rectify, later on if missed. And using it quickly helps teach trades the quality required for thermal performance.
Feel free to adjust it, and add and remove items to suit your build. Add it to your templates and plans as you see fit!
Each building stage has different challenges, with different control measures to be ticked off.
Let's follow the Builders Thermal Performance Checklist and go through each stage of the building process in standard construction. We will look at where buildings commonly fail thermally, what each of the Control Measures suggested refer to, and how to use the Builders Thermal Performance Checklist to avoid these issues.
LEGEND:
When looking through the following building stages, you will see 3 recurring icons.
Note: If you have time, and would rather watch a video of this, check out the highly recommended Net Zero Homes course in the Webinar Link Section at the end of this STEP
Ok, let's start the tour...
The Pre-Build Stage is all about setting up for success, and has 3 control measures to be ticked off:
The 1st Control Measure is for the Builder or Project Manager, or better yet the whole team, to watch the 3 Construction Stage Instructional Videos from Sustainability Victoria.
These videos, filmed by Efficiency Matrix, are short & excellent, and really make it clear what a good, tight, insulation install looks like.
Of course, once your team has watched it once, you don't need to do it every time!
...By the way, Efficiency Matrix have a really excellent YouTube Channel covering everthing to do with building tight. Highly recommended. Some of their photos even turn up in the following pages...
The 2nd Control Measure, is to check the plans have the House Energy Rating Stamp and the Building Permit Stamp on them. If not, you can almost bet something has changed and plans have been re-issued. You want to make sure you are building the plans the rating was done on, and the building permit was for.
Note: If there have been any changes in areas, volumes, materials, window/door sizing or locations, or no. of fans, lights, insulation, etc., it could effect the energy rating and the plans will need to go back to the assessor for a re-rating/check.
And the 3rd Control Measure is that you have noted the Pre-Handover Test Requirements at the end of checklist. These will be looked at further on, in Pre-Handover Stage.
These test requirements are not to make the builders life harder. (Remember you can adjust this template to work for you.) But having evidence of quality construction to show that that thermal build requirements have been met on the job is an important Risk Management strategy.
HOT TIP: Also, if you let trades know you will be taking thermal images and having a blower door test at the end of the build, you will be amazed at the overnight improvement in quality that you get in insulation, wrap taping, and caulking...
BUILDERS THERMAL PERFORMANCE CHECKLIST
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Pre-Build Stage (Current)
Though there can be thermal issues with slabs, most of these issues are to do with thermal bridging:
These issues are best dealt with in design stage, as they typically require inclusion in engineering plans. And, once detailed on engineering and drawings, due to the onsite building surveyor checks in many states, are usually installed reasonably well.
Timber subfloors on the other hand can have disastrous install outcomes, and can fail over time if not done correctly...
UNDERFLOOR INSULATION:
Pitfall: Why are the batts disintegrating!?!! The batts installed here are low density ceiling batts. Wrong product to use in the location, and not secured adequately. The almost certain sagging between joists from such an install, also introduces an air pathway above the insulation allowing potential wind washing...
Design Out Issue: Designers should specify correct product to use on the plans, as well as the R-rating. As can be seen with this Greenstuff product there is also a nice turn up detail, and fixing centers details depending on joist spacing. Such details on a section diagram help make sure they don't get missed on site.
Control Measure: In the absence of such details on the plans, check the tick-box items below have been carried out on site.
And here's a good example of an Underfloor Product installed from above to be flush with underfloor surface. No chance for wind washing under the floor along the top of the batts. Nice tight job!
BUILDERS THERMAL PERFORMANCE CHECKLIST
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Base Stage (Current)
The main thing to consider at frame stage is thermal bridging.
Prior to starting, builders should think through the structure with a eye for potential areas of thermal bridging.
You are looking for:
Hot Tip: Full loft thickness can be found on insulation product specifications, and should always be checked.
The following are some common problem areas...
Below is a thermal image. We will be using several of these throughout this Step as they are useful for showing areas of thermal weakness. (And in fact we recommend all builders get a thermal imaging camera to help pinpoint areas to be improved!) The dark/purple areas are cold, and the warm areas orange/red.
CORNER STUDS:
Pitfall: Look at the corner stud. Why is it cold?
As we saw in Step 1, Design Out Compression, this is a common weak point in standard construction because of the way we construct corners, primarily with the intent to pick up internal linings. This introduces a cavity, that once wrapped and clad, can no longer be accessed or insulated from the inside, and so typically will be left un-insulated. And this happens AT EVERY CORNER IN THE HOUSE! (See Stud Corner in pic below)
This image, taken at night, also shows a cold base plate.
While a cold slab edge may be contributing, no doubt cold heavy air from the un-insulated stud corner is falling down and pooling along the base plate as well.
Not only is this thermally problematic, but over time this "cold spot" can lead to surface mould (see pic A in STEP 3, What are we Worried About)
Design Out Issue: Detail 3 stud corners to be used on your plans. (This then allows insulation to be tucked behind from the inside during bulk insulation stage.)
Control Measure: Check the tick-box items below have been carried out on site.
Hot Tip: If using 3 stud corners, make sure insulation installers do insulate behind, and don't just butt up to the stud edge!
Hot Tip 2: If keeping standard framing and insulating cavity pockets from the outside prior to wrap, make sure you put this new insulation stage in your works flow chart, as insulation is not normally considered until after lockup, and it will get missed if not scheduled.
EXTERNAL JUNCTIONS:
INTERNAL JUNCTIONS:
Pitfall: The same happens at internal to external junctions.
Design Out Issue: Detail a nogged wall at 900 centres at every junction. Like the 3-stud corner, this allows insulation to be run behind at Bulk Insulation Stage later in the build. Alternatively, note insulation to be installed to external facing cavity pockets in the appropriate section detail.
Control Measure: Builder to check the tick-box items below have been carried out on site.
ROOF BATTENS:
Pitfall: This is a picture before ceiling plaster and batts, looking upward at a foil backed insulation blanket, under a metal roof with steel top-hat roof battens. It is a hot day outside, and you can see the foil backed blanket in the middle is cool (purple) and keeping the heat out. But the metal roof battens are glowing with heat.
What you are seeing is a thermal bridge.
Remember steel is ~1000 times more conductive than pine, so those metal roof battens are bringing in heat from the eaves, and circumventing the insulation of the foil backed blanket.
(Note, where no foil backed blanket is used, [ie. no bulk insulation above the roof membrane; common in above membrane ventilated cavity roof construction], then bridging is no longer an issue as there is no insulation for the battens to circumvent.)
Design Out Issue: Solution is easy. Specify timber roof battens on plans and engineering when using a roof blanket!
Control Measure: Check the tick-box items below have been carried out on site.
STRUCTURAL STEEL:
Pitfall: While the updated NatHERS software now takes into account thermal bridging through steel framing, structural steel is still not included in calculations, so represents a point where on-ground thermal performance can be different from predicted. While the effects are usually localised to the specific room, it can be broad in nature where a lot of structural steel exists in a design. And as steel is ~1000 times more conductive than pine, structural elements that pierce the external insulated shell of the house, effectively create a thermal superhighway for unwanted heat gain and loss. In cooler climates this can also introduce cold spots, condensation and mould potential.
Watch out for any steel structure that goes from the inside to the outside. Or less obvious, steel that takes up the whole thickness of an external wall or roof assembly, with internal and external lining on either side.
Design Out Issue: Check through engineering with an eye for any structural steel that bridges insulation, and ask the engineer to use timber, or specify structural breaks, or re-design to avoid the bridge. Where unavoidable, try to allow extra space for insulating around the steel or battening off cladding to at least create an air separation.
Control Measure: Check the tick-box items below have been carried out on site.
BUILDERS THERMAL PERFORMANCE CHECKLIST
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Frame Stage (Current)
The main thing to consider at Lockup stage, from a thermal point of view are:
Let's look at these in order...
WINDOWS:
Pitfall: As we saw in Step 1, Rule 2, windows are really important. It is therefore vital that the windows that were modeled in the rating are what gets installed, otherwise the on-ground performance will be markedly different from what the rating predicts.
Design Out Issue: Make sure the Window Manufacturer, Model type, Window Description, U-value & SHGC are detailed on the plans, along with the Window Schedule. Or, where leaving window choice to others and modeling with a generic window (not recommended), make sure that the U-value and SHGC of the windows rated, are written on the plans with the window schedule, so that the clients can then find windows of appropriate performance to suit. (See Step 1, Choosing a Window Manufacturer)
Control Measure: Check the tick-box items below have been carried out on site.
Hot Tip: Where a client wants to choose a different window than the one on the stamped plans substitution is possible. The rules of substitution are that the substituted window must have the same U-value or lower, and the SHGC must be no more than plus or minus 5% from what has been used in the report. Windows must also be the same opening type and size.
(Scan QR code on raters stamp to access the rating report and flick through to windows page.)
Where clients want to choose windows outside this tolerance, another rating will be required first. AND, for risk management, we suggest always getting the change in writing as a Variation.
WRAP CHOICE:
Pitfall: Wraps are easy get wrong. You need to confirm 3 things:
If there are any conflicts in numbers 1-3, contact the Designer to seek clarification in writing. If there is a need to change a foil wrap to a membrane, the designer will need to get another energy rating done, as membranes are not reflective.
Design Out Issue: Make sure the above 3 points are checked during design and that the appropriate membrane is clearly noted on the plans. Specify the product and matching proprietary tape.
Control Measure: Check the tick-box items below have been carried out on site.
WRAP INSTALLATION:
Pitfall: A non-compatible tape has been used with the membrane, and as you can see is already coming off. In this case a foil tape has been used with a material membrane. Membranes have along-term job to do, so it is important that the tape chosen seals for the life of the product. Look out for tapes that have been tested by an accelerated aging protocol to adhere for at least 100 years. You won't find this testing for reflective foil, so choose a quality foil tape with good adhesion. As a general rule, use the proprietary tape specified by the wrap manufacturer.
Design Out Issue: Specify both wrap and compatible tape brand and model on the plans.
Control Measure: Check the tick-box items below have been carried out on site.
WRAP WINDOW/DOOR DETAIL:
Pitfall: For wraps to be most effective they need to be taped at all penetrations. However this can sometimes be tricky at windows and doors, as there are many different window types, and different claddings and setups. Any taping needs to be long lasting, discreet, and work with any flashings. This takes some thought, and sadly is still often not done properly, where time to put tools down and stop and think can be at a premium.
Design Out Issue: The place to get the details right is on the plans during design phase. A detail showing how window, cladding, taping and flashings work together is a simple step, which makes it much more likely to be done properly on site.
Control Measure: While no Control Measure is specified, where window detail is not provided by the designer, the builder should consider what the detail for a tight, waterproof, well flashed window will need to be for the window and cladding type specified.
BUILDERS THERMAL PERFORMANCE CHECKLIST
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Lockup Stage (Current)
The wrap and claddings are on. Now the sparkies and plumbers come in. A successful roughin stage is about communication and fitting the pipes and wires in the cavities in such a way that subsequent insulation can be installed successfully.
WRAP PENETRATIONS:
Pitfall: Rips, holes, and cuts are not uncommon to find in wraps after sparkies and plumbers have been. These can defeat the whole purpose of the product and are simply the outcome of poor communication.
Design Out Issue: Proprietary gaskets are available and can be specified in the drawing documentation.
Control Measure: Check the tick-box items below have been carried out on site.
Hot Tip: Always have a roll of membrane and correct tape on site, and let the trades know that if they need to make penetrations in the wrap, they must be well sealed after, and there's the roll and tape to do it. And let them know you might be doing a blower door test...
PIPES & WIRES INSTALLATION:
Pitfall: One of the main downsides of standard construction from a thermal point of view is that there is no dedicated space for insulation, but instead it shares the cavity with the services. Without adequate planning this can lead to compression and spaces where it is almost impossible to install batts. (As in the pic). And remember, 5% holes reduces the value of the insulation by 50%, and facilitates wind washing. (More on this in Bulk Insulation Stage below.)
Design Out Issue: Consider a battened-out cavity, or bulkhead, where services can run, leaving full space for insulation.
Control Measure: Sort out on site with clear discussion. Check the tick-box items below have been carried out on site.
DOWNLIGHT INSTALLATION:
Pitfall: What you are looking at is not uncommon. You can see heat coming in from a hot room during the day, through missing insulation is at downlight locations. This is unfortunately typical in existing homes, and comes out of a history of halogen and incandescent downlights that required insulation to be removed 50mm all the way around the backside of each light to reduce fire risk. In reality no-one measures 50mm, but simply grabs and pulls out a wad of insulation. This totally ruins the insulation value of the batts and predicted performance from any energy rating will certainly not be met.
Design Out Issue: Do not design in downlights! Choose pendants, or surface mounted downlights instead. (And they use less power! See Step 4, Energy Efficient Lighting.)
Control Measure: Check the tick-box items below have been carried out on site.
Hot Tip: Beware, many IC-4 rated downlights only allow thickness up to R4 to go over the top, so check light specifications if having high R-value ceiling batts.
DUCTWORK:
Pitfall: Return air ducts, that are often just plastered stud cavities can be very leaky, as none of the internal junctions on the inside are sealed. In this picture you can see the warm air leaking out of the cavity "duct" to the wall and the rest of the roof beyond, to heat up the attic! This can be a huge energy waste, and make for a VERY leaky house.
Design Out Issue: Avoid ducted systems, and choose efficient single and multisplit airconditioners, which tend to be more efficient and also don’t suffer from ducting heat losses.
Control Measure: Where ducted heating/cooling is specified, check the tick-box items below have been carried out on site, to minimise system leaks.
BATH HOBS:
Pitfall: Can you see a pitfall here? Hobs are usually built, and baths placed before insulation. Where baths are on external walls, this makes it all but impossible to easily insulate and invariably insulation is missed behind the bath!
Design Out Issue: Maybe add a note on a wet area elevation?
Control Measure: Check the tick-box items below have been carried out on site.
BUILDERS THERMAL PERFORMANCE CHECKLIST
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Roughin Stage (Current)
INSULATION SUPPLY:
Pitfall: What's wrong here? Nothing, except, as you can see in the pic below, the insulation that turned up, was not what was specified!! Wrong insulation provided. In this instance R2.7 was specified for the walls, and an 80mm R1.8 foil backed blanket. Obviously if the wrong R rating gets installed the house will fail to perform to the rating.
Design Out Issue: Make sure R-values is clearly specified on plans.
Control Measure: Check the tick-box items below have been carried out on site.
INSULATION INSTALLATION:
Pitfall: What can I say? A job like this is completely unacceptable. Insulation needs to be complete, otherwise the heat flows through the weak points, which effectively become a myriad of thermal bridges; like having holes in an Esky and expecting drinks to remain cold at the beach! It is worth saying again: 5% of holes in insulation reduce its value by 50%.
Design Out Issue: None. This is an install issue to be sorted on site!
Control Measure: As per tick-box instruction below.
Hot Tip: Don’t pay until insulation looks like this!
WIND WASHING CHECK:
Pitfall: WHATS WIND-WASHING?
Glad you asked!
Batts are really good at limiting heat flow by conduction. However batts can be circumvented if air can find pathways through and across them.
That’s what you see in the picture here.
Ceiling batts are stopping short of the top plate (ie. Not overlapping as they should)
And wall batts near the corner look like they are not hard up to the plaster surface, but sit back a bit, allowing a gap for cold air to drain down between batt and plaster.
(Note: the corner stud also looks like it’s not insulated!)
Design Out Issue: None. This is an install issue to be sorted on site!
Control Measure: Check the tick-box items below have been carried out on site.
GAPS & CRACKS CHECK:
Pitfall: Small gaps like this are often left in standard construction, and there are many such small gaps in the average frame, which together can add up. Such gaps also facilitate wind washing (see above) and can create cold spots, and potential mould formation on the plaster. (See Condensation & IAQ - Control Strategy 1 below.)
Design Out Issue: None. This is an install issue to be sorted on site!
Control Measure: Check the tick-box items below have been carried out on site.
WINDOW & DOOR SURROUNDS:
Pitfall: Windows and doors are required to have gaps between jambs and house frame, to protect against jamming and glass cracking as the house moves over time. You can see the gap in this picture. This is usually 20mm around every window and door.
Doesn’t sound like much, but if you add all those gaps together you would get an un-insulated hole ~2m2 in the average home!
It needs to be filled. But importantly, it needs to be filled with something that allows for movement. And if using foam, will not expand so aggressively as to bow the frame and jam the operation.
Design Out Issue: Specifiy a low-expansion (and ideally low VOC) blow in foam around all windows and doors.
Control Measure: Check the tick-box items below have been carried out on site.
SERVICES CHASING:
Pitfall: NatHERS assumes batts are fully lofted and achieving their stated R-values. Batts achieve their high R-value per thickness by trapping a lot of smaller air spaces. Compressing the batt squashes and reduces these, and thus reduces the R-value of the product.
Design Out Issue: While you could consider a battened-out cavity, or bulkhead, where services can run, leaving full space for insulation, such issues will almost always be part of starndard construction to be dealt with primarily through site control measures.
Control Measure: As per tick-box instruction below. Note, if you want to see how this is done in action, watch the 3 introductory Construction Stage Instructional Videos from Sustainability Victoria.
POWERPOINTS:
Pitfall: Leaky, cold, powerpoints on external walls.
Traditionally insulation was removed behind powerpoints that needed more space. This however leaves holes, and allows air leakage through the switchplate as you can see in the thermal image.
Design Out Issue: Try and keep powerpoints off external walls. While still a leakage point on internal walls, the thermal impact is lessened.
Control Measure: Check the tick-box items below have been carried out on site.
NOTE: It is possible to buy (or make) air leakage free switch boxes. The worth of this depends on the tightness you are trying to achieve, but typically is only needed if aiming for very tight outcomes where full HVAC ventilation is to be installed.
INTERMEDIATE FLOOR PERIMETER:
Pitfall: Yikes! This is unfortunately a very common thermal failure on double storey homes.
The builder has follows the insulation notes on the plans, and asked the installers to insulate the groundfloor, and then do the upper floor, as per R-value specifications. Good stuff right?
Wrong.
This leaves the intermediate joist zone, which then haemorrhages heat in winter, and gains unwanted heat in summer. Also, if there are any gaps in the membrane, the intermediate joist zone acts as a superhighway deep into the home for airleakage and wind washing!
Design Out Issue: Detail this area to be insulated in section drawings with arrows pointing to show joist perimeter insulation. This is an area the industry needs to get used to insulating.
Control Measure: Check the tick-box items below have been carried out on site.
Top Tip: With insulation, always think about completeness of the external shell. Do this when thinking through the plan in your mind, and when organising installers. Do it again before signing off the job and starting plastering. Ask yourself, are there any un-insulated sections in the outside skin?
LINTELS:
Pitfall: Can you see what’s wrong here?
The insulation looks well installed...
...except that you can see the lintel.
Pine has an R-value of R0.1 per 10mm. So, the typical 45mm lintel will have an R-value of R0.45. This is a long way off the value of the batts going in the wall (commonly R2.5). If you add it up, there is a lot of lintel area in the average home. On a well insulated wall you can’t see the lintels! (This is discussed more fully in STEP 1, Thermal Bridging 1 - Frame.)
Design Out Issue: Put a note on plans to notch out insulation over lintels.
Control Measure: Check the tick-box items below have been carried out on site.
ROOF EDGES:
Pitfall: As discussed more fully in STEP 1, Design Out Compression, the limited volume in the attic-space at some roof edges severely compromises roof insulation around the perimeter, and makes installing the specified ceiling batt thickness impossible. Often batts are missed completely because they wont fit, or sometimes worse, batts are pushed against the roof line where they absorb condensation and become wet through, leading to mould and health issues. And batts that don’t overlap the top plate allow a pathway for wind washing, as seen above.
Design Out Issue: Confirm batt thickness specified will fit within the structure, and where space it too tight, introduce deep rafters or raised heel trusses as in the picture below.
Control Measure: Check the tick-box items below have been carried out on site.
BOX GUTTERS:
Pitfall: Another common area of where not enough space is allocated for batts to fit with full loft is under box gutters. The double whammy, as the picture shows, is that the ceiling blanket also never runs under box gutters, which as can be seen, can get very hot in the sun. Often this section will be completely missed, or batts will be compressed into place,where they can often accumulate condensation drips.
Design Out Issue: Check the insulation specified on the plans will fit within the structure at design stage, and upsize the rafter depth to compensate, or create a dropped bulkhead to accommodate the insulation thickness if required. Alternatively, specify denser insulation where a product exists. (See typical higher density insulation thicknesses in the pic below.)
Either way, this is one area that really needs consideration and problem solving at Design Stage!
Control Measure: Check the tick-box items below have been carried out on site.
NOTE: Where such an area loss continues across a wide span of a room, contact the Thermal Performance Assessor to confirm whether this has been allowed for in the rating.
300 CENTER JOIST SPACINGS:
Pitfall: What a MESS! What's going on here?
Batts typically come in 430 and 580mm widths, for 450 and 600 centre joist spacings respectively.
In this picture the engineer has specified 300mm centre joist spacings, perhaps due to a longer span requirement and thickness limitations - but no-one told the insulation installer!
Then, when the installer was on site, faced with the issue, instead of cutting the batts they have just been squashed in creating lots of gaps and cracks and compression.
This was a waste of time. Such an install needs to be pulled out and re-done!
Design Out Issue: Check the engineering. Where 300 centres are called for, insert a note in the relevant plan section for "batts to be cut to fit".
Control Measure: Check the tick-box items below have been carried out on site.
HOT TIP: Instruct ALL trades, that if they are unsure about something, or something is out of the norm, THEY MUST CONTACT THE BUILDER FOR INSTRUCTIONS BEFORE PROCEEDING. If this imderstanding becomes the norm for your build team, it will solve so many problems, in so many areas...
WALLS ABUTTING ATTIC SPACE:
Pitfall: Dwarf walls abutting roof spaces are very often left un-insulated. If you don’t insulate them, not only are you not complying with code, but the home will haemorrhage heat in the winter and be subject to unwanted heat gain in the summer through such a wall.
BUT, though it’s in the vertical plane, this ‘wall’ should be thought of as a ‘ceiling’, in that it abuts an unconditioned roofspace. As such it needs to be insulated to the same level as the ceiling, not the walls, otherwise a weak point is created.
In practice that means usually at least an R3.5 between the studs.
But be careful, such wide batts may not hold well between the studs, so will likely need to be stapled or stringed in.
Design Out Issue: If you are an architect or designer, please detail this on your plans. Though it is a code requirement, it is not yet common practice. Notes and diagrams on plans spread the message a makes sure it gets done.
Control Measure: Check the tick-box items below have been carried out on site.
ATTIC ELECTRICALS:
Pitfall: Missing batts in the roof around electricals...
Fans, lighting, flues, heaters, etc. Anything that gets hot in the roof will usually have a requirement for insulation to be offset, to control fire risk.
This is a good thing; we don't want fires in our roofs!
The typical offset is usually only 50mm. But does anyone take a tape measure into the roof? Not often, if ever...
In practice, insulation installers err on the side of caution and just miss out batts completely where there are any of these items. That is what has happened in the picture above.
NatHERS assessors on the other hand generally assume minimal offsets are adhered to. So,where they are not, the house will not perform to it's rating.
Design Out Issue: It is important to know the relevant minimum offsets required for specified plant in roof, and to detail offsets on the plans for builders to follow. You will find this information in relevant standards and in the product literature - or call the manufacturer tech department.
Control Measure: Check the tick-box items below have been carried out on site. To facilitate this Control Measure, builders will need to check standards and plant specifications, and instruct insulation installers as to what the minimum offsets to maintain are.
BUILDERS THERMAL PERFORMANCE CHECKLIST
Sub-Heading Navigation:
Bulk Insulation Stage (Currnet)
WIRE & PRE-PLASTER CHECK:
Pitfall: SO HERE'S A TIP for new builders wanting a quality insulation job.
Make sure you organise the electrician to drop by once the insulation is finished, and before plastering starts, to check all their wire ends are pulled to the front of the insulation and in the right position!
This step is worth it’s weight in GOLD.
If you don’t do it, there’s a very good chance come fitoff, that the electrician will have to BUTCHER the plaster and PULL OUT insulation, hunting for wires pinned on the backside…
...AND, use the time waiting for the electrician to arrive, to do a last run around, pushing back any batts that have slipped, and fitting offcuts into any gaps and cracks you notice.
So when the plasterers close up, you know the insulation is done, and done right!
(Like in the picture above. Look how tight those batts are cut and how flush they are to the front of the studs! WELL DONE LADS!!)
Design Out Issue: None, this is a site control issue.
Control Measure: Check the tick-box items above and below have been carried out on site.
BUILDERS THERMAL PERFORMANCE CHECKLIST
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Pre-Plaster Stage (Current)
As discussed in Step 1, Tightness Layer 2 - the plaster/internal lining, plaster is one of the important layers to get right in making our homes tight, and below the ~10 airchanges per hour @50pa that NatHERS assumes. But the plastered surface, common in standard construction, is also the finished surface and has junctions and numerous items that penetrate it.
While you don’t need to have a perfectly complete surface to achieve the NatHERS tightness assumption (10ACH@50pa is not that tight), you do need to minimise and caulk penetrations where they exist.
In the section we will look at some of the main areas that represent gaps in the plaster.
NOTE: Before embarking on all the measures to tighten the plaster we recommend you do a blower door test on one of your jobs to see how tight you are already building. With tight plaster and especially on slab construction, it is reasonably easy to get down to tightness levels where HVAC is required to maintain indoor air quality. (See Pre-Handover Stage, Blower Door Test below. And Healthy IAQ & what if it's too tight?)
SKIRTING BOARDS:
Pitfall: As can be seen in the picture, the squareset cornice is nice and tight, but there is a gap at the wall/floor junction. This gap is purposeful, and is inserted to allow for expansion and contraction of the framing without popping or cracking plaster. Such expansion gaps are common in construction and required by code.
But as you can see from the thermal image (thanks Efficiency Matrix!), this junction can often be an area of unwanted leakage.
It is possible however to seal this leakage point and allow for movement, by using a flexible caulking for a tight finish.
Design Out Issue: Apart from perhaps a note, this is a site control issue.
Control Measure: Check the tick-box items above and below have been carried out on site.
SHADOW LINES:
Pitfall: One of the downsides of internal brick walls, which by their nature are not completely flat surfaces, is creating a tight junction with plaster. Here’s one solution.
Design Out Issue: Detail a p50 plaster bead and caulking seal. This creates a reasonable long term seal, while also providing a sharp pleasing shadowline.
Control Measure: Check the tick-box items above and below have been carried out on site.
(This is a general note to builders to make sure the wall/ceiling junction is a tight sealed joint, The method is up to builder depending on the wall and ceiling material and cornice used.)
CAVITY SLIDERS:
Pitfall: While cavity sliders can be very useful when space for door swings is at a premium, the gap into the slider pocket represents a large hole in the plaster at each and every slider door. It is not uncommon to walk by a cavity slider on a windy day and feel wind coming out of the opening!
Design Out Issue: The easiest way to remove the issue is to design out cavity sliders where possible!
Control Measure: Where cavity sliders remain in the plans follow the control measure as per the tick-box instructions below. Alternatively, pre-seal slider pockets, as detailed in STEP 1, Tightness Layer 2 - the plaster/internal lining.
CABINETRY BEHINDS:
Pitfall: It is not uncommon for the plaster behind cabinets to be left out, especially at the ceiling line. Visually it is fine as it gets covered up. However once cabinets are placed, it is almost impossible to caulk them.
And cabinets are not air seals!
Air will find a way through gaps, cracks and joins to get to that plaster hole behind. The best approach is to make sure that plaster is complete, and junctions sealed and caulked prior to placing any cabinetry. This note is to help remind the builder before the cabinetry arrives and makes sealing impossible...
Design Out Issue: None, this is a site control issue.
Control Measure: Check the tick-box items below have been carried out on site.
REAR CABINETRY PENETRATIONS:
Pitfall: Even when plaster is complete a lot of built in cabinetry will have cables, pipes and ducting run into it, typically from the walls. Such penetrations are usually not caulked as they wont be seen, but each of these services represents a hole in plaster that ‘loosens’ the airtightness of the home.
All these penetrations are easy to caulk, and don’t even need to be done all that neatly, as they are to be covered by cabinetry. The hard part is to remember to do it! Once the cabinetry is there, it is too late to go back, and the leakage point is locked in.
This, of course, is the benefit of using a thermal quality control reminder checklist!
Design Out Issue: Mostly a site control issue. Perhaps a note on the cabinetry internal elevations to caulk plaster penetrations behind cabinetry?
Control Measure: Check the tick-box items below have been carried out on site.
BUILDERS THERMAL PERFORMANCE CHECKLIST
Sub-Heading Navigation:
Plaster Stage (Current)
The final stage when caulking will be your friend…
ARCHITRAVES HEADS:
Pitfall: Ideally windows and doors are foam sealed at Lockup Stage, though this does not always close all pathways for air infiltration.
A secondary line of defense against air leakage is to caulk the architrave. This is usually done as part of a neat painting job.
HOWEVER, what you can’t see often gets ‘forgotten’, and in this case it is the window and door heads, which if not sealed can be a source of leakage.
Easy to fix however with a bit of vigilance and a checklist! Make sure all architraves and skirts are caulked back to the plaster prior to final painting.
Design Out Issue: None, this is a site control issue.
Control Measure: Check the tick-box items below have been carried out on site.
MANHOLES:
Pitfall: As mentioned in STEP 1, Insulation & Tightness, the man-hole is usually left uninsulated...
But remember, hot air rises and the uninsulated, and ventilated attic is just beyond. It needs to be insulated! And sealed tight!
One standard detail as below, put on the plans is all it takes.
Good job for the apprentice.
‘Nough said really!
Design Out Issue: Make this a standard detail on your plans!
Control Measure: Check the tick-box items below have been carried out on site.
BUILDERS THERMAL PERFORMANCE CHECKLIST
Sub-Heading Navigation:
Fix Stage (Current)
As we promise higher performance Risk goes up. This stage is all about compiling evidence to prove the house has been built as required to meet the energy rating. (Just in case fingers start getting pointed...)
PHOTOGRAPHIC RECORD:
Pitfall: NO PITFALL - JUST OPPORTUNITY!
Good insulation installation pictures are the best evidence you can have to show that you did your job, as complete pictures like this point to a level of thermal detailing likely applied throughout. Take pictures (or videos) of each room. Similar pictures can be taken of well-sealed wraps. And both can be really useful marketing for establishing performance building credibility!
Design Out Issue: None. Other than making sure this QA template is in your plans for the builder to complete as part of the tender obligations!
Control Measure: As per tick-box instruction below
THERMAL IMAGES:
Pitfall: AGAIN, NO PITFALL - JUST OPPORTUNITY!
A thermal camera for a few hundred dollars, is one of the best investments you can make as a builder, and is another good tool for checking performance. Whether you give the photos to the owner, or keep them to look over with your team is up to you. (Remember this checklist is for you to modify as you see fit.)
It doesn’t take long with a thermal camera to isolated problem areas in your builds and improve thermal outcomes. It is also one of the tools that makes the invisible, visible for the team, so that they understand why they are taping,caulk, sealing, and insulating as they go, and where they should be focusing effort.
The other tool is the blower door test. (See below…)
Design Out Issue: None. Other than making sure this QA template is in your plans for the builder to complete as part of the tender obligations!
Control Measure: Check the tick-box items below have been carried out on site.
BLOWER DOOR TEST:
Pitfall: How tight are the homes you build? Do you know? If not, you should get a blower door test on your next build. A blower door is simply a big fan that gets put in the front door opening of a home, with other doors and windows shut. The fan is connected to a computer. The fan is turned on to either suck air out of the house (a negative pressure test), or blow air in (a positive pressure test). It sucks or blows to 50 pascals(50pa) of pressure (equivalent to a 30km/hr breeze blowing over the house). How hard it is for the fan to work, depends on how well sealed the house is.
The computer measures the resistance to the airflow through the turn of the fan blades. And by that resistance can work out the amount of air leakage.
As mentioned previously, NatHERS assumes <10ACH@50pa (Air Changes per hour at 50 pascals of pressure). So, if you are tighter than that you comply with the tightness assumption of the NatHERS model. And the tighter the better, from an energy saving point of view...
However, you don’t want to go too tight – at least not without mechanical ventilation!
The NCC, under verification method H6V3, suggests not going under ~5ACH@50pa without mechanical ventilation, otherwise indoor air quality may suffer. We will look at this in further detail a little later in Healthy IAQ, but for now, a reasonable rule of thumb is aim for less than 10 and more than 5, unless planning for mechanical ventilation…
...in which case you can try and go as low as you can!
Design Out Issue: None. Other than making sure this QA template is in your plans for the builder to complete as part of the tender obligations!
Control Measure: As per tick-box instruction below. Builders should keep this record for themselves, even if not shared with the client.
WANT HOMES THAT PERFORM TO THEIR NatHERS RATING?
YOU CAN’T DO IT ALONE. IT TAKES A TEAM.
SUCCESS DEPENDS ON CREATING A HIGH-PERFORMANCE CULTURE. AND THERE ARE 2 EASY STEPS…
When you first start trying to build comprehensively insulated tight structures, the biggest impediment is the status quo of standard practice. Trades tend to do what they have always done.
And as we come from a history where performance was not a priority, that is what you get. Low performance builds - again and again.
It is incredibly frustrating for an aspiring sustainable builder to come to site and see gaps in insulation, or poorly executed wraps, or penetrations to cavities not caulked behind cabinetry. Or worse yet, mysterious batts on the floor at plaster completion!
And you know you had that toolbox talk!!!
What they hell happened?
YOU HAVEN’T CHANGED THE CULTURE OF YOUR BUILDING TEAM.
So here are 2 EASY STEPS that will change things OVERNIGHT. (Or at least get them moving in the right direction 😉)
1) Get a Thermal Imaging Camera. Give it to your Project Manager. Get them to take lots of pictures. Get them to talk to every trade who comes on site and show off this fancy new gadget. Show how it can SEE THROUGH PLASTER…
2) Tell trades that you are doing a BLOWER DOOR TEST on the job. Explain what that is, and why its super important that all taping, caulking, sealing gets done right. Explain that, "We don’t want to fail and have to redo later…"
That’s all you have to say.
Trades will put 2 & 2 together.
What you have told them is that INSULATION and TIGHTNESS and THERMAL PERFORMANCE is as important to your company as the finish.
AND YOU HAVE THE ABILITY TO CHECK!
Even better, invite trades to the blower door test when you have one! It’s an educational and team bonding day, so the more the merrier.
Do this for the next couple of jobs and watch your culture change before your eyes…
We hope you enjoyed the tour of the stages of standard construction!
Next we look at potential issues that can be associated with high performance construcion...and what to do to make sure they don't happen on your site!
BUILDERS THERMAL PERFORMANCE CHECKLIST
Sub-Heading Navigation:
Pre-Handover Stage (Current)
TIME TO ADDRESS ANOTHER ELEPHANT IN THE ROOM - CONDENSATION AND AIR QUALITY.
IS IT TRUE THAT 7 STAR+ ENERGY EFFICIENT HOMES ARE AT RISK OF DANGEROUS CONDENSATION LEVELS, MOULD, AND INDOOR AIRQUALITY ISSUES?
The answer is No
And Yes
And Sometimes.
Ultimately an unsatisfactory, “It depends”.
But understanding what it depends on is the key to builders and designers mitigating risk and creating high performance healthy structures.
So, what are the issues?
What is the answer?
And what are the practical things we need to do going forward?
To cut to the chase,
Yes, we can do it all - Performance & Health.
This section will attempt to break it all down.
Step by step.
From building physics to the practical, implementable solutions you need for a healthy outcome.
Let's start by looking at the potential problem...
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
THERE ARE 3 MAIN CONCERNS RAISED IN RESPONSE TO THE MOVE TO 7+ STARS AND BUILDING TIGHT:
A) Tighter homes have less air exchange with the outside & so as we breathe & live in the homes, the humidity could build up, making condensation & surface mould more likely
B) More insulation in our homes keeps more of the heat in, consequently less heat escapes into wall/roof cavities, which are then cooler compared to poorly insulated older homes, making condensation more likely to occur. In turn, potentially leading to hidden mould/rot, & even early structural failure.
C) Tighter homes with less air exchange, means chemicals can build up to unhealthy levels, potentially negatively influencing occupant health.
YOU SCARED YET?
Ready to go back to 5 star? Or 3? Or just give up on insulation and tightness all together?!?
Relax, there’s a lot of strategies we can employ to significantly reduce risk.
But first we need to understand a bit of building physics...
Let's start with the Condensation that leads to potential issues A & B.
Sub-Heading Navigation:
What are we Worried About? (Current)
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
WHAT IS CONDENSATION?
The air is like a sponge. When it warms it expands and can soak up a lot of moisture. When it cools it contracts and can't hold as much. Condensation is what happens when warm moist air hits acold surface which cools the air next to it and which consequently contracts. If it contracts enough, it can no longer hold the water vapour in it, and droplets condense out onto the cold surface.
This often happens in cold climates when relatively warm moist internal air, hits cold single glazed windows. The air touching the window contracts, can’t hold the water vapour in it anymore, and the moisture condenses as droplets on the glass.
It can of course do the same anywhere in the house, on any surface where thermal bridging creates cold spaces, or in cold cavities...
And this brings us to the 4 control strategies that we will be looking at in this series.
1 & 2 are good strategies against Surface Mould
2, 3, & 4 are good strategies against Cavity Mould & Rot.
We will start by looking at Strategy A in the next post, along with a little bit more building physics…
Sub-Heading Navigation:
Condensation Control Strategies (Current)
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
THE GOOD NEWS IS, ALL ELSE BEING EQUAL, 7STAR+ HOMES SHOULD HAVE WARMER SURFACES THAN 6 STAR HOMES, AND SO BE LESS LIKELY TO GET SURFACE MOULD.
Have a look at the attached graph. It shows how relative humidity changes with temperature.
What's relative humidity you ask?
It is simply a ratio between how much moisture is in the air, vs how much it can hold.
100% Relative Humidity = Condensation.
Each column in the graph represents the same amount of water in the air. The dark blue is the moisture typically already in the air. The light blue, moisture added by people living in the home.
As the air gets warmer it's relative humidity goes down. As the air gets colder it goes up. In this Eg. when the air reaches 13 deg next to a cold surface, relative humidity reaches 100%, and condensation occurs.
A couple of important takeaways from this graph:
1) If we avoid thermal bridges, (which we want to do for energy efficiency too!), our surfaces will be warmer, and surface condensation less likely
2) If we can cut the habitation moisture (which we will look at later in the series) condensation potential also reduces
3) You don’t actually need condensation to get mould germination. Extended periods above 70% humidity can do it (below 20deg in this example)
4) 40-50% humidity is the ideal zone, to also avoid dust mites, but is very hard to achieve year-round in many climates without dehumidification
So, simply put, consistent warmer surfaces should help reduce surface mould.
Limiting thermal bridging and quality insulation installation to achieve this, also improves thermal performance.
Win-win! 😁
But what if you build too tight? Won’ t that lead to problem-level humidity buildup?
Possibly.
Particularly in smaller homes and apartments where the buildup can be quicker.
But we are getting ahead of ourselves...
We’ll look at tightness and ventilation later in the series.
The point here is all else being equal, well-built 7+star homes should help reduce internal surface mould.
Cavity condensation however is another story.
We will look at that and some related building science in the next section.
Before we do however, here are some examples of mould and condensation that can happen in any home regardless of star rating if thermal bridging and poor insulation practices are allowed.
If we build to minimise thermal bridging, and install insulation to achieve on ground thermal performance, we also minimise cold spots and surface mould opportunities. Win-Win.
Also note: Mould is VERY OFTEN caused by actual water leaks!! Always check that possibility first. (Thermal imaging cameras can be useful for that too!)
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues (Current)
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
IS CAVITY CONDENSATION (KNOW AS INTERSTITIAL CONDENSATION) & ASSOCIATED MOULD/ROT MORE PREVALENT IN 7STAR+HOMES?
Not necessarily - depends on how you build of course!
But here’s the logic of the argument & a bit of building science...
COLD CLIMATE Example:
Insulated homes, while energy efficient, have colder cavities than uninsulated homes. This is particularly true on the cladding side of the cavity, which will be close to the outdoor temperature (& can be significantly colder for roofs when radiating to a clear night sky!)
If warm moist air gets in an insulated cavity, & moves towards the outside, it will be prone to condensation in cold weather.
But why would warm moist internal air enter the cavity in the first place?
Answer: Vapour Drive.
Warm moist air has a drive to move through the fabric of our homes. The more gaps & cracks you have in your linings & membranes the faster it will enter the cavities, & the greater will be the amount of condensation that occurs when temperatures & humidity levels are right.
Condensation is further exacerbated if the air vapour meets an impervious surface & moisture loads are able to build up in cavities.
The following 2 slides show the consequence of long-term condensation & inadequate drying, for both cool/cold and tropical climates.
Obviously adverse health implications &structural damage can occur from this dynamic.
But are 7 star+ homes more likely to get interstitial condensation compared to 5 or 6 star homes?
All else being equal, the difference should be marginal. Under the previous 6 star regime, cavities were generally already fully insulated. And even if we can upgrade the R value, (& good design is a much better way to get to 7+), the extra coldness in cavities will be minimal (in line with the diminishing performance returns you get adding extra insulation).
Does that mean nothing to worry about?
Unfortunately, NO.
The issue is already out there in the houses we currently build. The Genie has already escaped the bottle years ago.
But solutions exist!
Which brings us to Control Strategy No. 2
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues (Current)
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
THE EASIEST AND MOST IMPORTANT MEASURE TO REDUCING CONDENSATION AND MOULD POTENTIALON SURFACES AND IN CAVITIES, IS TO REMOVE WATER VAPOUR AS IT IS MADE.
We can’t limit breathing.
But we can do a lot to limit water vapour from other larger sources which add to humidity build-up in the home.
Less added water vapour = less condensation issues. It’s that simple.
Start with good, ducted extraction fans.
Gone are the days of extracting into roof spaces. All extraction fans under NCC2022 must duct externally and have backdraft dampers incorporated.
Undercut bathroom doors 20mm, to supply adequate makeup air.
And instruct owners in the handover manual that they must run their fans 10 minutes after they have had a shower/bath to remove the moisture that evaporates off walls/floors over time. Better yet, install run on timers and humidity-sensing fans for a more guaranteed outcome.
NCC2022 now also requires non-condensing dryers to be fitted with external ducting kits. This saves ~5 litres of water being added to the air on each use.
And in the kitchen a good rangehood saves ~3 litres of water vapour a day, and another 2 per day (total 5) if you have induction cooking.
(Yes, burning gas creates water! Another good reason to move away from gas cooktops!!)
And if you install reverse cycle A/c, you effectively have dehumidifiers in the house!
Make sure you show owners how to use the ‘Dry’ button. This can be really useful to dehumidify in humid shoulder seasons. And the cool function is useful to dehumidify in humid hot weather.
Unfortunately, in cool climates you don’t get dehumification from the heating cycle. (Unless you are fortunate to have the old Daikin US7).
What then can you do in these climates?
Consider HRV.
Because the outdoor air is colder, if you bring it inside with an HRV, the relative humidity of the new air goes down as it warms up, effectively lowering the home's humidity over time.
For a cheaper less refined version of the same principle, the Whisper Green extraction fan pictured has the added functionality that you can set a dial for low powered continuous extraction. This extraction creates a slight negative pressure in the house, which in turn draws air in through infiltration pathways.
As the incoming air heats, it’s relative humidity goes down, and so does the home’s overall humidity. While this doesn’t have the same heat recovery as an HRV, it could get you out of trouble if moisture levels ever become an issue for owners down the track…
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home (Current)
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
WANT LESS CONDENSATION IN YOUR CAVITIES? BUILD TIGHT!
Less moisture in = less condensation in the cavity. It’s not rocket science.
And the good news is building tight also increases energy efficiency. Yay! Win-Win!
So, keep doing all that good stuff we’ve been banging on about:
Caulking
Sealing cavity sliders
Limiting downlights and other penetrations
Foam filling around windows, doors and pipes
Taping membranes
Sealing manholes and junctions
Etc.
But will this stop all moisture ingress in standard construction? Plaster doesn’t stop water diffusion, does it?
No.
But look at this picture.
Vapour air leakage transport through holes brings moisture into cavities much faster - 90 times, according to the slide! If you seal gaps, water still diffuses through…but much more slowly.
And speed matters.
Slower vapour loading allows cavities more time to dry if condensation does occur, through vapour permeable membranes on the outside. (Which we will look at in the next Strategy)
The aim is to achieve greater drying potential than wetting potential.
And plaster also has the benefit of being able to dry to the inside, which can be a handy feature, especially in mixed or tropical climates where vapour drive can come from the outside.
If you want to go the next step however in minimising vapour ingress and are in climates where vapour drive from the inside predominates, consider non-permeable internal membranes. And look out for membranes with some smart back-drying capability. Not only do such membranes minimise condensation risk considerably, but the installation is usually combined with battening off plaster, allowing a convenient service channel for pipes and wires, and a dedicated insulation space.
More effort yes, but worth considering, especially if you are in a more susceptible microclimate.
This begs the question; how do I know if my building is at high risk of condensation?
Read on...
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities (Current)
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
THIS IS AN IMPORTANT QUESTION TO ASK WHEN CONSIDERING WALL & ROOF ASSEMBLIES (which we look at next). WHAT FACTORS SHOULD WARN YOU CONDENSATION MAY BE AN ISSUE TO BE ADDRESSED ON YOUR JOB?
Let’s start with CLIMATE:
This useful table adapted from the excellent Pro Clima Australian Study, is a great firstport of call. The added arrows show predominant vapour drive direction, coloured by risk.
Other than the dry Climate Zone 3, most areas have risk, usually seasonal.
In the tropics the risk is HIGH during the wet summer in airconditioned buildings, as water vapour drives from the moist outdoors towards the back of cooled internal linings.
In Cool climates the risk is CERTAIN, especially in winter when temperatures near the external cladding get colder.
See Understanding Cavity Condensation Issues.
Next consider MICRO-CLIMATE:
Is your build:
…all these suggest Condensation Risk goes up.
Lastly consider WHAT YOU ARE BUILDING:
If having asked these questions you find your project is likely at higher risk consider:
Now that we know what to look out for, let's look at the last Control Strategy: Creating wall and roof assemblies that allow for the condensation that will happen, to do so where it can drain away and dry out Safely...
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Risk factors for Condensation (Current)
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
YOU HAVE MINIMISED WATER VAPOUR IN THE HOME (See Control Strategy 2) & LIMITED WATED VAPOUR ENTERING CAVITIES BY BUILDING TIGHT (Control Strategy 3) – WELL DONE. SOME MOISTURE HOWEVER WILL STILL MAKE ITS WAY INTO YOURWALL CAVITIES…
And if you are in cooler climates, that moisture will cool as it moves through the walls under vapour drive. And then, if outdoor temperatures are right, condense…
…in your wall!
This is especially so where non-breather wraps are used trapping the moisture in.
Or, where a breather membrane is used, but direct fix cladding stops them being effective in letting vapour out.
This situation can lead to wet insulation and stud rot.
Especially on the bottom plate, where water gathers. (See above pic.) Especially on the colder South side of the house, where things don't dry well at the best of times.
So what's the solution?
There are a couple of levels of control you can apply.
The first below is to include a ventilation/drainage cavity with a permeable Class 4 membrane.
This allows water vapour to pass through, and if it condenses on the cladding, to drain away.
But don't forget to put a ventilation opening at the top (subject to BAL ratings & cladding requirements).
On a cold night water may still condense out before reaching the Class 4 membrane, and you want it sitting in your studwork for as short a time as possible. Ventilation at the top of the cavity will allow the stack effect to get going when the sun warms up, and turbo-charge evaporation and the drying potential in the wall.
Ok, but what is a Class 4 membrane?
And what if I'm in the Tropics?
And what are the other levels of control you hinted at?
Good Questions.
Let's look at them in order...
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out (Current)
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
YOU’VE PUT IN A DRAINAGE CAVITY. GREAT. BUT WATCH OUT WHEN ORDERING WRAPS! IF YOU ARE IN CLIMATE ZONES 4-8 CODES HAVE CHANGED. HERE’S WHAT YOU NEED TO KNOW.
To address condensation in walls NCC2022 now requires that wraps (where used) in climates where vapour drive is primarily from the inside to the outside, be more permeable to moisture vapour than in the past. Which is what you want, so moisture vapour can get out and not build up and condense in the wall.
Look at the following slides.
In climate zones 4-5 wraps need to be not less than 0.143 µg/N.s, and in zones 6-8 not less than 1.14 µg/N.s
But what are these funny units?
It’s Micrograms per Newton per second. And is the amount of water vapour in micrograms that passes through one square meter of material in one second under a pressure difference of one newton per square meter. Tested when the temp is 23deg at 50% relative humidity.
And a microgram is 1 millionth of a gram.
Luckily you don’t need to remember that. Just remember:
0.143 to 1.14µg/N.s = Class 3
1.14+µg/N.s = Class 4
And make sure you check wrap Class when choosing!
For Class 4, the bigger the number, the more water vapour can get through.
Let's look a wrap types.
Both wraps in the picture are Class 4, but always look at the Specs.
The wrap on the right is more permeable. (4.5µg/N.s vs. 1.45µg/N.s)
Importantly it is also a WATER BARRIER, which stops liquid water coming in from the outside. (Think Gore Tex jacket, waterproof, but breathable.)
A water barrier must be used where there is no drainage cavity in most cladding systems, or when specified by the cladding manufacturer. (Again, ALWAYS check the Specs.)
The 'breather' foil is NOT a water barrier, as permeability is created by making holes in the weave that liquid water CAN get through. But it does give a handy +0.5R value facing a cavity. So, if choosing to use foil, you have to trust the cladding system, as the cladding now becomes the water barrier.
It is also important to note most 'breather' foils are Class 3. And non-breather foils are Class 1 and don't breath at all. Installing the wrong wrap Class for your climate can exacerbate condensation issues, rot and mould. Also, many products require the wrap to be a Water Barrier and to have Class 3 or 4 permeability, and only material membranes will currently meet this.
So, check specifications, understand product benefits and limitations and choose wisely!
Ok, but what about the Tropics – where vapour drive and what you have to do all turns around…
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Climate and Membrane Choice (Current)
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
THE NCC GIVES WALL PERMEABILITY REQUIREMENTS FOR CLIMATE ZONES 3-8. BUT WHAT ABOUTZONES 1-2? WHAT ABOUT THE TROPICS?
“Chirp”…“Chirp”…
…CRICKETS…
If you are in the Tropics there is no Deemed to Satisfy in the NCC!
What do you do?
Back to Performance Provisions instead.
NCC2022 H4P7 states, “Risks associated with water vapour and condensation must be managed to minimise their impact on the health of occupants.”
Ok. Great. How?
H5V6states, “Compliance with Performance Requirement H4P7 is verified for a roof or external wall assembly when it is determined that a mould index of greater than 3, as defined by Section 6 of AIRAH DA07, does not occur on—
1.the interior surface of the water control layer; or
2.the surfaces of building fabric components interior to the water control layer.
A water control layer is either a wrap that is classed as a Water Barrier, or a very trusted cladding (say a well flashed metal roofing profile), separated by a drainage cavity.
A mould index of 3, means not more than 10% of the surface can show any mould to the naked eye.
GOT IT?
Know what to do?
I thought not.
Unfortunately, the code doesn’t tell you what to do.
Technically it’s Performance Solution time.
Because Vapour Drive is generally from the outside in, in Zones 1-2, you want a well taped LOW Vapour permeable membrane on the outside, to keep the warm moist air out of the usually cooler inside. If you condition your building and don’t keep it out, you can get condensation and mould on forming on the backside of the plaster. (See Vapour Drive above)
Solution– choose a Class 1 foil on the outside of the studs, and tape well.
Well…Maybe.
But stuff happens.
You may get a seasonal turn around in vapour drive.
Rain might get in somewhere.
A pipe might spring a leak.
It is generally considered good practice to have some drying ability to the outside of the wall, which a Class 1 wrap would inhibit.
So, the rule of thumb from those in the know is:
“As breathable as you can and only as low permeability as you need.”
Clear yet?
I thought not.
This is where you should probably check with an expert that your proposed assembly will not lead to a mould index >3.
Advice - Unless you are very sure about the suitability of construction systems for your climate, you should probably get a WUFI or similar to try and cover risk.
A what?
A WUFI.
Yes, dear reader, we are down the rabbit hole…
Will try and explain, and you get out...
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics (Current)
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
WHATIS W.U.F.I? AND WHY IS IT USEFUL?
WUFI is one of several leading hygrothermal analysis simulation software. It is also the one most commonly used in Austalia.
WUFI stands for Wärme Und Feuchte Instationär which, translated from German, means heat and moisture transiency.
It is used to calculate heat and moisture transfer through multi-layer building components exposed to local climatic conditions, to assess the likelihood of interstitial condensation and mould growth within proposed building assemblies at the subject site.
A WUFI report will then tell you what the Mould Index on the outer surface of the insulation layer is likely to be. If greater than 3 (see above), you have a problem. And while only a prediction, it would be a brave builder or designer who built against the advice of the report.
So why do a WUFI?
1.As we saw in last post there are some climate zones in Australia where there is no Deemed to Satisfy (DtS) detail to follow.
2.There has also been some discussion that in colder climate zones, the proposed DtS in the NCC may not be good enough…
3.And lastly you may want to make up your own assembly, or use a material or product not discussed in the National Construction Code (NCC).
If you are doing anything outside of the DtS you should cover yourself by getting a WUFI report.
You may even find in the future that Building Surveyors will require proof that your proposed non-DtS assemblies meet the performance requirements of the NCC. In which case a WUFI can form the evidence in a Performance Solution to show compliance with the NCC.
At the end of the day, a WUFI is just another consultant report, but one that can save thousands in rectification costs.
When in doubt, get yourself a WUFI.
You can search for qualified practicioners across Australia here.
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
WUFI Analysis (Current)
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
WHAT ABOUT ROOFS? CAN ROOFS HAVE CONDENSATION PROBLEMS?
You Betchya!
Indeed, as heat tends to rise, & moisture tends to follow heat, roofs can have an increased risk, especially in cool climates.
This tendency can be further exacerbated by air leakage pathways through the plaster to the roofspace: at downlights, extraction fans, ducts, cavity sliders & ducted heating return vents, etc. Such pathways represent a lot of potential moisture loading through vapour transmission.
If you add in a cold roof surface under night sky cooling, you can start to understand how standard construction has the potential to lead to a lot of condensation on the underside of the roof surface, or single foil as in the picture.
Such condensation leads to dripping & wetting of batts. And in more steeply pitched roofs (where drips run down the under-surface), it can lead to the soaking of any batts incorrectly jammed up to the underside of the roof sheets at external walls. (As in the pink wall picture below.)
Over time, these high levels of wetness will quickly turn to mould, rot… & headaches for the builder!
SO, WHAT TO DO?
Just like with walls, Control Strategies 2 & 3 will have a big impact on limiting the problem in the first place.
For instance:
Control Strategy 2 - Minimise water vapour in the home, by ducting quality extraction fans to the exterior, &,
Control Strategy 3 – Limit water vapour entering cavities, by designing out penetrations of the internal (plaster) lining, & then using site Control Measures during construction to caulk & seal.
REMEMBER: Less moisture loading =less problem.
However, some water vapour will still get into roof spaces, & IT WILL condense IF it cools enough.
This is where we need to look to Control Strategy 4.
And, similar to walls, best practice is to have a ventilated cavity to remove the buildup of moist air & enhance drying where condensation does occur.
Indeed, under NCC2022, if you are in Climate Zones 6-8 then the code NOW REQUIRES such ventilation.
And there are 2 broad systems you can implement to achieve this.
We will look at what they are and if they work below...
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
What about roofs? (Current)
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
THE NCC2022 INTRODUCES A VENTILATED CAVITY FOR ALL ROOFS IN CLIMATE ZONES 6-8, TO MINIMISE MOISTURE VAPOUR BUILDUP AND CONDENSATION
Let’s look at the RULES…
CLAUSE 3.8.7.4 (1):
In climate zones 6, 7 & 8, a roof must have a roof space that is located immediately above the primary insulation layer;
OR
immediately above sarking with a vapour permeance of not less than 1.14 μg/N.s, which is immediately above the primary insulation layer;
AND
has a height of not less than 20mm.
AND
is either - ventilated to outdoor air through evenly distributed openings in accordance with Table 10.8.3;
OR
located immediately underneath the roof tiles of an unsarked tiled roof
In other words, there must be an airflow pathway directly above ceiling insulation in a roofspace
[I’m calling this System 1]
Or an airflow pathway above a Class 4 vapour permeable membrane directly over the insulation
[I’m calling this System 2]
And the roofspace must be at least 20mm high at all points.
The idea is that moisture drive coming from the house can pass through the insulation into a ventilated space & get away.
OK, SO HOW MUCH VENTILATION IS RECOMMENDED?
Look at Table 10.8.3 in the above picture:
You will see that it depends on the pitch of the roof.
This is because the amount of airflow that you get under a roof depends on the stack effect. The more height, the bigger the temperature differential you can get between bottom & top of the roof, which induces a stronger airflow, & thus more ventilation pull through.
At low roof pitches (<10deg) the stack effect is less effective & so you need a bigger opening at both eave & ridge to make up (25,000mm2 in Table 10.8.3 = 25mm high along each meter length of eave & ridge).
At higher pitches the stack effect is more effective, & so openings can be smaller.
NOTE:
AND THAT’S THE RULES.
We’ll look at System 1 & 2 setup,
And pros & cons below…
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space (Current)
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
WE’VE LOOKED AT ROOF VENTILATION & THE NCC RULES. BUT WILL THEY WORK?
Have a look at the picture above taken from the excellent, “Condensation in Buildings – Tasmania Designers Guide 2.”
This shows the basic elements for ventilating roof spaces:
1. Vent at the eave
2. Air pathway at least 20mm high above the insulation
3. Vent at top of ridge for the air to come out.
System 1a above shows an attic.
System 1b below shows a cathedral ceiling.
Sizes for openings can be determined from the NCC 2022 Roof Ventilation Rules above.
BUT WILL IT WORK?
YES
Generally...
But there are also potential failure points that need to be accounted for:
1. NCC Deemed to Satisfy (DtS) rules are not always enough.
Pitch, direction, colour & climate are BIG factors. If you have a low pitched, south facing, light coloured roof, daytime stack effect will be severely reduced. And DtS ventilation not necessarily enough to alleviate problems.
SUGGESTION: In a cold climates, when considering such a roof, check with a WUFI.
2. The stack effect severely reduces at night.
Indeed, a cold roof may cause cool air to fall onto the insulation & between any cracks. As in the picture below.
This is exacerbated if the insulation is not installed tightly, or if you have insulation removed around downlights & vents, etc. Such cold spots can create mould potential.
SUGGESTION: Install batts tightly (& completely) to minimise this problem.
3. On cold still nights many roofs will still get cold enough to create condensation on the underside, or on the membrane next to them.
SUGGESTION: This can be minimised by,
i. Anticon roof blankets which reduce condensation potential by insulating roof air away from the colder roof surface. OR,
ii. Above Membrane Ventilation strategies (see System 2 below.)
4. In cathedral ceilings check your space.
Rafter sizes will often need to be upsized to fit the insulation depth required AND a clear ventilation pathway.
SUGGESTION: Check rafters in engineering can fit insulation R-value depth AND air pathway. Allow ~40mm extra & PLACE BATTS CAREFULLY to achieve the 20mm min. air pathway, accounting for batt variation.
Such considerations should avoid most issues for ventilating attic roofs with System 1, Below Membrane Ventilation.
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs (Current)
System 2 Above Membrane Ventilation
Tropical Roofs and Final thoughts (Under construction)
IS HEAVEN ABOVE? HERE'S WHAT YOU NEED TO KNOW ABOUT ABOVE-MEMBRANE VENTILATION
As discussed, in the new NCC, climate zones 6-8 now need roof ventilation either:
1. Above the insulation layer (System 1 - see above),
OR
2. Above a Class 4 membrane over the insulation layer (System 2, in above picture).
If System 2 sounds familiar, it is.
It is just like wall ventilation requirements. And the same logic applies.
Vapour driving from the inside in cool climates, passes through the assembly & membrane, to condense on the underside of a cold roof in cool weather. Water then drips back onto a tightly sloped membrane, to flow away to the gutter, or down the wall drainage cavity to the ground (when roof and wall membranes are connected.)
Of course, in reality, water vapour isn’t so obliging. Just like with walls, if vapour travelling through an assembly hits the right temperature before the membrane it will condense out, or condense on the membrane like on a spider’s web and not travel through.
But, like walls (& System 1), once the sun comes out, stack effect ventilation will typically be enough to dry the assembly in most cool climates; working better, the higher the pitch and darker the colour.
SWEET.
And the big PRO is that the ventilation IS NOT MIXING with insulation, which is the main downside of Under Membrane Ventilation (System 1).
DOUBLE SWEET.
The CON however is that you can’t use a foil backed blanket to up the roof insulation R-values, so in flat framed roofs to achieve equivalent system R-values, rafters will typically need to be upsized to fit a thicker batt to compensate. (This is not usually a problem in attic roofs which usually have space to spare.)
At lower pitch you also have the issue of keeping the membrane on grade for drainage. And, as with System 1, achieving enough ventilation flow at low angles.
Luckily there are systems and RULES OF THUMB to account for low pitch, for those wanting to use Above Membrane Ventilation.
And they are distilled and assembled here below, for your consideration...
Sub-Heading Navigation:
Condensation Control Strategies
Control Strategy 1 – Minimise Cold Internal Surfaces
Understanding Cavity Condensation Issues
Control Strategy 2 – Minimise Water Vapour in the Home
Control Strategy 3 – Limit Water Vapour Entering Cavities
Control Strategy 4 – Let Moisture Condense where it can Drain Away Safely and/or Dry Out
Condensation Control in the Tropics
NCC 2022 Rules for a Ventilated Roof Space
System 1 Ventilated Attic Roofs
System 2 Above Membrane Ventilation (Current)
Tropical Roofs and Final thoughts (Under construction)
WE BANG ON ABOUT THE NEED TO BUILD TIGHT FOR ENERGY EFFICIENCY – BUT IS THERE A DANGER? CAN WE BUILD TOO TIGHT?
This is an important question.
And the answer is Yes.
Definitely.
And No.
Confused?
Let's break it down.
Tight, is good for energy efficiency.
Tight is also good for minimising vapour transport and moisture loading in building cavities.
But a house is more than energy efficiency.
The number one function of a home is to keep the occupants comfortable & healthy.
And in a tight, well-sealed home, there IS the potential for unwanted chemical buildups if there isn’t adequate replacement of fresh air.
As we breath we use up oxygen and create carbon dioxide…
And create moisture vapour…
And the glues, binders and finishes in the materials of our homes can leach out over time into internal air…
And then there’s chemicals released from gas combustion (for those still with gas cooktops and appliances...)
If a home is tight enough, all these things can build up to unhealthy levels.
This is even recognised in the Code. NCC2022 Table H4V3 gives maximum contaminant levels for indoor air quality (which comes into play when following a Performance compliance pathway through the code.)
But don’t despair.
You CAN have an energy efficient AND healthy home!
There are 3 strategies you can take to make sure the homes you create are efficient AND healthy:
Strategy 1 - Minimising Off-gassing Materials
Strategy 2 - Know your Tightness, (& Aim for the “Goldilocks” zone.)
Strategy 3 - Introduce Ventilation, (and go as tight as you want to.)
The first strategy is a general one all should employ.
But whether you answer Yes or No to the starting question, “Can we Build Too Tight?”, will depend on whether you choose to follow Strategy 2 or 3.
Still not clear?
Don’t worry. We will go through the strategies below, and look at the Pros & Cons of both 2 and 3.
And in so doing, give you a pathway to navigate risk and maintain good Indoor Air Quality, when building homes to perform…
HEALTHY IAQ (Current)
Sub-Heading Navigation:
Strategy 1 - Minimising Off-gassing Materials
Strategy 2 - Know your Tightness
Strategy 3 - Introduce Ventilation
Constant Volume Extraction Fans
**Note: Minimising offgassing materials was already covered in STEP 2, under Healthy Material Selection. It is repreated here however as it is important to consider for this TOPIC.
THE FIRST STRATEGY FOR GOOD INDOOR AIR QUALITY IS ONE ALL SHOULD FOLLOW - BECAUSE IT’S EASY. CHOOSE LOW OFF-GASSING MATERIALS!
This is even more important as homes get tighter & the potential for chemical buildup increases.
Very few of the materials we use in construction these days are single material products.
Most are amalgams, held together with glues & binders made from petrochemicals. Unfortunately, many of these chemicals leach out of the products over time & build up in the indoor air. In some cases, this can effect health, especially for those who are more chemically sensitive, & contribute to what is known in the industry as, 'Sick Building Syndrome'.
Unfortunately, the science in this area is in its infancy.
Though regulators put limits on the amounts many chemicals are allowed to off-gas, these are usually based on short term exposure testing. Very few studies look at chemical effects with long term exposure. Added to that, these chemicals are not often inert, meaning they may be reacting and forming new chemistry with each other.
Even less is known about that...
We certainly don't want to fearmonger, but suggest taking a precautionary approach when specifying materials.
There are 2 things to look for:
1. For things that are wet and come in cans and tubes (eg. paints, glues, waterproofing, etc.), choose products that are either 'low VOC' or 'zero VOC'.
VOC stands for Volatile Organic Compound,and is a measure of how much a product off-gasses.
2. For manufactured timber products (eg. cabinetry, particleboard, mdf,etc.), look for 'E0' rating, or 'Super E0'.
E0 is a measurement of how much formaldehyde offgassing a product has. And the Australian legislated standard is E1, containing a higher level.
Most board products use a urea-formaldehyde glue to bind together the small bits of wood fibre. Unfortunately, formaldehyde is a known human carcinogen. So again, it is worth just taking a precautionary approach & swap to lower offgassing versions where practical.
The other side of the health coin is providing adequate ventilation.
We look at that in Strategy 2 below...
Sub-Heading Navigation:
Strategy 1 - Minimising Off-gassing Materials (Current)
Strategy 2 - Know your Tightness
Strategy 3 - Introduce Ventilation
Constant Volume Extraction Fans
TIGHT ENOUGH TO MEET ENERGY RATINGS. LOOSE ENOUGH TO MAINTAIN REASONABLE INDOOR AIR QUALITY… IS THERE A GOLDILOCKS ZONE?
At the last measure, the average new home was found to have an air tightness of 15.4 airchanges/hr at 50 pascals of pressure (15.4ACH@50pa). NatHERS ratings however assume an tightness of <10ACH@50pa, (the actual tightness depending on inputs dialed in by the assessor). This means most new homes are not meeting their energy rating & most owners are not getting what they paid for!
We need to tighten up & NCC 2022 has recognised this introducing H6V3 which requires the building envelope to be sealed at an air permeability of not more than 10 m3/hr.m2 @50pa (~10ACH@50pa), when following a performance pathway through the code.
But when going tight, at what point do we need to introduce mechanical ventilation?
While there is no definitive scientific agreement on this, with ranges from 3-6ACH+, H6V3 again gives us guidance, stipulating that mechanical ventilation must be provided below 5 m3/hr.m2 @50pa (~5ACH@50pa).
This leaves a “Goldilocks Zone” less than 10 & greater than 5, where energy efficiency can be met & IAQ can be adequate for MOST users, MOST of the time. It is also an easy target to achieve with only minimal effort. And there are also some extra efficiency gains to be realised as you head from 10 ACH to 5. Win-Win!
But be careful.
There are some things a builder adopting this strategy needs to know:
This risk however CAN be moderated…
…by adding TECHNOLOGY & VENTILATION OPTIONS.
And we will look at those from simple to advanced, below...
Sub-Heading Navigation:
Strategy 1 - Minimising Off-gassing Materials
Strategy 2 - Know your Tightness (Current)
Strategy 3 - Introduce Ventilation
Constant Volume Extraction Fans
YOU ARE AIMING FOR THE GOLDILOCKS ZONE ON YOUR BUILDS... (5-10ACH@50pa)... GREAT! BUT WHAT IF YOU BUILD TOO TIGHT? OR WHAT IF YOU WANT TO AIM LOWER THAN 5 FROM THE OUTSET? WHAT ARE THE RULES AND WHAT ARE THE OPTIONS?
Let’s start with the rules:
If you go under 5ACH a continuous ventilation strategy is highly recommended. In fact, H6V3(2) requires mechanical ventilation when following a performance pathway through the NCC. So, ignore ventilation in a tight house at your peril!
This then raises the first question...
How much ventilation do homes need?
As mentioned previously there is not yet scientific consensus on this.
H6V3 however gives a formula based on the American standard, ASHRAE 62.2, of 5 litres of fresh air per second per 100 sqm of floor area plus 3.5 litres per second per occupant, so that seems a reasonable place to start.
(Note, the no. of occupants is derived by the no. of bedrooms in the home +1, allowing for 2 people in the master bedroom).
Let’s do an example.
For a 200m2, 3 bedroom home, that would be:
2 x 5 + 4 x 3.5 = 24L/s of continuous ventilation required for good indoor air quality.
Pretty easy to work out!
There are 2 provisos though...
1) This is a minimum. Actually, the current version of the ASHRAE 62.2 standard now requires 15L/s/100m2 +3.5L/person. Significantly higher than the NCC2022 level!
So we have included a lookup table below, taken from the standard if you want to work to that level. If you are heading down to Passivhaus tightness, this is probably a better level to aim for as there is less infiltration to subsidise ventilation the tighter you build the fabric.
2) If having a whole house system designed by an HVAC consultant, they will be looking at distribution and load requirements in different areas of the home. These are important considerations when going very tight, so get professional advice.
Once you know how much ventilation you need, the next question is what are the ventilation options?
We will look at the 3 most common mechanical ventilation options for maintaining ventilation requirements, in order of cost and complexity, below...
Sub-Heading Navigation:
Strategy 1 - Minimising Off-gassing Materials
Strategy 2 - Know your Tightness
Strategy 3 - Introduce Ventilation (Current)
Constant Volume Extraction Fans
WE’VE LOOKED AT HOW MUCH VENTILATION A HOME NEEDS. BUT WHAT ARE THE CHOICES? WELL, IT DEPENDS ON APPROACH…
Let’s start with those aiming for the Goldilocks tightness zone of 5-10ACH@50pa .
The danger with this approach is that we might inadvertently build TOO tight!
Though we harp on about tightening up building practice, the truth is with a little knowledge, & minimal effort, 5-10ACH is easy to achieve. So much so, that the unwary builder trying to meet thermal performance tightness criteria, could find themselves in the 3-4 ACH range, or even lower. This is a very real risk for the industry as we push for efficiency, especially for small homes. But thankfully there are solutions.
Let’s start with the most cost effective – efficient Constant Volume Extraction fans (like the Panasonic FV-24A3 pictured above).
These fans work just like standard intermittent bathroom/laundry extraction fans, cost only a little more, & can be just swapped out for the extraction fans currently used. The difference is that these fans can, if needed, be set for 24hr extraction at various air volumes. Their motors are such that they guarantee the set extraction rate, which in turn creates a slight low pressure inside the home. This consequently increases the infiltration rate through gaps & cracks in the structure, upping passive air changes in the house. Though reasonably crude, this can bring a slightly too tight home, back above 5ACH, & is a nice bit of insurance & risk mitigation to have installed if needed, should a blower door test find the house a bit too tight, or should the owner want more ventilation.
To set the extraction rate, just dial up the required air volume. Best practice would be to spread the required volume over the different units in the home to try & even up the distribution.
Though cheap & energy efficient, the downside of this approach is that it relies on uncontrolled infiltration through the fabric which is hit & miss in regards to distribution; has no proper filtering; & has NO heat recovery.
Such limitations however can be addressed (for a cost) by moving to Heat Recovery Ventilation units,
which we will look at next…
Sub-Heading Navigation:
Strategy 1 - Minimising Off-gassing Materials
Strategy 2 - Know your Tightness
Strategy 3 - Introduce Ventilation
Constant Volume Extraction Fans (Current)
DON’T LIKE THE IDEA OF EXTRACTING CONDITIONED AIR OUT OF THE HOUSE WITHOUT RECOVERINGTHE HEAT/COOL? BUT CAN’T/DON’T WANT TO RUN DUCTWORK? DE-CENTRALISED HEAT RECOVERY UNITS MIGHT BE FOR YOU…
Here’s a relatively new technology that’s very clever.
Put a pipe hole through the wall, insert de-centralised heat recovery ventilation unit (DHRV).
Connect power. Caulk/seal.
Ventilation done!
At least for one space…
Let's look at how they work.
Inside the DHRV unit is a fan with a ceramic core the air must pass through.
The fan initially extracts air from a space like an extraction fan. As it does this the internal air passes through a ceramic core. In winter, when the internal air is warmer than outside, the warmer air heats up the ceramic core as it flows out of the house. Most units take 1-2 minutes to charge the core to room temperature.
Then they reverse!
And draw in new air from the outside.
As the cooler air comes in it passes back through the ceramic core picking up the heat just left there by the outgoing air, thus pre-heating the incoming air.
NEAT!
And through this process they can typically recover 80-90% of the heat in the air & keep it in the house.
In summer, they keep in the cool in exactly the same way.
VERY NEAT!
Got a bigger room?
No problem.
Many models can pair with wifi to sync up, so that one extracts while the other ventilates, & then they reverse roles together, keeping a balanced pressure in the home.
EXTREMELY NEAT!
These units work well for small homes, apartments, or single rooms (like bedrooms that can become a bit musty overnight…)
BUT...
They are generally not a replacement for extraction fans in high-vapour-load wet areas.
And, as you generally need at least one per room, become expensive for the standard-sized Aussie home.
SO, great for:
BUT, for standard size homes wanting to go tight....
...AND for those wanting more control with better filter options...
THEN ducted HRV’s are usually the way to go.
So, let's turn our attention to those…
Sub-Heading Navigation:
Strategy 1 - Minimising Off-gassing Materials
Strategy 2 - Know your Tightness
Strategy 3 - Introduce Ventilation
Constant Volume Extraction Fans
De-Centralised HRV (Current)
GOT AN AVERAGE TO LARGE HOME AND WANT TOTAL CONTROL OVER THE AIR YOU BREATH INSIDE? TIME TO TALK CENTRALISED HRV SYSTEMS…
Centralised HRV units are considered by many the gold standard for ventilation control…
Balanced systems with the highest efficiencies in heat recovery on the market. (Up to 95%).
Low energy use (starting from 35W).
And guaranteed ventilation levels 24/7.
But what really sets them apart from other strategies previously discussed (apart from the price 😉), is that if you combine them with a super tight build (below 1ACH@50pa), and use appropriate filters, you can screen out a wide variety of particulates from the air,
…from dust, to pollen, to mould spores, bacteria, smoke & more…
And take almost full control of IAQ!
Filter choice unfortunately gets pretty techy, pretty fast, with a plethora of standards and grades.
The following table can help with selection, with the recommendation for superior home air quality starting from 7 / MERV 13, and up.
And for the filters to be most effective. the tighter the shell the better!
Then virtually all the air for the home comes through the unit, making filters placed in them very effective for regulating airborne particulates.
If you are concidering using Centralised HRV, you will need to allow for 75-90mm duct running to most rooms, typically behind bulkheads or under 120mm+ dropped ceilings, within the conditioned zones of the home, to minimise heat loss/gain.
The motor itself is the size of a small wall cabinet with 2 fans inside, usually put in the laundry or utility room, with one fan bringing filtered fresh air from outside to habitable rooms, and one exhausting return air from the wet/utility areas.
The picture below will give you an idea of the typical layout… but get a professional to help with design.
Inside the motor unit is the core where the magic happens, as incoming air is run in a labyrinthine baffle next to outgoing air. The very thin walls between, allow heat to pass from the warmer air stream to the cooler.
In this way the heat or cool from conditioned internal air is passed to the incoming air and retained within the home.
And that’s it. Almost complete IAQ control!
“Almost” because humidity can still be an issue in some months in some climates.
For that, those in mixed to humid climates, you will need to turn to ERV’s. We've put some recommendations for climates where ERV's should be considered below.
But if you want to know more about ERV's & HRV's, you will find out all you need to know on the Webinars resource page.
(Scroll down to, "The Importance of Ventilation in your Home", and let Joel Segren from Fantech give you a great rundown of everything HRV!)
Sub-Heading Navigation:
Strategy 1 - Minimising Off-gassing Materials
Strategy 2 - Know your Tightness
Strategy 3 - Introduce Ventilation
Constant Volume Extraction Fans
Centralised HRV (Current)
SBA Thermal Quality Assurance Checklist
(What to consider at each stage of construction - Issues & Solutions)
Building a house is difficult, with many trades, jobs, and supplies that need to be coordinated and work together. And each building stage poses particular challenges for thermal performance. To help, SBA has created the following Thermal QA Checklist to make achieving a good outcome straightforward. Feel free to modify it to suit your build types and use as you see fit.
It also goes hand in hand with the following video presentation that explains each point on the checklist, and gives a pictorial tour of where buildings can fail thermally in standard construction and what to do about it. It is highly recommended that you watch the video before using the checklist.
SBA gives the checklist in good faith, but have no specific knowledge of your build. All responsibilities and liabilities remain with the builder and design team. Where any conflict with NCC, Standards, or plans exist, codes take precedence and the builder should check with the designer where unsure.
The Net Zero Homes builders' course is a partnership between Design Matters National and the SA Gov, and was created by the author of this website (Jeremy Spencer - Positive Footprints), so covers much the same ground.
If you like listening rather than reading, this course will be for you. The good news is that all 5 modules are completely FREE at the moment under a funding grant to professionals in the industy.
Highly recommended - register while funding lasts at
https://www.solar.vic.gov.au/net-zero-homes-skills-training
Though we suggest watching the videos in order, Module 2, deals with the topics discussed in this step most closely. In particular it goes over each step of the Builders Thermal Performance Checklist tool downloadable above, and so is a highly recommended resource to watch if you are considering using the tool. Module 3 is also linked below, as it deals with Condensation, Air Quality and Risk, also covered in this Step.
FREE ICANZ insulation installation training tools
http://icanz.org.au/installers/
Construction Stage Instructional Videos from Sustainability Victoria.
https://www.youtube.com/playlist?list=PLh7JO_MR5_CJN9eH2BosGYUjQ1cuC7pVT
As Built Verification Checklist to follow during the Construction
https://assets.sustainability.vic.gov.au/susvic/Factsheet-energy-ABV-Zero-Net-Carbon-Homes.pdf
Efficiency Matrix Youtube Channel - great resource on building tight
https://www.youtube.com/channel/UCzLaC1JZUVwvHAxt1FzyloA
To find a Blower Door Tester (& Thermal Imaging)
https://www.bcta.group/attma/members/air-tightness-testers/australia/
Choosing Low VOC & EO materials
http://www.ecospecifier.com.au/
https://www.globalgreentag.com/
ABCB Condensation in Buildings Handbook and Video
https://www.youtube.com/watch?v=loDH86dgXHU
Tasmanian Condensation in Buildings Designer's Guide (Cool/Temperate Focus. Wall & Roof section examples)
Pro Clima 'Australian Based Study on Air Tightness and Moisture Management' (Comprehensive explanation of condensation in Australian buildings).
https://proclima.com.au/pro-clima-australia-study/
To find a WUFI practicaioner
https://www.wufi.com.au/wufi-professionals/
Passivhaus
https://passivehouseaustralia.org/
Collection of various Energy Efficiency Training videos & resources (Notably "Wrapping & Sealing a Building" series)
https://www.energy.gov.au/business/energy-efficiency-skills-and-training/energy-efficiency-training
Master Builders Getting Energy Efficiency Right videos
https://www.mbav.com.au/getting-energy-efficiency-right
Green Building Institute - full range of industry training
https://www.greenbuilding.org.au/
And for a complete rundown of all Ventilation Options read this excellent article by Alex Zeller
Only fans – A draft guide to continuous ventilation of homes in Australia and New Zealand
