Key takeaways
- Air sealing is a sequence and detailing problem rather than a product selection problem, so the penetrations and junctions have to be planned before the frame is closed.
- The most common failure points are the junctions between different materials and trades, not the middle of a sheet or a panel.
- Services rough-in is the stage where most of the damage is done, because downlights, exhaust fans, plumbing and cabling punch through the sealed layer after it has been finished.
- Inspection has to happen before linings go on, because once the plasterboard is fixed the defects are invisible and the cost of correction multiplies.
- A certificate or a BASIX commitment does not install the seal, and the builder carries the practical and commercial risk when the completed envelope does not perform.
Air sealing is the cheapest energy efficiency measure in a house and the one most often done badly. The materials cost very little. The labour is small compared with the frame and fit-out programme. Yet most houses leak, and most of the leaks are at junctions the trades never treated as part of the envelope.
The reason is sequence. Air sealing is not a product you buy and install at one point in the programme. It is a layer that runs continuously around the conditioned volume, and every trade that opens that layer has to know how it gets closed again. If the seal is applied before services rough-in, the rough-in destroys it. If it is applied after linings, the junctions are already hidden.
This article covers the sequence and detailing of air sealing during the frame and fit-out stages. It does not cover blower door testing as a subject in its own right. Testing tells you whether the envelope performs; getting the sequence right is what makes it perform in the first place.
Why the envelope leaks in the first place
Houses leak because air moves through paths of least resistance. Those paths are almost never in the middle of a sheet of plasterboard, a sheet of sarking or a panel of insulation. They are at the edges, where one material stops and another starts, and where one trade stops and another starts.
The Your Home guidance on sealing your home is blunt about this: the gaps and cracks that matter are the ones you can see if you look for them, and they occur at junctions rather than in the field of a material. That is the whole game. A wall that is perfectly sealed in the middle and open at the bottom plate leaks as badly as a wall with no seal at all.
The second reason is that the trades that create the leaks are not the trades that are asked to fix them. The electrician drills through a top plate. The plumber cuts a penetration for a waste. The air conditioning installer runs a duct through a wall. None of them see themselves as working on the envelope, and unless the builder makes the envelope their responsibility at the point of work, the leak is created and left.
Plan the sealed layer before the frame goes up
The first decision is which plane is the air barrier. In some houses it is the internal lining. In others it is an external membrane or a rigid sheathing. In others it is a combination, with the barrier changing plane at different parts of the building.
You do not need to be a building physicist to make this decision, but you do need to make it explicitly and put it on the drawings. If the barrier is the internal lining, then every service that runs in the ceiling space and drops into a wall has to be sealed where it crosses the lining plane. If the barrier is external, then the internal lining is a secondary barrier and the detail changes.
What matters for the builder is that the answer is written down before the frame is erected. The related article on BASIX certificates for NSW builders covers the commitment side of this. The commitment assumes an envelope; the frame stage is where the envelope is either built or lost.
Once the plane is decided, mark it on the frame drawings. Not on a detail sheet that nobody opens on site, but on the plan the carpenters and the services trades actually work from. The mark should show where the sealed layer runs and where it is interrupted.
Frame stage: the junctions that matter most
Frame stage is where the bulk of the permanent air sealing work happens, because the frame is open and every junction is accessible. Once the linings are fixed, the cost of correcting a missed junction goes up by an order of magnitude.
The junctions to treat are the ones between different building elements, not the fields of the elements themselves. The bottom plate to slab or floor structure. The top plate to the ceiling structure. The wall frame to the floor frame. The wall frame to the ceiling lining support. Each of these is a linear junction, and each one needs a continuous seal rather than a series of patches where someone noticed a draught.
Bottom plates and the floor junction
The bottom plate to slab junction is the single most common leak path in a residential envelope, and it is almost always accessible at frame stage. The detail is straightforward: a continuous bead of sealant or a compatible tape between the plate and the slab, run before the plate is fixed down or as part of the fixing sequence.
The failure mode is not the product. It is the sequence. If the plate is fixed and then the sealant is run afterwards, the sealant sits on the outside face of the plate rather than between the plate and the slab. It looks sealed and it is not. The same applies to the junction between the wall frame and a suspended floor, where the seal has to run between the plate and the floor structure.
Top plates and the ceiling junction
The top plate to ceiling junction is the second most common leak path, partly because it is where the ceiling space connects to the wall cavity and partly because it is where services often run. The seal needs to be continuous along the top plate and around any penetration.
Where a dropped ceiling or a bulkhead interrupts the wall, the junction changes plane and the seal has to follow it. This is a detailing problem, not a product problem. If the detail is not resolved on paper, it will not be resolved on site, and the ceiling space will be connected to the conditioned volume through a gap that nobody sees.
Openings and the frame-to-rough-opening gap
Window and door openings are part of the sealed layer, not exceptions to it. The gap between the window or door frame and the rough opening is a leak path, and it has to be sealed as part of the envelope rather than left to the architrave or the reveal.
The detail depends on the window system and the wall construction, and it changes with the weatherproofing strategy. What does not change is the principle: the opening has to be treated as a junction in the sealed layer, with the seal continuous around the frame and compatible with the flashing and weatherproofing details.
Frame stage air sealing sequence
- 1Plan the sealed layerMark on the drawings which plane is the air barrier at each junction before any framing starts.
- 2Seal bottom and top platesRun the sealant or tape between plates and the slab or floor structure and between plates and the ceiling structure.
- 3Seal wall and floor junctionsTreat the junction between the wall frame and the floor or slab as a continuous line, not as a series of patches.
- 4Seal wall and ceiling junctionsSeal between the top plate and the ceiling lining support so the ceiling plane is continuous with the wall plane.
- 5Seal around openingsTreat window and door openings as part of the sealed layer, including the gap between the frame and the rough opening.
- 6Photograph before liningRecord every sealed junction with the frame exposed so the work can be verified after the linings are fixed.
Services rough-in: where most of the damage happens
If frame stage is where the envelope is built, services rough-in is where it is most often destroyed. Every service that penetrates the sealed layer is a potential leak, and there are a lot of them: downlights, exhaust fans, plumbing, electrical cabling, air conditioning, vacuum systems, data cabling.
The problem is not that these services exist. It is that they are installed by trades who are not thinking about the envelope and who are working to a programme that does not include sealing. The electrician drills a hole for a cable and moves on. The plumber cuts a penetration and moves on.
The fix is procedural. Before rough-in starts, the builder identifies every penetration through the sealed layer and specifies how it will be sealed. During rough-in, the trades either seal as they go or leave the penetration accessible for sealing. After rough-in, the builder inspects before linings go on.
Downlights and exhaust fans
Recessed light fittings and exhaust fans are the classic envelope failure. A standard recessed downlight is an opening in the ceiling plane, and unless it is a sealed unit or is installed in a sealed box, it connects the ceiling space to the room. The same applies to exhaust fans, which are often installed with a backdraft shutter that does not seal when the fan is off.
The solution is specification, not site improvisation. Choose sealed fittings or specify a sealed enclosure for each one, and make that part of the electrical package rather than a variation. If you leave it to the electrician on the day, you will get whatever is in the van.
Plumbing and waste penetrations
Plumbing penetrations are more varied than electrical ones, because they include waste pipes, water supply, and vent pipes. Each one is a hole through the sealed layer, and each one has to be sealed with a product compatible with the pipe material and the surrounding construction.
The detail that catches builders is the vent pipe, which often runs up through the ceiling and out through the roof. The penetration through the ceiling plane is the one that matters for the envelope, and it is often missed because the focus is on the roof flashing.
Cabling and conduits
Cabling is the least visible and most numerous source of leaks. Every cable that passes through a top plate, a bottom plate or a stud is a potential path. In practice, the volume of cabling in a modern house means that the cumulative effect is significant.
The practical approach is to treat groups of cables as a single penetration and seal them together. Grommets, sealant and tape all have a role, depending on the substrate and the number of cables. The principle is that the penetration is sealed, not that a particular product is used.
Sealing after lining is a losing game. Once the plasterboard is fixed, you cannot see the junctions, you cannot reach them without cutting, and the cost of correction is several times the cost of doing it at frame stage. The inspection has to happen before the linings go on, not after.
Fit-out: protecting the layer you have built
Fit-out is where the envelope is most vulnerable to accidental damage, because the trades are working inside the sealed volume and the consequences of a mistake are hidden. The two risks are physical damage to the sealed layer and new penetrations created by fit-out trades.
Physical damage is usually caused by moving materials, installing joinery or running services in the ceiling space. The seal at the top plate is particularly exposed if anyone is working in the roof space or installing a ceiling-mounted item. The bottom plate seal is exposed if flooring or skirting is installed with fixings that go through the plate.
New penetrations are more predictable. Any fit-out item that requires a fixing into a sealed junction, or a hole through the sealed layer, has to be treated as a penetration and sealed. This includes range hoods, bathroom exhausts, wall-mounted televisions, and any ceiling-mounted fixture.
Joinery and cabinetry
Kitchen and bathroom cabinetry is often fixed to walls that contain the sealed layer. The fixings themselves are small, but a series of fixings along a wall can create a leak path if they pass through the seal. The practical approach is to know where the sealed layer is and to fix into structure rather than through the seal wherever possible.
Where a fixing must pass through the seal, seal it. This is a small task that is almost never included in a cabinetry trade’s scope, so it has to be either added to the scope or done by the builder’s own labour.
Wet area exhaust and ventilation
Bathroom and kitchen exhaust systems are a fit-out item that connects directly to the envelope. The duct has to run from the room to the outside, and every joint in the duct, every penetration through a wall or ceiling, and the termination itself are part of the envelope.
The common failure is a duct that is connected to the fan but not sealed at the point where it passes through the ceiling or wall. The fan works, the air moves, and the envelope leaks. The detail is to seal the penetration where the duct crosses the sealed layer, not just at the fan.
Pre-lining air sealing checks
- Every bottom plate is sealed to the slab or floor structure along its full length.
- Every top plate is sealed to the ceiling structure or the ceiling lining support.
- Wall, floor and ceiling junctions are continuous, with no gaps behind packers or noggings.
- Window and door openings are sealed between the frame and the rough opening.
- Services penetrations through the sealed layer are sealed with a compatible product.
- Recessed light fittings and exhaust fans are sealed or specified as sealed units.
- Photographs of every sealed junction are taken before linings are fixed.
Inspection before lining: the checkpoint that saves the job
The single highest-value action in the whole process is a documented inspection of the sealed layer before the linings go on. At that point every junction is visible, every penetration is accessible, and every defect can be corrected at frame-stage cost.
The inspection should be systematic. Walk the perimeter of the conditioned volume and check each junction in turn. Check the bottom plates, the top plates, the wall and floor junctions, the openings, and every services penetration. Photograph each one. The photographs are not for the client; they are for you, so that when a defect appears later you can establish what was done and when.
The inspection is also the point at which responsibility is assigned. If a penetration is not sealed, the question is which trade owns it. If the answer is not clear, the builder owns it by default, because the builder is the one who has to fix it before the linings go on.
How this connects to BASIX, NatHERS and the NCC
Air sealing is not a standalone requirement. It sits underneath the thermal performance commitments that are made under BASIX and modelled under NatHERS, and it is affected by the changes in the National Construction Code.
The HIA summary of the NCC 2025 changes for residential builders is a useful starting point for understanding what has changed at the code level. The practical point for a builder is that the thermal performance of a house depends on the envelope performing as modelled. A house that is modelled with a sealed envelope and built with an unsealed one does not deliver the performance that was committed to.
The related article on NatHERS assessments covers the modelling side. The frame and fit-out sequence covered here is what makes the model real. If you want the broader context of how these commitments sit within CPD and licensing, the PointCPD guide to NSW builder CPD explains how the requirements fit together.
Worked example: a builder in western Sydney
A builder in western Sydney is completing a two-storey dwelling with a suspended timber floor on the upper level and a slab on ground below. The design uses an internal lining as the primary air barrier, with a sealed ceiling plane and sealed wall-to-floor junctions.
At frame stage, the carpenter seals the bottom plates to the slab and the top plates to the ceiling structure. The wall-to-floor junction on the upper level is sealed between the plate and the floor structure. The window and door openings are sealed between the frame and the rough opening. The builder photographs each junction.
Services rough-in begins. The electrician drills through top plates for cabling and installs recessed downlights in the ceiling. The plumber cuts waste penetrations through the floor structure and the wall frames. The air conditioning installer runs a duct through the ceiling space and penetrates the sealed ceiling plane.
Before linings, the builder inspects. The bottom plates and top plates are intact. The wall-to-floor junction is intact. The window openings are sealed. But the services penetrations are not: the cabling through the top plates is unsealed, the downlights are standard recessed fittings, the waste penetrations are open, and the duct penetration is unsealed.
The builder corrects the defects at frame-stage cost: sealing the cabling penetrations, replacing the downlights with sealed units, sealing the waste penetrations, and sealing the duct where it crosses the ceiling plane. The cost is a fraction of what it would have been after lining. The inspection photographs document what was done.
If the builder had waited until after lining, the same defects would have required cutting plasterboard, relocating fittings, and re-finishing. The envelope would have leaked in the same places, and the builder would have carried the cost of correction. The sequence is what made the difference.
Commercial reality: who pays when the envelope fails
Air sealing defects surface in one of three ways. They surface at blower door testing, if the house is tested. They surface as comfort complaints from the owner, usually in the first heating or cooling season. Or they surface as a defect claim, often years later, when the consequences are more expensive.
In all three cases, the builder carries the practical risk. The trades that created the leaks have moved on. The products used are rarely the cause. The cause is the sequence and the detailing, and the builder is the party who controls both.
The commercial argument for doing the work at frame stage is straightforward. The cost of sealing at frame stage is small. The cost of correcting after lining is several times larger. The cost of correcting after occupation, when the owner is in the house and the defect is disputed, is larger again.
There is also a compliance dimension. The commitments made under BASIX and the performance modelled under NatHERS assume an envelope that performs. A builder who signs off on a house that does not meet those commitments is exposed, regardless of whether the defect is visible. The related article on strip footings and pad footings in NSW is a reminder that the structural elements of a house are inspected and certified; the envelope deserves the same discipline.
For builders looking to formalise their understanding of these requirements, the PointCPD course catalogue includes content on envelope performance and the NCC changes. The knowledge is not complicated. The discipline is.
What to change on your next job
Air sealing is not a new trade and it does not require new materials. It requires a change in sequence and a change in who owns the junctions.
Start by marking the sealed layer on the frame drawings. Make the carpenters responsible for the frame-stage seals and inspect them before services start. Add sealing of services penetrations to the scope of the trades that create them, or accept that you will do it yourself. Inspect and photograph before linings. Treat every penetration during fit-out as a penetration, not as a fixing.
None of this is expensive. Most of it is a matter of knowing where the envelope is, who is responsible for it at each stage, and when it has to be checked. The builders who get this right do not lose points on air sealing, because they never let the envelope out of their control.
Questions builders ask
Do I need to seal the bottom plate if the slab is already covered by a moisture barrier?
Yes. A moisture barrier under the slab is not an air seal at the bottom plate. The junction between the plate and the slab is a leak path regardless of what is under the slab, and it has to be sealed separately. The seal is between the plate and the slab surface, not between the slab and the ground.
Can I use expanding foam to seal services penetrations?
It depends on the substrate, the size of the penetration and the product. Expanding foam is one option among several, and the key requirement is that the product is compatible with the materials it touches and remains in place over time. The important thing is that the penetration is sealed, not which product is used.
How do I stop the electrician from drilling through the top plate?
You probably cannot stop the drilling, so the practical approach is to make the sealing part of the electrical scope. Identify the penetrations before rough-in, specify how they will be sealed, and inspect before linings. If the electrician seals as part of the rough-in, the sequence works.
Do recessed downlights always leak air?
Not always, but standard recessed fittings are an opening in the ceiling plane unless they are sealed units or are installed in a sealed enclosure. The specification is the control point: choose sealed fittings or specify an enclosure, and make it part of the electrical package.
Should I test the envelope with a blower door before linings?
A blower door test is usually done on the completed house, not before linings. The pre-lining inspection is a visual and documented check of the sealed layer, not a pressure test. Use the inspection to find and correct defects, and use the completed-house test to verify the result.
Does air sealing change the BASIX or NatHERS outcome?
Yes. The thermal performance commitments under BASIX and the modelling under NatHERS assume an envelope that performs as designed. If the envelope leaks, the house does not deliver the modelled performance, and the builder carries the practical risk of that gap. The sealing work is what makes the model real.
Sources
- Sealing your home · Your Home (Australian Government)
- National Construction Code 2025: what’s changed for residential builders · Housing Industry Association
Published 27 June 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
