You've switched to LED globes, bought an efficient fridge and you turn the lights off. So why does the air conditioner still run flat out all summer?
Often the answer is the house itself. Every gap around a pipe, downlight, window frame or skirting lets air move in and out. In Queensland that usually means hot, humid air getting in and the air you've paid to cool getting out. You can't see it, but you pay for it every quarter.
The good news is that it's far cheaper to fix during the build than after you move in.
How leaky are Australian homes?
CSIRO has done the most thorough testing here. Its 2015 study of 125 new homes found an average of about 15 air changes per hour when tested at 50 Pa of pressure (written as 15 ACH@50Pa). CSIRO suggests new homes should aim for around 5. A later CSIRO study in 2024 found newer homes had become markedly tighter, so things are improving, but there's still a wide spread between good and poor builds.
Where the leaks are
Air leaks happen wherever materials meet or something passes through the building envelope:
- Junctions: where walls meet the slab, ceilings meet walls, and at corners.
- Penetrations: pipes, wiring, ducts and exhaust fans passing through walls and ceilings.
- Ceilings: downlights, the manhole and gaps around cornices let air into the roof space.
- Windows and doors: gaps between the frame and the wall framing, and poor seals on the sashes and doors.
- Power points and switches on external walls.
- Exhaust fans and range hoods without self-closing dampers.
Most Queensland homes are slab-on-ground, so the old "draughty subfloor" problem is less common. In elevated Queenslanders and homes on stumps, though, gaps in the floor are still a big one.
Why it matters beyond your power bill
When warm, humid air leaks into a wall or roof space and meets a cooler surface (like the back of plasterboard in an air-conditioned room), it can condense. Over time that means mould, staining and deterioration you won't see until it's a problem. This is why sealing and ventilation have to be thought about together.
What the NCC now requires
NCC 2022 tightened the rules for new homes:
- Building sealing (Housing Provisions Part 13.4) covers things like sealing around windows and doors, exhaust fans, roof lights and gaps in the building envelope.
- Condensation management (Housing Provisions Part 10.8) covers things like pliable membranes, exhaust fans discharging outside the roof space, and roof space ventilation.
- 7-star NatHERS for most new homes means your insulation, glazing and shading all have to work harder.
These are minimums. Building only to the minimum leaves comfort and savings on the table, and you can ask for more at design stage.
Insulation and windows: use Australian ratings
Be careful with overseas advice. US insulation figures like "R-10" or "R-30" are in imperial units and don't translate directly to Australian metric R-values.
- Insulation: your required levels come from your energy assessment or the NCC tables for your climate zone. In Queensland, ceiling batts are commonly in the R3.5 to R6.0 range and wall batts R2.0 to R2.7.
- Windows: Australian windows are rated under WERS by U-value (how much heat passes through) and SHGC (how much solar heat gets in). In Queensland, a low SHGC and good shading usually matter more than double or triple glazing.
What to ask your builder
- How do you seal penetrations for plumbing, electrical and ducts through the external walls and ceiling?
- How are window and door frames sealed to the wall framing?
- Will you do an air-tightness (blower door) test? Ideally once the building is sealed but before the linings go on, so leaks can still be fixed easily.
- Are the downlights and exhaust fans sealed or fitted with dampers?
- How will ventilation work once the house is sealed? Exhaust fans ducted outside, and possibly a mechanical ventilation system.
The best time to check all this is at the rough-in and enclosed stages, before the plasterboard hides everything. My C10 Rough In and C11 Enclosed checklists cover what to look at.
Don't forget ventilation
A tighter house needs planned ventilation. Fresh air should come in where you choose, not through random gaps. That can be as simple as good exhaust fans ducted outside, or as complete as a heat or energy recovery ventilation system. I've written more about that in 12 benefits of installing a ventilation system in your new home.
The bottom line
Gaps and cracks don't look like much, but together they make a house harder and dearer to keep comfortable. Ask how your builder seals the house, whether they'll test it, and how it will breathe once it's sealed. Those three questions will tell you a lot.
