Water on your windows on a winter morning is the visible part of condensation. The bigger risk is the moisture you can't see, inside your walls and roof, where it causes mould and rot. Here's what causes it and how to stop it in a new home.
Why it's a bigger problem than it used to be
Older homes were draughty. Air leaked through every gap, which wasted energy but also carried moisture away. Modern homes are better insulated and better sealed, which is a good thing, but it means moisture has fewer ways out. Visible condensation on windows has reduced, but the problem has moved to places you can't see: roof spaces and wall cavities.
Hidden condensation causes:
- Mould, which affects health, especially for people with asthma and allergies.
- Rot and corrosion in timber framing, fixings and steel.
- Wet insulation, which loses much of its value, so your energy bills go up.
- Damage you won't find until it's well advanced.
How condensation happens
Air holds water vapour. Warm air can hold more than cold air. Relative humidity is how much water vapour the air is holding compared with the most it can hold at that temperature.
Cool that air down and its relative humidity rises. At the dew point, it can't hold any more and the water condenses on the nearest cold surface. That's why a cold drink "sweats" on a humid day.
Water vapour is invisible and moves easily through gaps and through many materials. Wherever moist air meets a surface below its dew point, you'll get water.
The Queensland twist
Most condensation advice comes from cold climates like Canberra, Melbourne and Hobart. There, warm, moist indoor air moves out into the walls in winter and condenses on the cold outer layers.
In south-east Queensland the risk often runs the other way. Warm, humid outside air meets cool, air-conditioned walls and condenses inside the wall. That's why the vapour permeance of your wall wrap and how airtight your plasterboard layer is both matter here, and why plastic vapour barriers on the inside of the wall, copied from cold-climate practice, can cause problems.
The right approach depends on your climate zone and the wall and roof design. There's no one-size-fits-all answer, so ask your designer to show how the design deals with it.
Ventilation versus air leakage
These two get confused, but they're different:
- Ventilation is deliberate: exhaust fans, rangehoods, openable windows and mechanical ventilation systems, moving air where and when you want it.
- Air leakage is accidental: gaps around downlights, windows, pipes and cables, and unsealed junctions. It's uncontrolled, and it carries moisture into places it shouldn't go.
The aim is to minimise air leakage and provide enough controlled ventilation. "Build tight, ventilate right."
What NCC 2022 requires
NCC 2022 strengthened the condensation rules for houses in the ABCB Housing Provisions, Part 10.8:
- Wall wraps must have a minimum vapour permeance in cooler climate zones (4 to 8), so walls can dry out.
- Exhaust fans from kitchens, bathrooms, toilets and laundries must discharge outside, not into the roof space.
- Roof spaces in the coldest climate zones need ventilation.
These are minimums. Ask your builder how your design meets Part 10.8, and what they've done beyond it.
How to prevent condensation in a new home
1. Reduce the moisture load
Every day, a household puts litres of water into the air from showers, cooking, washing and breathing. Get it out at the source:
- Bathroom, ensuite and laundry exhaust fans ducted outside. Not into the roof space, and not just to the eaves soffit where the moist air can come straight back in.
- Rangehood ducted outside. A recirculating rangehood filters grease but leaves the moisture in the house.
- Clothes dryer vented outside, or use a heat pump or condenser dryer.
- Fans sized for the room and run long enough. A fan on a timer or humidity sensor that keeps running after the shower is far more effective.
- Openable windows placed for cross ventilation when the weather suits.
2. Stop moist air getting into the walls and roof
The hidden spaces inside walls, floors and the roof (building scientists call them interstitial spaces) are where condensation does the most damage. Keep moist air out of them:
- Sealed downlights and ceiling penetrations. Every unsealed downlight is a hole into the roof space.
- Sealed penetrations where pipes and cables pass through walls and ceilings.
- A well-sealed plasterboard layer, with attention to gaps at cornices, skirting and around windows.
3. Let any moisture dry out
Some moisture will always get in. The design must let it out:
- Wall wrap with a vapour permeance class suited to your climate zone, installed and taped to AS 4200.2.
- A drained, vented cavity behind the cladding so water and vapour have somewhere to go.
- Roof space ventilation where the design calls for it.
- No plastic vapour barriers on the inside of the wall unless a building scientist has designed the wall for it.
4. Ask for a condensation risk check
For anything beyond a standard design, ask whether the wall and roof build-ups have been checked for condensation risk. Read why your builder should be doing a condensation risk analysis.
5. Check it on site
The best design fails if it isn't installed properly. Before the plasterboard goes on, check wall wrap laps and tape, exhaust fan ducting run to the outside, and sealed penetrations. Our C7 Frame and C10 Rough In checklists cover what to look for.
Living in the home
Once you've moved in:
- Use the exhaust fans every time you shower or cook, and leave them running for a while afterwards.
- In humid weather, run the air conditioner on dry mode or use a dehumidifier. Aim for indoor relative humidity of roughly 40 to 60%.
- Don't dry clothes indoors without ventilation.
- Watch for early signs: musty smells, mould spots in corners and wardrobes, or peeling paint.
The bottom line
Condensation is manageable if moisture, air sealing, ventilation and insulation are treated as one system. Get the moisture out at the source, keep moist air out of the hidden spaces, and let anything that gets in dry out. Do that, and you'll have a healthier, more durable and more energy-efficient home.
Read more on understanding and managing condensation in buildings.
