Key takeaways
- Condensation happens when warm, moist indoor air meets a surface colder than its dew point. New homes are better sealed, so moisture from living has fewer ways out unless it is designed for.
- Surface condensation on windows and cold corners is often driven by how the home is used: unflued gas heaters, drying clothes indoors, and exhaust fans left off.
- Hidden condensation inside walls and roofs is a building issue: the wrong membrane for the climate, no drainage cavity, exhaust ducted into the roof space, unventilated roof spaces and gaps in insulation.
- Since NCC 2022, the housing provisions have required exhaust from kitchens, bathrooms and laundries to discharge outdoors, membranes suited to the climate zone and, in some climate zones, roof space ventilation.
- Diagnose before you argue: record where and when it happens, measure indoor humidity, check exhaust discharge and insulation, and rule out leaks.
In July and August, builders’ phones ring with the same complaint. The client’s new home has water streaming down the windows in the morning, black spots on the ceiling in the ensuite, and a musty smell in the wardrobe on the southern wall. They want to know what the builder is going to do about it.
Sometimes the answer is that the home is being lived in a way that produces more moisture than any home can handle. Sometimes the house has a genuine defect: an exhaust fan ducted into the roof, a wall without the right membrane, missing insulation. Often it is a bit of both. This guide explains the mechanism, how to tell which situation you are in, and the details that prevent condensation in the first place.
Why condensation happens
Air always holds some water vapour. Warm air can hold more than cold air. When warm, moist air is cooled, it reaches a temperature, the dew point, at which it can no longer hold all its moisture, and the excess condenses into liquid water on the nearest cold surface.
In winter, the inside of a home is warm and the outside is cold. Every surface on the boundary between them, including windows, external walls, ceilings under the roof, and anything in contact with them, is colder than the room air. If the indoor air carries enough moisture, it condenses on those surfaces. Glass is usually the coldest surface in a room, which is why windows show it first.
Where does the moisture come from? From living. People breathe and perspire. Showers, baths, cooking, kettles, dishwashers, washing machines and indoor clothes drying all release water vapour. Indoor plants and fish tanks add more. An unflued gas heater releases water vapour as it burns gas, and it can add a surprising amount to a room.
Why new homes seem to have more problems
Older homes leaked air everywhere: gaps around windows and doors, open fireplaces, vented skirting boards, unsealed floors. That was terrible for energy efficiency, but it meant moist air was constantly replaced with drier outside air, whether anyone wanted it or not.
New homes are built to much higher energy standards. They are better insulated and better sealed, with better windows. That keeps heat in, but it also keeps moisture in unless the home is designed to remove it deliberately, through exhaust fans that actually vent outdoors, ventilation, and walls and roofs that let any moisture that gets in dry out.
That is why the National Construction Code has increasingly focused on condensation. NCC 2022 introduced significant condensation provisions for housing, and NCC 2025, released in May, strengthens them further.
Two kinds of condensation
Surface condensation
This is the kind the client sees: water on windows, damp patches in cold corners, mould behind wardrobes and on ceilings. It happens on the room side of the building. It is strongly affected by how the home is used, but building factors, such as missing insulation creating cold spots, make it much worse.
Interstitial condensation
This is the kind nobody sees until damage appears. Warm, moist air gets into the wall or roof, through gaps around downlights, pipes and cables or by vapour diffusion through linings, and condenses on a cold layer inside the construction, often the back of the wall wrap or the underside of the roof. Over time it can wet insulation, stain linings, corrode fixings, and cause mould and timber decay inside the wall. This is a building design and installation issue, and it is what the NCC provisions primarily target.
The building factors that cause problems
- Exhaust ducted into the roof space. An exhaust fan that dumps moist bathroom air into the roof, or into a closed eave, moves the problem rather than solving it. The moist air condenses on the cold underside of the roofing and drips back onto the insulation and ceiling. The NCC housing provisions now require exhaust from kitchens, bathrooms and laundries to discharge outdoors.
- The wrong membrane for the climate. Wall wraps and sarking are rated for how easily water vapour passes through them. In colder climates, a membrane that resists vapour too strongly on the outside of the wall can trap moisture inside it. The NCC now sets vapour permeance requirements for membranes by climate zone.
- No drained cavity. Cladding fixed directly against the wrap leaves nowhere for moisture to drain or dry. NCC 2025 extends the circumstances in which drained and ventilated cavities are required.
- Unventilated roof spaces. In some climate zones the NCC requires roof space ventilation, so moisture that reaches the roof can escape before it condenses.
- Gaps in insulation. A missing batt, a compressed section at the top plate, or an uninsulated corner creates a cold spot on the inside surface. Mould often maps the missing insulation exactly.
- Thermal bridges. Steel frames, metal window frames without thermal breaks, and slab edges conduct heat out, creating cold lines where condensation forms.
- Air leaks into the construction. Downlights, pipe penetrations and cable holes let warm, moist air into walls and roofs.
The occupant factors
Builders should be careful here. It is tempting to blame the client, and clients are rightly sceptical when every problem is put down to how they live. But some occupant factors genuinely overwhelm a well-built home:
- Unflued gas heaters, which release water vapour and combustion products into the room.
- Drying clothes indoors, especially on racks in closed rooms.
- Showering without the exhaust fan on, or switching it off as soon as the shower stops.
- Keeping the house closed up all winter with no ventilation.
- Furniture and wardrobes pushed hard against cold external walls, creating pockets of still, cold air.
- Heating only some rooms, leaving others cold enough for moist air to condense in them.
The fair approach is to check the building first, fix anything that is wrong, and then explain clearly and in writing what the occupant can do.
How to diagnose a complaint
Diagnosing a condensation complaint
- 1Ask when and whereMornings in winter on the coldest walls and windows points to condensation. After rain on one wall points to a leak.
- 2Measure humidityLeave a simple hygrometer in the worst room for a week. Persistently high indoor humidity tells you moisture is not getting out.
- 3Check the moisture sourcesUnflued gas heaters, clothes drying, fish tanks, many occupants, showers without the fan running.
- 4Check the buildingDo exhaust fans actually discharge outside? Is insulation complete? Is the roof space ventilated where required?
- 5Look for cold spotsA thermal camera or careful inspection finds missing insulation and thermal bridges where mould grows first.
- 6Write it upRecord findings and advice in writing, with photos. It protects you and helps the client.
The first question, when and where, does a lot of the work. Condensation follows cold: it appears on cold mornings, on the coldest surfaces, and it tends to be spread across similar surfaces. A leak follows water: it appears during or after rain, below a specific defect, and it is usually localised. Rising damp follows the ground: it appears at the base of walls.
Condensation, leak or rising damp?
| Clue | Condensation | Leak | Rising damp |
|---|---|---|---|
| When | Cold mornings, winter | During or after rain | All year, worse in wet periods |
| Where | Windows, external corners, behind furniture, ceilings under cold roofs | Below a specific defect: flashing, window head, roof penetration | Bottom of walls, from the floor up |
| Pattern | Spread across cold surfaces | Localised, often tracks down | Tide mark at a fairly even height |
| What fixes it | Less moisture in, more moisture out, warmer surfaces | Fix the water path | Damp-proof course and drainage |
A cheap digital hygrometer left in the worst room for a week is one of the most useful tools a builder has for these complaints. If indoor humidity stays high day and night, moisture is not getting out. That points you to the exhaust fans, ventilation and moisture sources. If humidity is normal but mould is growing in one corner, look for a cold spot: missing insulation or a thermal bridge.
Always check where the fans go. The single most common building cause of condensation complaints is an exhaust fan that does not actually discharge outside. Get into the roof space and follow the duct. A duct that ends in the roof space, is disconnected, or discharges into a closed eave is a defect to fix before any conversation about how the client uses the house.
Fixing it
The fix depends on the cause, but most solutions fall into three groups:
- Less moisture in. Remove unflued gas heaters, vent clothes dryers outside, and use lids when cooking.
- More moisture out. Make sure exhaust fans discharge outside, are powerful enough for the room and run long enough after use. Run-on timers and humidity-controlled fans help. Ventilate the house regularly.
- Warmer surfaces. Fix missing insulation, address thermal bridges, and improve glazing where windows are the main problem.
For hidden condensation inside walls and roofs, the fixes are harder and more expensive after the fact, which is why getting the details right during construction matters so much.
Building it out: the details that matter
Building condensation out of a new home
- Exhaust fans in kitchens, bathrooms and laundries ducted to outdoors, not into the roof space or eaves where air cannot escape.
- Pliable membranes with the vapour permeance the NCC requires for the climate zone, installed and lapped correctly.
- A drained and ventilated cavity behind cladding where the design calls for it.
- Roof space ventilation where the climate zone requires it, with vents kept clear of insulation.
- Insulation continuous at corners, top plates, around windows and behind services, with no compressed or missing sections.
- Air sealing at downlights, pipe and cable penetrations so moist air does not get into walls and roofs.
- A handover sheet telling the owner how to use the fans, ventilate and avoid unflued gas heaters.
These are not exotic details. They are the ordinary components of a wall and roof, installed with an understanding of what each layer is for. The builders who have the fewest condensation callbacks are the ones whose supervisors and trades understand the mechanism, not just the minimum.
A worked example: the ensuite ceiling
A builder receives a call in the second winter after handover. The ensuite ceiling has black mould spots in one corner and along the line of the cornice, and the paint is peeling above the shower. The owners say they always run the fan.
The builder visits on a cold morning. The ensuite window is streaming, but so is the bedroom window next to it, which suggests high humidity through the house. A hygrometer left for a week shows indoor humidity staying high overnight. The owners use a portable unflued gas heater in the living room and dry washing on racks in the spare room.
In the roof space, the builder follows the ensuite exhaust duct. It has come away from the eave vent it was meant to connect to and is discharging into the roof space above the ensuite. The ceiling insulation directly around the fan is damp and compressed, and there is a gap in the batts at the corner where the mould is worst.
The honest answer is that both the building and the use of the home contributed. The builder reconnects and secures the duct to discharge outside, replaces and completes the ceiling insulation, and treats and repaints the ceiling. They also give the owners a written explanation of the other moisture sources, recommending they stop using the unflued heater and dry washing outside or in a vented dryer. The mould does not return the following winter.
Handled that way, the builder fixed what was theirs, explained what was not, and kept a client. Handled by arguing on the phone that condensation is always the owner’s fault, the same job becomes a complaint to the regulator.
Materials that tolerate moisture, and those that do not
Some materials handle occasional moisture far better than others. Masonry and concrete can absorb and release moisture without damage, although they can grow surface mould. Timber tolerates short-term wetting if it can dry out, but decays if it stays wet. Plasterboard and paper-faced products lose strength and grow mould quickly once wet. Glasswool and polyester insulation lose performance when damp and can hold moisture against the frame.
That is why hidden condensation inside a lightweight wall or ceiling is more serious than a wet window. The materials in the concealed spaces of a modern home are the ones that suffer most from moisture, and they are the ones the owner never sees until the damage shows through.
The handover conversation
A short, plain-English handover sheet on moisture helps clients look after their home and reduces callbacks. It should explain that new homes are well sealed and need to be ventilated, that exhaust fans should be used during and after showers and cooking, that unflued gas heaters should not be used, that clothes should be dried outside or in a vented dryer, and that furniture should be kept slightly away from external walls. It should also say who to call if they see persistent condensation or mould, so small problems are investigated before they become big ones.
Given at handover, that sheet is advice. Given after a complaint, it can sound like an excuse. Timing matters.
The commercial reality
Condensation complaints are expensive for builders in time, reputation and sometimes rectification. Hidden condensation inside walls and roofs can surface as a defect claim years after handover, when the cost of opening up walls is high and the argument about responsibility is harder. The cost of getting the membrane, cavity, exhaust and ventilation details right during construction is small by comparison.
Winter is the season when the problems show. It is also a good time to walk your current jobs with the condensation checklist in hand, before the linings go on.
Questions builders ask
Why is there condensation on the windows of my new house?
Warm, moist indoor air is meeting cold glass. New homes are well sealed, so moisture from showers, cooking, breathing and drying clothes has fewer ways out. Using exhaust fans, ventilating and avoiding unflued gas heaters usually helps. If it persists, have the builder check that exhaust fans discharge outside and insulation is complete.
Is condensation a building defect?
Not always. Surface condensation is often driven by how the home is used. But building faults, such as exhaust fans ducted into the roof space, missing insulation, the wrong wall membrane or an unventilated roof where one is required, can cause or worsen it and are defects to fix.
Where must bathroom exhaust fans discharge?
Under the NCC housing provisions introduced with NCC 2022, exhaust from kitchens, bathrooms and laundries must discharge outdoors, not into the roof space.
How can I tell condensation from a leak?
Condensation appears on cold mornings across cold surfaces such as windows and external corners. A leak appears during or after rain, below a specific defect, and is usually localised.
Does NCC 2025 change condensation requirements?
Yes. NCC 2025 strengthens condensation management, including changes affecting external wall construction and roof ventilation. It applies from each state’s adoption date, which is 1 May 2027 in NSW and Queensland.
Sources
- Condensation in buildings handbook · Australian Building Codes Board
- NCC 2025 Condensation in buildings handbook · Australian Building Codes Board
- NCC 2025 has been released · Australian Building Codes Board · 2026-05-01
Published 7 August 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.
