How to stop windows getting mouldy in winter.

Mould on window frames and reveals is condensation nobody wiped up. Glass and frames are the coldest surfaces in a room, so moist indoor air condenses there first and stays wet long enough for mould to establish. Stopping it means lowering the moisture in the air, raising the surface temperature, and getting the water off before it settles — in that order.

The useful thing to understand first is that the window is rarely the problem. It is the indicator.

Glass is non-porous, so mould does not grow on the pane itself. It grows on the frame, the reveal, the sill, the sealant bead and the curtain hem — wherever the water runs to and sits. And because the window is the coldest surface available, it is where condensation appears earliest. If it is forming there, the same conditions exist elsewhere in the room, on surfaces you cannot see.

Why it happens on windows first

Air holds water vapour, and warm air holds more of it than cold air. Cool a parcel of air far enough and it reaches the dew point — the temperature at which it can no longer carry what it is holding — and the excess becomes liquid water on whatever surface did the cooling.

In most Australian houses the coldest interior surface on a winter night is a single-glazed window, and the second coldest is the frame around it. Aluminium conducts heat readily, which is why an aluminium frame often runs wetter than the glass it holds.

Three things have to be present for mould to follow: moisture in the air, a cold surface, and still air. Remove any one and it stops. Most households can influence all three.

Which side of the glass gets wet, and why

There is one rule underneath all of this: water forms on whichever surface is colder than the dew point of the air touching it.

In winter the driver is the opposite of what it looks like: the room is warmer and wetter than the air outside, heat flows out through the glass, and the room-side face ends up the coldest surface in the room. That is where the water forms.

So for each face of the glass there are only two things to know — how cold that surface is, and how much moisture is in the air against it.

A window sits between two bodies of air with different temperatures and different humidity. Heat flows through the glass from the warm side to the cold side, which means the surface temperature of each face lands somewhere between the indoor and outdoor air temperature. Where it lands is decided by how well the glazing insulates — and that single variable determines which face gets wet.

On the inside — the winter case

Glass is a poor insulator, so on single glazing the room-side surface sits much closer to the outdoor temperature than to the room's. On a cold night that face can be well below the temperature of the air in the room.

Warm, moist indoor air touches it, cools past its dew point, and lets go of the water it was carrying. That water lands on the glass, runs down to the frame and sill, and sits there.

This is the case that causes mould, and it is telling you something specific: the moisture level in the room is too high for how cold that surface is. Both halves are fixable — lower the humidity, or raise the surface temperature.

On the outside — usually good news

Condensation on the outer face of the glass surprises people, and it is generally a sign the window is performing rather than failing.

A well-insulated double or triple glazed unit lets very little indoor heat reach its outer pane. That pane therefore sits close to outdoor temperature — and on a clear, still night it can drop below the surrounding air temperature, because it radiates heat away to the open sky. It is the same effect that puts dew on a car roof and frost on a lawn while the air above stays above freezing.

Once that outer surface falls below the outdoor dew point, dew forms on it. Nothing is wrong. It clears within an hour or two of the sun reaching it.

The same thing happens in reverse during humid summers: an air-conditioned room chills the glass, and warm humid outdoor air condenses on the outside face.

Between the panes — the one real fault

If the moisture is inside a double-glazed unit and cannot be wiped from either side, the perimeter seal has failed. The desiccant in the spacer bar has absorbed all it can, and humid air is now moving into the cavity.

This has nothing to do with your ventilation or your habits, and no amount of airing the house will touch it. The sealed unit has reached the end of its life and needs replacing.

Why it builds up overnight

Condensation is not an event. It is an accumulation — and the reason you find it at seven in the morning rather than at nine the night before is that every variable moves the wrong way while you sleep.

The glass keeps getting colder. Outdoor temperature falls from sunset onward and usually reaches its minimum shortly before dawn. There is no sun on the glass to offset it. So the room-side surface is at its coldest at the exact hour the night's moisture has finished collecting on it.

The heating goes off. The room cools, but the water vapour already in the air does not leave with the heat. Same moisture, colder surfaces.

The escape routes close. Doors shut, windows shut, exhaust fans off. For eight hours almost nothing is removing vapour from the room.

And moisture keeps arriving. Two people breathing and perspiring through a night in a closed bedroom release a significant amount of water into a small volume of air. This is why bedrooms are so often the worst room in the house.

So the moisture input continues while every route out is closed and the surface temperature falls. The water on the pane at dawn is the sum of everything that condensed since roughly midnight, and none of it has gone anywhere, because the surface has stayed below the dew point the entire time.

Which is the real reason wiping works. Leave it, and as the room warms through the day much of that water evaporates straight back into the air inside the house — where it waits to condense again the following night. Wiping it up and taking the cloth away physically removes the moisture from the building. Letting it dry only recycles it.

Why it starts at the bottom

Look at a window that has just begun to fog and the water is almost always a band along the bottom edge, creeping up the corners.

Two things cause that. Cold air is denser than warm air and sinks, so the coldest air in the room pools against the lower part of the pane. And the edge of the glass is colder than its centre, because the frame and the spacer bar around the perimeter conduct heat away far faster than the glass itself does — metal spacers considerably more so than insulated ones.

The practical use of this: the height of that band is a rough gauge of severity. A thin line along the bottom on the coldest mornings is ordinary. Water reaching halfway up the pane most mornings means the room needs attention.

It also explains why the frame and reveal go mouldy before the glass does. They are colder, they stay wetter, and unlike glass they are porous enough for mould to take hold.

Where the moisture is coming from

A household generates a surprising volume of water vapour without anyone noticing.

• Showering and bathing — the single largest source in most homes.
• Cooking, particularly boiling and simmering without a lid.
• Drying clothes indoors. A single load of wet washing releases litres of water into the house as it dries.
• Unflued gas heaters. These produce water vapour as a direct product of combustion and release it into the room. If you have one, it is often the largest contributor in the house during winter.
• People. Breathing and perspiration, which is why bedrooms are frequently the worst room in the house by morning.
• A damp subfloor, evaporating moisture upwards continuously.

What to do first

Before changing anything about the house, change the routine. These cost nothing and work quickly.

1. Wipe the windows dry each morning. Unglamorous and completely effective. Mould needs the surface to stay wet; removing the water each day denies it the one condition it cannot do without — and takes the moisture out of the house rather than letting it evaporate back in.
2. Run the bathroom fan during the shower and for a good while after. Most of the moisture leaves the room after the water stops, not during. A timer switch is the single best small purchase for this problem.
3. Use the rangehood every time you cook, and put lids on pots.
4. Stop drying clothes indoors, or if you must, do it in one room with the door closed and a window open.
5. Purge the house daily. Windows and doors wide open for a few minutes, ideally on opposite sides for cross-flow. Short and sharp loses less heat than a window cracked all day and shifts far more air.
6. Open internal doors so air moves between rooms instead of sitting still in the cold ones.

Raising the surface temperature

The other half of the equation is how cold the glass and frame get.

Heat the house more evenly. A single hot room surrounded by cold ones drives moisture into the cold rooms, where it condenses. Bedrooms left unheated all winter are the classic case.

Watch what your curtains are doing. Heavy curtains hanging against the glass trap a pocket of cold air and make the surface colder still, which is why mould often appears on the reveal behind a curtain. A pelmet at the top with a gap at the bottom is better, or curtains held clear of the glass.

Move furniture off external walls, and leave a gap. Wardrobes on external walls are a common mould site — leave the doors ajar occasionally and do not pack them solid.

Longer term, double glazing and thermally broken or timber frames raise the internal surface temperature substantially, which reduces condensation at the same indoor humidity. It reduces it rather than eliminating it — if the air is wet enough, it will still find the coldest surface available.

Cleaning what is already there

Deal with existing growth before it establishes further, and take a little care doing it.

• Do not dry-brush or dry-scrape mould. It disperses spores into the room air. Wipe with a damp cloth so the growth comes away wet.
• Clean the surface rather than bleaching the colour out of it. Something that removes the growth physically is more effective than something that merely lightens the stain — a bleached patch that was never cleaned will regrow.
• Ventilate the room while you work, and wear gloves.
• Never mix cleaning products, particularly anything chlorine-based with anything ammonia-based.
• Dispose of the cloth rather than wringing it out into a bucket you then wipe with.
• Do not paint over it. Paint applied over mould fails, and it hides the evidence of a problem that is still running.

Two situations warrant more than a cloth: growth over a large area, and anyone in the household with asthma, a respiratory condition or a compromised immune system. Your state health department publishes guidance on mould in homes, and that is the right source — it is a health question rather than a maintenance one at that point.

If it keeps coming back

Persistent mould after all of the above means something structural is feeding it. Work through these.

1. Find out where your bathroom fan discharges. If it terminates in the roof space rather than outside, it is not removing moisture from the building — it is relocating it into a cold cavity, from which it returns. This is one of the most common defects in Australian housing and a ten-minute check with a torch.
2. Check the rangehood the same way. Recirculating rangehoods filter grease and return the moisture to the room.
3. Look at the subfloor — damp ground, blocked vents, poor drainage against the building.
4. Count the windows affected. One window suggests a local issue: a cold corner, a curtain sitting against the glass, or a blocked drainage slot in the frame. Every window in the house suggests whole-house humidity.
5. Note which rooms. Bedrooms and bathrooms point to occupancy moisture. A single unheated room points to temperature.
6. Check whether it is seasonal. Condensation mould appears in winter and eases in warmer months. Mould in the same spot year-round is a water leak, not condensation.

When it is not condensation at all

A few patterns look similar and need a different response.

Staining running down from above the window suggests water entering around the head flashing rather than forming on the glass.

Damp at sill level that persists through summer may be water ingress through the frame junction or the wall below.

Fogging between the panes of a double-glazed unit that cannot be wiped away means the seal has failed and the unit has lost its cavity. That is a glazing repair, not a ventilation problem.

All three are worth distinguishing early, because no amount of wiping or airing will touch them.

Common questions

Is window mould dangerous?

Small amounts on a frame, cleaned promptly, are a maintenance matter. Larger or recurring growth is worth taking more seriously, particularly where anyone in the house has asthma or another respiratory condition. Your state health department publishes guidance on when mould in a home needs professional assessment.

Why is it worst in the bedroom?

Bedrooms combine two people breathing for eight hours with a door that stays closed and, often, no heating. That is high moisture, still air and a cold surface in the same room — all three ingredients at once.

Why is it always worst first thing in the morning?

Because that is the end of the accumulation, not the start of it. Outdoor temperatures bottom out shortly before dawn, so the glass is at its coldest just as the night's moisture finishes collecting on it. By mid-morning the sun and the heating have raised the surface temperature and the water has gone — which is exactly why it is worth wiping up before it does.

My new windows sweat more than the old ones did. Why?

Older windows usually leaked, and that leakage was ventilating the house accidentally. New, well-sealed windows stop that air movement without adding any deliberate ventilation to replace it, so indoor humidity rises. The answer is not worse windows — it is ventilation you control.

Does double glazing stop condensation completely?

It reduces it substantially by raising the internal glass temperature, but it does not eliminate it. If indoor humidity is high enough, moisture will still condense on the coldest surface available — which may become the frame, or a wall corner, rather than the glass.

Should I leave a window open through winter?

A short, deliberate purge with windows wide open moves far more air and costs less heat than a window cracked open all day. Do it once or twice daily, and always after showering or cooking.

Will a dehumidifier fix it?

It will manage a specific problem room and can be genuinely useful there. It does not address why the moisture is accumulating, and needing to run one continuously usually means ventilation or a water source needs attention.

Where to go next

Window mould is the visible edge of a whole-house moisture picture, and it is worth understanding the rest of it.

Read next: winter condensation and mould for how the same problem shows up elsewhere in the house. Or start at Seasonal Maintenance for the wider picture.