Why mould keeps coming back on window sills and frames
Every winter the same thing happened at our place. A line of black speckle along the bottom of the coldest window, on the sill and up the frame, wiped away on Sunday and back by the next weekend. It is one of the most common mould spots in an Australian home, and it is almost always the same cause.
Short answer: mould returns on window sills and frames because of condensation. Warm, moist indoor air hits the cold glass and frame, the moisture turns to water on the surface, and that water is what mould needs. Wiping without dealing with the morning condensation just resets the clock. Deal with the water and the airflow and it stops.
Why do window sills and frames grow mould?
Think about the coldest surface in a room on a winter morning. It is the window. Indoor air always carries some moisture, from cooking, showers, drying washing, and simply breathing. When that air touches cold glass or a cold metal frame, it cannot hold the moisture any more and it condenses into water. The sill sits right underneath, catching it. Aluminium frames are the worst offenders, because metal is a cold bridge and pulls a lot of condensation. That daily film of water, sitting on a sill that never quite dries, is a standing invitation.
This is not a rare or exotic problem. Coulburn and Miller's 2022 Australian review notes the last national survey to measure it found almost 18% of Australian homes had mould or mildew. Condensation on windows in winter is one of the most ordinary ways it starts.
Why does it keep coming back after I wipe it?
Because wiping deals with the mould you can see, not the water that grew it. Tomorrow morning the air hits the cold glass again, the sill gets wet again, and any spores in the room settle onto that damp surface and start over. To really understand the returning pattern it helps to read how mould works and why it comes back, but the short version is: no dry, no mould. As long as the sill gets wet every morning, cleaning is a holding action.
What is the morning routine that holds it back?
The fix is undramatic and it works. Wipe the condensation off the glass, frame and sill in the morning, before it sits there all day. Open the window for a few minutes to let the moist air out. Keep curtains and blinds off the glass overnight where you can, because a curtain pressed to a cold window traps a pocket of damp air right where it condenses. And keep the room's general moisture down: lids on pots, the exhaust fan running after showers, washing dried outside rather than on a rack under the window.
Condensation risk by frame and glazing
| Window type | Condensation tendency |
|---|---|
| Single-glazed with aluminium frame | Highest. Cold metal and cold glass both condense heavily |
| Single-glazed with timber or uPVC frame | Glass still condenses, frame less so |
| Double-glazed | Lowest. The inner pane stays warmer, so less water forms |
When is it the building's fault, not yours?
Sometimes the morning routine is not enough, and that usually points at the building. Old single glazing, no trickle vents, a room with no real ventilation, or a frame with failed seals letting water track in will keep condensing no matter how diligent you are. If you rent, that side of it can be a maintenance matter rather than something you caused, and it is worth raising in writing. Our renter mould data gives a sense of how damp your city runs and where the responsibility tends to sit.
For the full prevention method, see our guide to how to stop mould.
Common questions
Why only in winter? Because the temperature gap between warm indoor air and cold glass is biggest then, so the most water condenses. In a humid summer you can get it too, but winter is the classic.
Is the black mark on the frame dangerous to the window? This guide is about the mould and the damp, not the glazing. Left wet long enough, timber frames can suffer, which is another reason to dry the sill.
Will a dehumidifier help? In a closed room with a bad condensation problem, yes, because it lowers the moisture in the air that condenses on the glass. Ventilation does the same job for free when the weather allows. As of 2026 that trade-off has not changed.
Related guides
- Why mould keeps coming back in shower silicone and grout
- Black mould on a bathroom ceiling, and why it keeps coming back
- How to stop mould in a bathroom with no window
For more rooms and surfaces, see our guide to tackling mould room by room.
Ben, founder of Aurala Naturals.
