Mould around windows: what it means and whose job it is

Why windows are where mould appears

Condensation forms when warm, moist air touches a surface cold enough for the moisture to settle out as water. Government damp and mould guidance for England says it forms on the colder parts of a building, such as window frames and the wall behind furniture. Around a window that means the glass, the frame, the reveal (the strip of wall that returns to the frame) and the sill. Where the reveal or the lintel above the window has little insulation, that strip of wall sits colder than the plastered wall beside it and collects water first.

Water runs down the glass overnight, pools on the bottom of the frame and sill, wicks into the sealant and plaster at the lower corners, and mould follows. Heavy curtains or blinds pulled tight hold a pocket of still, cold air against the glass. A trickle vent that is closed or painted shut takes away a route moist air had to leave. A bed or wardrobe pushed against the wall under the window does the same to the wall.

None of this means the building is faulty or that you are doing anything wrong. It means the window is the weakest point in a room producing more moisture than it gets rid of. The question is why: too little ventilation, too little steady heat, or a window and reveal colder than they should be.

Condensation, a failed seal, or water getting in: telling them apart

Different problems produce mould around a window and need different responses. Look at when the wetness appears, where it starts, and what the surfaces feel like.

What to photograph: the four views

Photographs are only useful if they let someone else see the pattern. Take the same four views of every affected window and repeat them after the next cold spell and the next heavy rain.

• The whole window from across the room, with the reveal, sill, curtains and any furniture below it in shot. This shows the setting.
• The affected reveal or sill from a metre away, with the frame edge in shot for scale. This shows the extent and pattern: bottom corners, one side, all the way round.
• A close-up of the growth or staining, with the flash off and the room lit. This shows what it is: spotting, a tide mark, a run, blistered paint, soft timber.
• The outside of the same window, if you can safely see it: the sealant line, the sill, and any cracked render or missing pointing above the frame.

What a resident can reasonably do

Government guidance for landlords in England says damp and mould should not be blamed on tenants' lifestyle. Even so, there are things any household can do that reduce condensation at the window, and doing them shows you have done your part.

• Wipe the glass, frame and sill dry each morning and wring the cloth out in the sink.
• Open the trickle vents and leave them open. If they are painted shut, tell the landlord and ask for them to be freed; if the window has no vents at all, say so in your report, because how the room is ventilated is part of what the landlord should assess.
• Ventilate at the source: lids on pans, extractor on and door closed when cooking and showering.
• Heat steadily rather than in short blasts, as far as your budget allows. If it does not allow much, say so in writing.
• Pull furniture a hand's width off the wall under and beside the window, and stop curtains sealing against the glass overnight.
• Once it has been photographed and reported, small patches on hard surfaces can be wiped with a mould and mildew product (the government guidance for England prefers these to bleach), wearing gloves and a mask, and left to dry. Larger areas are for the landlord's contractor, and cleaning does not replace fixing the cause. Wipe it rather than scrubbing it dry.

What needs the landlord, or someone qualified, to look at

Some things cannot be managed with a cloth and an open window. If you see any of the following, the first step is a written report asking for an inspection.

• A double-glazed unit that has misted between the panes.
• Cracked, shrunk or missing sealant between the frame and the wall, inside or out.
• A timber sill that is soft, split or crumbling, or a uPVC sill that has come away from the frame.
• Mould across the reveal and the wall above the window, not just the bottom corners, which points to a cold bridge at the lintel.
• Trickle vents that are painted shut, extractor fans that do not run or do not vent outside, and a room with no ventilation at all, which the landlord should assess.

When it becomes the landlord's repair, by nation and tenure

The line is similar in practice: managing moisture is something a household can be expected to do, while the window, its frame, sill and seals and the wall around it are part of the structure and exterior, which the landlord must keep in repair in England, Wales and Scotland. In Northern Ireland the tenancy terms come first, with default obligations where they are silent. Where the law puts that duty differs.

How to report window mould so it gets dealt with

Report it in writing, name the window, say what you have seen and when, and attach the four photographs. Describe the pattern rather than diagnosing it: "black mould along the bottom of the rear bedroom window reveal and sill, back within a week of cleaning, trickle vent painted shut, glass misted between the panes" tells the landlord exactly what to send someone to look at. Ask for an inspection of the window, seals and reveal, and for the findings in writing.

If the reply is "open the windows more", answer with your record of what you have already been doing and ask again for the inspection. In social housing in England the Awaab's Law timescales run from when the landlord becomes aware of the problem, however it is reported; a written report is the clearest way to fix that date. In Scotland the Awaab's Law timescales apply to reports made from 6 October 2026. Everywhere else a written report fixes the date from which a reasonable time is measured.