Most people file condensation under midwinter problems. It isn't one. Condensation on windows in autumn starts in the second or third week of September, weeks before anyone touches the thermostat, and by the time the first black spots show up in November it has been quietly running for two months.
We manufacture paint and cleaning chemicals in Glasgow, and our inbox tells the same story every year. Silence through September. A trickle in late October. Then a wall of emails in the first fortnight of November from people whose bathroom ceiling has gone black seemingly overnight. It didn't go black overnight. It started when the nights first got cold.
Why does condensation on windows in autumn start so early?
Air holds water in proportion to how warm it is. Cool it down and at some point it can't hold what it's carrying, so it dumps the difference onto the nearest cold surface. That temperature is the dew point, and it's the only number that matters here.
Run it for an ordinary living room. At 20°C and 60% relative humidity, the dew point is roughly 12°C. Every surface in that room colder than 12°C will start collecting water. Now take a Scottish September night, where outdoor temperatures sit at 6 to 8°C. The inner pane of an older double-glazed unit tracks closer to the outside temperature than the inside one. It drops under 12°C without much trouble.
January feels worse but often isn't. By then the heating has been running for six weeks, the fabric of the building has warmed through, and surface temperatures are higher than people assume. September is the gap in the middle: cold nights, no heating, and a house still carrying the moisture load it built up over summer. Nothing is drying out, and nothing is being warmed up.
That gap is the point at which it's worth doing something, and it's why we'd rather talk about prevention now than sell you our Legacy Anti-Mould Paint for walls and ceilings in December when the damage is already visible.
Why do the bottom corners go first?
Look at any window that's had a condensation problem and the staining starts at the bottom two corners of the reveal, then creeps up the sides. There's a reason for that, and it isn't just gravity.
The edge of a sealed glazing unit contains a spacer bar. On older units that bar is aluminium, which conducts heat straight through the unit and makes the perimeter of the glass measurably colder than the centre. Add the frame and the plaster reveal behind it, both of which bridge cold from outside, and the coldest surface in the whole room is the 50mm band around the edge of the glass. That band hits dew point first, holds water longest, and sits directly above porous plaster and paint that soak it up.
So the corners aren't where the problem is worst. They're where it shows first.
If the mould doesn't appear until November, why act in September?
Because mould doesn't need standing water. It needs sustained humidity at the surface, somewhere above 75 to 80%, and it needs time.
When we test the fungicide package in a batch of anti-mould paint, we inoculate treated and untreated panels and hold them in a humidity cabinet under conditions far more aggressive than any bathroom. Even then, visible growth on an untreated control panel takes days. On a cold window reveal sitting at 10 to 15°C, the same process takes weeks. What you notice at Christmas germinated in September and has been colonising the plaster ever since, under the paint film, out of sight.
That's the whole argument for treating this as an autumn job. Once growth is established in porous plaster, you're no longer preventing a problem. You're excavating one.
What do people get wrong when they finally do something?
The single most common complaint we deal with is a variation of "your paint failed". Almost every time, it's the same sequence: the customer has painted over growth that was still alive.
An anti-mould coating works at the surface. The fungicide sits in the dried film and stops spores landing on it from germinating. It does not reach down into plaster and kill what's already rooted there. Paint over live mould and the hyphae carry on working underneath, break the film within four to eight weeks, and reappear as a faint grey bloom that looks exactly like the paint has failed. Kill it and clean it back to sound substrate first, then coat.
The practical numbers we'd give anyone doing this in autumn:
- Two coats, always. One coat gives you patchy film thickness, and film thickness is what the biocide performance depends on.
- Around 12m² per litre on properly prepared surfaces. Bare or previously distempered plaster will drink more than that.
- Touch dry in 1 to 2 hours, recoat after 4. Touch dry is not cured. Cure is what gives you a washable, coherent film, and in a cold room it takes considerably longer than the tub implies.
- Don't apply below about 10°C, and don't apply to a surface that's still holding moisture. A water-based emulsion that can't lose its water properly never forms a sound film.
And the honest limits, because they matter more than the sales pitch: anti-mould paint will not fix a leak, will not lower the humidity in your house, and will not compensate for an extractor fan that doesn't work. If the moisture source is still there, all you've bought is time.
What actually helps in September?
None of this costs much, and all of it is more useful in September than in January.
- Buy a hygrometer. They're about a fiver. Aim to keep indoor relative humidity under 60%. You cannot manage what you aren't measuring, and most people are surprised by the reading.
- Wipe the glass in the morning. Thirty seconds with a cloth removes water that would otherwise sit on the reveal all day and soak into plaster.
- Open the trickle vents. Nearly every window fitted in the last twenty years has them, and a remarkable number have never been opened.
- Run the extractor for longer. A bathroom fan that stops when the light goes off has removed maybe a third of the moisture from a shower.
- Set unused rooms to low, not off. A steady low background temperature keeps surfaces above dew point far more effectively than blasting the heating for two hours in the evening.
- Leave a gap behind furniture on external walls. Around 50mm. A wardrobe against a cold north wall creates a pocket of still, cold air that will be the first place mould appears.
If you're drying washing indoors, that alone can put several litres of water a day into your air, and it's worth sorting out before the weather turns. If you've already got black spotting around the frames, deal with the growth itself before you think about redecorating, and don't reach for the bleach.
Get ahead of it this year
Legacy™ Anti Mould Paint is a solvent-free acrylic emulsion with the fungicide built into the paint film, made by us in Glasgow. It's designed for bathrooms, kitchens, bedroom walls and anywhere condensation keeps coming back, and it carries a 5 year guarantee against regrowth on the treated surface. Clean and kill first, then two coats.
Shop Legacy Anti-Mould Paint →
Written by the people who make it: how anti-mould paint actually works and how to get rid of black mould before you paint.
Frequently asked questions
Is condensation on windows in autumn normal?
A small amount on the coldest mornings is normal and not a sign of anything wrong with the building. What isn't normal is condensation every morning, water pooling on the sill, or damp patches appearing on the reveal. That points to too much moisture in the air, too little ventilation, or both.
Should I wipe condensation off the windows or leave it to dry?
Wipe it. If you leave it, it doesn't disappear, it evaporates back into the room and condenses again the following night. Wipe it into a cloth and wring the cloth out at the sink, and that water has actually left the house.
Does anti-mould paint stop condensation?
No, and any manufacturer telling you otherwise is overselling. Anti-mould paint stops mould growing on the painted surface. Condensation is caused by moisture and cold surfaces, and only ventilation, heating and insulation change that. The paint protects the decoration while you deal with the cause.
What humidity should I keep my house at to prevent mould?
Below 60% relative humidity is a sensible working target for the whole house. Mould needs sustained surface humidity above roughly 75 to 80% to germinate, and keeping room air under 60% generally keeps surfaces clear of that threshold except at genuine cold spots.
Why did condensation start after I had new windows fitted?
Because the old ones were ventilating your house whether you liked it or not. New units are far more airtight, so the moisture your household produces has nowhere to go. It's one of the most common calls we get, and the answer is almost always trickle vents and extraction rather than anything wrong with the windows.
