Condensation on windows: what a curtain can do, and what it makes worse
Condensation on a window does not come from the window: it comes from the room's air, loaded with moisture, meeting a cold surface. A curtain does not create it, but it can make it worse, because it stops warm air from reaching the glass.
The remedy is three habits: air the room for five to ten minutes a day even in winter, leave a gap between curtain and glass rather than pressing it flat, and deal with the moisture source before the window.
Where the water on the glass comes from
The air in a home always holds water vapour, and it holds more of it the warmer it is. When that air meets a cold surface, it can no longer keep the vapour: it settles as droplets. The glass is almost always the coldest surface in a room, so it is the first to fog up.
That is why condensation appears in autumn and not in summer: humidity has not risen, the glass has cooled. A shower, cooking, or drying laundry indoors is then enough to tip the balance.
What a curtain can do, and what it makes worse
We should be straight about this, because it is counter-intuitive: a thick curtain pressed against the glass increases condensation. By insulating the glass from the room's warmth, it leaves it colder, and therefore more prone to fogging.
| Situation | Effect on condensation |
|---|---|
| Curtain flat against the glass, closed day and night | Makes it worse. The glass stays cold and air stagnates between fabric and pane. |
| Curtain with a gap behind it, open during the day | Neutral to favourable. Air circulates and the glass warms up by day. |
| Curtain that also covers the wall around the window | Favourable for comfort: it reduces the cold-wall sensation without trapping air against the glass. |
In other words, a curtain solves a thermal comfort problem, not a humidity problem. The two are treated separately.
The three habits that work
None of them costs anything, and all three are daily.
| Habit | Why |
|---|---|
| Air the room 5 to 10 minutes, wide open, every day | Cold outdoor air is drier. Brief and full renews the air without cooling the walls, unlike a window left ajar for an hour. |
| Treat the source before the glass | Laundry dried indoors, cooking without a lid, a bathroom with no extraction: the glass is only the messenger. |
| Do not press the curtain against the pane | Let the space between fabric and glass breathe, and open it by day when the sun is out. |
When the window itself is at fault
If condensation appears between the two panes of a double-glazed unit, ventilation is no longer the issue: the seal has failed, and only the joinery can answer for it. On the inner face, by contrast, it is almost always an air problem, not a glazing one.
The cold you feel is a separate subject. According to ADEME, windows account for 10 to 15% of a home's heat loss, and the Union des Fabricants de Menuiseries notes that this estimate leaves out air leakage around windows already installed. That is the share a dense curtain works on.
Frequently asked questions
Does a thermal curtain cause condensation?
It does not create it, but it can accentuate it if pressed against the glass and left permanently closed: the pane then stays colder. A gap behind the panel and an opening during the day are enough to avoid that.
Should curtains come down in winter because of damp?
No. The problem comes from the room's air, not the fabric. Removing the curtain would cost thermal comfort without addressing the cause, which is poor ventilation or an unmanaged moisture source.
Why does condensation appear mostly in the morning?
Because heating drops overnight and the glass reaches its lowest temperature, while the occupants' breathing loads the air with vapour in a closed room. Airing on waking is the single most effective habit of the day.
Does condensation damage curtains?
It can, if the bottom of the panel rests on a permanently wet sill: the fabric stains and mould can appear. A curtain that stops above the sill, or a sill wiped regularly, settles the question.
To prepare for the cold season, our autumn curtains page gathers the September steps, and our made-to-measure thermal curtains are made to your dimensions in 620 gsm.
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