Last Updated on October 24, 2026 by Umar Farooq
Condensation behind curtains forms because closed curtains seal a pocket of still air against cold glass overnight, so the pane falls below the room’s dew point and collects water that the rest of the room never sees. The curtain is not making the water. It is just hiding the wettest spot in the house from you until the hem goes damp.
If you have ever drawn the curtains at 8am and found the sill puddled, you have met it. The room feels dry, the walls look fine, and yet the one square metre of glass behind the fabric is streaming.
Why condensation behind curtains forms overnight
Water vapour in room air turns to liquid when it touches a surface colder than its dew point. In winter, the glass is the coldest surface you own, so it is where the vapour gives up first. The window itself works as a free humidity meter, and the fabric simply stops you reading it.
Here is a worked example, and the arithmetic is mine rather than a measurement. Room air at 20 C (68 F) and 60% relative humidity has a dew point of about 12 C (54 F). Any surface colder than 12 C collects water. A pane on a freezing night is far colder than that, so it wets. The question is only how much, and the curtain decides that.
Three things a closed curtain does to a cold window
Each one is small. Together they make the glass behind the fabric wetter than the same window left open.
- It cuts off the heat. An uncovered pane gets warmed a little by room air moving across it. Close a lined curtain and that warm air cannot reach the glass, so the pane sits close to the outdoor temperature all night. Thermal and blackout linings make this stronger, which is the point of them.
- It traps still air. The pocket between fabric and glass barely moves. Vapour leaks in round the edges, the air in the pocket cools, and it has nowhere to go. Moving air carries moisture away before it condenses. Still air does not.
- It stays shut for hours. Eight hours of sleeping people breathing out water vapour, in a closed bedroom, with the curtains drawn, is about the best recipe there is. Andersen Windows puts the cause plainly: interior condensation forms because indoor humidity is too high, not because the window is faulty, and its advice is to open drapes or shades by day so warm air can reach the glass.
My own view: the curtain is a symptom amplifier, not a cause. If the glass behind the curtains is the only wet place in the house, the room is only slightly too humid. If the walls and ceilings are damp as well, that is a bigger problem and the condensation on walls and ceilings guide is the one to read.
Read the signs before the mould does
You do not need to lift the curtain to know. Look for these, roughly in order of how early they appear.
- A wet or beaded sill in the morning, or a ring of water under the window.
- Damp hems and lining on the side facing the glass, which feel cold and slightly clammy when you pinch them.
- A musty smell when you open the curtains, from fabric that stayed damp.
- Peeling paint or swollen timber on the sill and frame, which means the wetting is regular rather than occasional.
- Black speckles in the frame corners and along the seal, where water sits longest.
Black spots are the point where this stops being a damp-glass problem. The EPA’s brief guide to mould, moisture and your home says to fix the moisture problem first, and treats growth larger than about 10 square feet (roughly 1 square metre) differently from small patches. A few spots on a seal are nowhere near that. For the frame side, see mould around windows, and for the fabric side, mould on curtains.
Do blinds, sheers and curtains over radiators behave the same?
Not quite. This part is judgement, not measured data, so treat it as a guide.
| Covering | What it does to the glass | Condensation risk |
|---|---|---|
| Thick lined or blackout curtains, closed | Seals a still pocket, blocks room heat | Highest |
| Sheers or voile, closed | Lets air move through the weave, little insulation | Lower |
| Roller blind, down | Flat barrier close to the glass | High, and drips collect at the bottom edge |
| Venetian slats, tilted open | Air passes between slats | Lower |
| Long curtain over a radiator | Heat escapes up behind the fabric, wasting it | Glass drier, room colder |
The pattern is simple: the more airtight the covering, and the closer it sits to the glass, the wetter the pane. Blinds fit tight to the frame, so they trap just as well as curtains. They are also easier to miss, because the water collects out of sight behind a slat or at the bottom rail.
A two-minute morning routine that keeps the pane dry
The fixes for condensation on windows in general cover the whole house. For the curtain pocket specifically, these are the moves that matter.
- Open the curtains as soon as you are up. Warm room air reaches the glass and starts drying it. Leave them open while the sun is on that side of the house.
- Wipe the sill and the bottom of the glass. A dry cloth takes seconds. The Barking and Dagenham council guidance on preventing condensation says to dry windows and windowsills every morning if they are wet, and to keep internal doors open to ventilate.
- Hang the curtain clear of the glass. Aim for a hand’s width of air behind it, with the fabric ending above the sill rather than draped across it. Tie-backs help by day.
- Leave a gap at night in the worst rooms. A pencil-width opening at the centre, or a trickle vent left open, lets some air circulate. In a bedroom where the sill is wet every morning, it is worth trying for a week.
- Do not push furniture against the curtain. A bed or a sofa back against the fabric is one more thing blocking air.
Check the room’s humidity before you blame the curtains
A curtain cannot add water to a dry room. If the pocket keeps filling, the air is carrying too much, and you can measure that instead of guessing. The EPA advises keeping indoor humidity below 60%, ideally between 30% and 50%. A cheap hygrometer on the bedroom wall will tell you where you are, and checking it against a second one stops a bargain unit misleading you.
In cold weather the safe number sits lower still, because the glass is so cold. The main sources are the usual ones: breathing, showering, cooking and drying washing indoors. Cut the source, add an extractor fan or a trickle vent, and the pocket behind the curtain stops filling. A dehumidifier is the last resort for a room that stays above target, not the first.
Wet glass behind the curtains is a ventilation problem, not a curtain problem
Closed curtains turn a cold pane into a damp pocket, which is why condensation behind curtains shows up first and worst on the sill. You do not need new curtains. Open them in the morning, keep them off the glass, wipe what formed, and bring the room’s humidity down to where the dew point sits below the pane’s temperature. If black spots keep returning in the frame corners after that, the cause is the air, and that is where to keep working.
FAQ
Why is there condensation behind my curtains but not in the rest of the room?
The glass is the coldest surface, and the curtain stops warm room air reaching it. The rest of the room stays above the dew point, so only the glass behind the fabric gets wet.
Do curtains cause condensation on windows?
Not by themselves. Water comes from humid indoor air. Curtains make it worse by trapping still air against the cold glass and blocking the heat that would warm the pane.
Should I leave curtains open at night to stop condensation?
Fully open helps most, but is not always practical. A gap, tied-back fabric or a trickle vent left open lets air reach the glass while keeping some privacy.
Is condensation behind curtains dangerous?
The water is harmless, but regular wetting can lead to mould on the sill, frame and curtain hem. Fix the moisture first, then clean any growth. For anything large, follow the EPA guidance linked above.
How much space should there be between curtains and a window?
I use a hand’s width as a working rule, with the hem clear of the sill. It is my rule of thumb, not a tested figure, but the principle is air movement across the glass.
