Window condensation
Why do Velux windows get condensation?
Droplets on a roof window usually point to moisture in the room, not a fault in the glass. Below: the causes, how to read the signs and what to change.
The short answer
Velux windows get condensation because warm, damp air in the room meets glass that is colder than the air. The droplets form on the room side, and the usual sources are cooking, bathing, drying washing indoors and too little ventilation. Wet between the two panes is a different fault, a failed glazing seal, and a drip that keeps coming back after dry weather may be a leak.
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Where the moisture comes from
Indoor air carries water vapour, and the warmer it is, the more vapour it can carry. When that air touches a cold surface it cannot keep all its moisture, and the surplus turns to droplets. A roof window can be the coldest surface in a loft room, so the droplets may show there before anywhere else. The room itself is the source, not the window.
Everyday living puts a great deal of vapour into a house: boiling a kettle, running a shower, cooking, and drying clothes on a rack. A bedroom under the roof collects the breath of whoever sleeps there overnight as well. Where the air cannot leave, the moisture settles on the glass, the frame and the lining, and in time it can mark the timber or feed mould.
BSI lists BS 5250 as current and under review. That standard deals with moisture in buildings, and it treats condensation as a matter of how much moisture is made, how well the building is ventilated and how warm the surfaces are, which is why a new pane alone may not cure it. A window that fits well and is glazed to a high standard still shows droplets when the room is damp enough, so the habits in the house count as much as the product on the roof.
Reading the signs on the window
Where the wet appears tells you more than how much of it there is. The table sets out what each sign usually points to, so you know whether to open a window or call for a survey.
| What you see | What it suggests | First step |
|---|---|---|
| Droplets on the room side, clearing by midday | Damp air in the room | Ventilate and cut vapour |
| Mist or a film between the panes | A failed glazing seal | Ask about reglazing |
| Wet frame or lining after rain only | A leak, not condensation | Book a roof survey |
| Dark marks along the lower rail | Moisture sitting on the timber | Dry it, then check the seals |
Wiping the glass dry in the morning and watching it for a week gives a good clue, and a short note of the weather on each day makes the pattern easier to read. Droplets that return on cold nights and vanish on mild ones are condensation. A stain that spreads after a storm is something else entirely, and worth showing us.
Changes that cut the moisture
Most of the cure lives inside the house, and none of it needs a ladder. These steps help a loft room with a roof window:
- run the extractor in the bathroom and kitchen during use and for a while after
- dry washing outdoors, or in a room with a window open and the door shut
- leave the window on its trickle vent or in the ventilation position for a short time each morning
- keep furniture and curtains a hand's width clear of the glass, so air can move across it
- open any blind fully once the room is warm, since a closed blind holds damp air against the pane
None of this is a promise that the droplets will stop, because the weather and the household both play a part. It does lower the amount of moisture the glass has to deal with. If the window still weeps daily after a fortnight of trying, the cause may be built into the roof, and that is the next section. It also helps to look at the lining panels and the reveal around the frame with a torch, and to feel for cold air moving across them on a windy day.
When the cause is in the roof
Sometimes the glass is cold because the work around the window lets heat escape. A gap in the insulation behind the lining panels, a lining that does not meet the frame, or a draught through a poorly sealed opening will chill the frame and the reveal, and moisture collects there first. That is a fitting fault, and the fix is to open up the lining, restore the insulation, reseal it and check the flashing at the same visit, so that the window is warm and weathertight together.
Misting between the panes is a separate question. It means the sealed glazing unit has failed, and our page on reglazing a Velux window explains what can be done. Drips that follow rain are covered in why a Velux window leaks.
A survey looks at the room, the lining and the roof outside in one visit, then gives you one written total price for whatever the roof needs, with the scaffold, if any, included in it. The wider roof window and skylight service shows how the work is put together.
More questions
Does condensation mean my Velux window needs replacing?
Not by itself. Droplets on the room side come from the air in the room. A new window in the same room meets the same air. Replacement is worth discussing when the frame is soft, the sash no longer seals or the glazing has misted between the panes, and a survey will say which of these applies.
Why is the condensation worse in a bedroom or a bathroom?
Both rooms make more vapour. People breathe out moisture all night in a bedroom, and a shower fills a bathroom with warm steam that meets the cool glass at once. A room with the door closed and no way for damp air to leave will show droplets on the window sooner than a well-ventilated one.
Tell us what's wrong with the roof.
You'll get one written total price.