The short answer
Condensation on the inside of a Velux forms when warm, damp room air meets cold glass. To stop it, lower the humidity in the room, ventilate by opening the window or its vent, keep the blind from trapping cold air against the pane, and keep the frame area warm. Droplets between the panes mean a failed sealed unit, which is a separate problem.
Water on the glass is a sign of damp air, not usually a sign of a leak. Where it forms and when it appears tell you what to change.
Why a roof window mists up inside
Air always holds some water vapour, and the warmer it is, the more it can hold. Once that damp air reaches a cold surface, part of the vapour turns back into liquid. A roof window is often the coldest surface in the room, because glass loses heat faster than a plastered ceiling. The droplets collect where the air meets the glass or the frame.
Is condensation the same as a leak?
No, though the two are easy to confuse. Condensation shows up as a fine film or beads on the inside surface, often in the morning and on cold nights. A leak shows as a staining or a drip that follows rain, usually near a corner of the frame or the lining. If the marks appear after rain and stay on dry mornings, the cause is water getting in, not damp air, and the flashing needs a look. If the water appears on cold mornings and vanishes by lunchtime, ventilation is the more likely answer.
Both can happen together in a bedroom with a leaking corner and a damp atmosphere. We check from outside as well as inside, because treating one while the other continues gets you nowhere.
Cutting the moisture in the room
Every household produces water vapour. Breathing, cooking, showering and drying washing indoors all add it. A room in the roof gathers more of it than the rooms below, because warm air rises and meets the cold glass overhead.
What can I change in daily habits?
Run the extractor fan in the bathroom and the kitchen while you use them, and for a while afterwards. Keep doors shut so steam does not drift upstairs. Dry washing outdoors or in a room with an open window, not on a radiator under the sloping ceiling. A dehumidifier is a practical help in a room that stays damp all winter.
Keep the heating steady rather than blasting it for an hour. A very cold room warmed quickly pushes moisture onto the coldest surfaces. If the same pattern repeats each cold snap, the answer is more often steady ventilation than a change to the window itself. BS 5250:2021 is the standard on moisture in buildings, and it treats ventilation as part of the answer, not an optional extra.
Vents, openings and blinds
Many roof windows have a small vent flap near the top of the frame that can be opened without opening the sash. Using it lets damp air leave at the highest point in the room, which is where it collects. Check the controls on your window or the maker's guide.
Does a blind make condensation worse?
It can. With a blind drawn down at night, the air between the blind and the glass is trapped, and it cools quickly. Damp air that slips past the blind meets cold glass in a still pocket, and the droplets stay. Raising the blind a little in the morning, or leaving the vent open, lets that pocket clear. Our answer on blinds for an existing Velux explains how blinds are matched.
Opening the sash for ten minutes each morning does more than any product. A window left on a safe night latch gives a little air through the day. If you cannot open it safely or the mechanism sticks, mention it when we visit, since a sash that will not open is its own problem. For the wider picture, see the main page on roof windows and skylights.
When the cause is the roof, not the room
Sometimes the glass is cold because the area around the window is poorly insulated or badly ventilated. Gaps in the lining or missing insulation round the frame let cold reach the inner surface, so moisture settles on the lining and the glass together.
Could my roof ventilation be at fault?
It might. Where the loft is left cold, with insulation laid across the ceiling below it, air has to keep moving through the space or damp builds up. Marley's guidance for a roof with a non-breathable HR underlay is eaves ventilation of 25mm along the eaves for pitches of 15 degrees or less, and 10mm for steeper roofs. Blocked eaves vents, often covered by loft insulation, are a common reason for dampness.
We survey the outside, go up into the loft, send you pictures of what we see, and put a clear recommendation in the written price. If the glass itself has water trapped between the panes, that is a different fault, and our page on a Velux window misted between the panes deals with it.
More on damp and roof windows
Why is there water on the outside of my Velux in the morning?
Dew can form on the outer pane on clear, cold nights, because the glass radiates heat to the sky and falls below the air temperature. It is harmless and not a fault. It clears as the day warms, and nothing needs to be done.
Will a new Velux stop condensation for good?
A newer window usually keeps its inner surface warmer, which helps, but a damp room will still show moisture. The cure is balance: ventilation, steady heat and lower humidity together. If a window mists often in a room that is otherwise dry, it is worth having us look at the frame and the lining.
The full picture: Velux roof window and skylight installation in Taunton ยท More short answers