Why does my Velux window get condensation?

Air in a lived-in room carries water vapour from breathing, cooking, showers, kettles and drying clothes, and heated air can carry a good deal more vapour than cool air. When that air touches a cold surface it chills quickly, can no longer hold the vapour and leaves it behind as droplets. A roof window sits in a sloping ceiling, close to the outside, with warm air rising straight into it, so it is often the first surface in the room to fog.

That is why a bedroom window mists overnight and a bathroom window runs after a shower. A little on a cold morning is normal, and checking on a still, chilly morning shows most clearly whether the room or the window is at fault. What matters is whether it clears quickly and whether the frame or the lining stays wet afterwards, because lasting damp is where mould begins. If you are weighing it against a real leak, our guide to a leaking roof window lists the signs.

Wet on the room side of the inner pane, which wipes off, is ordinary condensation. A cloudy film trapped between the two panes, which will not wipe away, means the seal round the sealed unit has given up and damp air has crept in. No amount of airing cures that, and the unit or sometimes the whole window needs replacing.

The third case is a leak. Water that appears on the sill or lining during or after rain, leaves a tidemark, or runs from one corner of the frame is arriving from outside, usually past a flashing. Condensation is worst when the room is damp and cold, and a leak is worst when it has rained. If you cannot tell which you have, note the times it happens for a week and take photographs each day. Our answer on flashing kits describes the part that most often lets rain in.

An older roof window, especially one with single glazing or a worn seal, has a far colder inner surface than a modern one. The glass is the coldest part, but the frame can be nearly as bad, and if insulation was packed carelessly round it when it was fitted, a cold strip runs round the edge and the plaster beside the window sweats. Approved Document L 2021 Table 4.2 limits a roof window installed in an existing house to a U-value of 1.4, or accepts Window Energy Rating Band B, so a new unit keeps its inner pane warmer.

Blinds trap damp air against the glass, and one left down at night leaves the pane colder and wetter by morning. In a loft bedroom the roof window is often the only cold surface in a warm room, so every breath and every bit of damp air heads straight for it.

Start with the moisture itself. Run an extractor fan during and after showers and cooking, keep lids on pans, dry washing outdoors when you can, and keep the door of a steamy room shut. Open the roof window a little each day, or use its trickle vent if it has one, so damp air leaves before it reaches the glass. Heating the room evenly rather than in short bursts also helps, because cold surfaces are what turn vapour into water.

If the window is the weak link, a replacement with modern glazing will do more than any amount of wiping. The guide to replacing an old roof window covers like-for-like swaps and upgrades. Meanwhile, wipe water off the frame and lining each morning, because damp that is left standing is what lets mould settle into plaster, paint and the timber underneath.

Sometimes the cause sits beyond the window. In a pitched roof with a cold loft, air has to move through the roof space to carry moisture away, and Marley's cold-roof ventilation advice for a high-resistance underlay is a 25 mm opening at the eaves up to a 15 degree pitch and 10 mm on anything steeper. Blocked eaves or insulation pushed hard against the underlay can send moist air toward the cold window instead.

Poorly packed insulation around the frame can leave a cold bridge that no ventilation will fix. If you are unsure what sits behind your lining, we open it up on the visit and take pictures of what is behind, so you see the cause before you decide what to spend. Our skylights and roof windows page describes how we approach a roof window, and a written quote follows the visit, with one total price.