The short answer
Condensation forms when moist indoor air touches a colder surface, and a roof window is often the coldest surface in the room. The usual causes are too much moisture, too little ventilation, a ventilation bar kept shut, or old glazing that no longer insulates well. Misting on the room side is a humidity problem. Water between the panes, or droplets on the outside, points elsewhere.
Where the moisture on your roof window comes from, which kinds of misting are harmless and which need a roofer.
Why a roof window mists up
Air at room temperature holds water vapour from cooking, washing, breathing and drying clothes. When that air touches a surface colder than the dew point, the vapour turns to liquid. A roof window is a pane of glass in a sloping roof, so it is exposed to the sky, and on a still, cold night it is usually the coldest surface in a bedroom or landing. Misting is the visible result.
The position matters. Warm air rises, so the moisture collects at the top of the house, right where a roof window sits. A loft bedroom with the door shut, a landing above a kitchen and a bathroom under the roof slope are the classic places to see it. Mild misting that clears by mid-morning is common and is not a fault in the window.
Persistent water running down the glass, black mould on the frame or a damp reveal is another matter. Those say the room is holding too much moisture or the window is too cold, and they are worth acting on. The standard code on moisture in buildings is BS 5250, and ventilation is the practical answer for a damp room. Our roof windows and skylights page explains the wider work we do.
Using the ventilation bar properly
Many roof windows have a ventilation bar or flap in the top of the frame, and it is often closed. Opening it gives a small, constant flow of fresh air with no draught and no chance of rain coming in, so it is usually the least costly fix for misting. If your window has a bar, check the position of the control before you buy anything else.
Open the bar for the hours when the room is occupied, and especially overnight in a bedroom, when breathing adds moisture. If the room has a bathroom or kitchen extractor, run it during and after use, and keep internal doors shut so steam does not drift up the stairs to the roof window. Wipe the glass and frame with a cloth in the morning, as standing water can stain timber frames.
Do not seal the bar shut with tape. It does the job it is designed for, and sealing it only moves the moisture to the next cold surface. If the bar is blocked with dust or paint, a roofer can clear it.
Glazing age and failed seals
Older glazing insulates less, and a colder inner pane collects more condensation. A modern double-glazed unit keeps its inner surface warmer, so the same room may mist on an old window and stay clear on a new one. Part L sets maximum heat-loss values for new windows and rooflights in existing homes, which is one reason replacement glazing performs better than the pane it replaces.
Check where the moisture sits. Condensation on the room side of the glass is about the room. Condensation on the outside face is normal on a clear night and means the glass is insulating. Moisture trapped between the two panes, or a milky haze that never clears, means the sealed unit has failed, and the answer is a replacement unit, not a cleaner. See our answer on Velux replacement glass.
The checks are worth doing before spending money. Photograph the glass in the morning, wipe it, and see where the water comes back. A roofer can then tell you quickly whether you have a humidity problem, a failed unit, or a leak. If the frame is damp from the outside, our guide to a Velux window leaking helps you tell the difference.
Bathrooms, kitchens and loft rooms
A bathroom with a roof window is a high-moisture room with a cold surface, so misting there is expected. A shower fills the room with vapour within minutes, and the glass above it catches the lot. A working extractor fan, a ventilation bar kept open and a door that closes behind you cut most of it. Our answer on a roof window in a bathroom covers the choices for glass and fixings.
A kitchen or open-plan loft behaves in the same way. Cooking, a clothes airer and a sealed room all add moisture. Leave the window on its ventilation setting, run the cooker hood, and dry washing outdoors where you can. Keep furniture a little way from the reveal so air can move around the frame.
If the misting continues after those steps, or the plaster around the window is staining, ask for a survey. We check the window, flashing, reveal and insulation around the opening, send photos of what turns up, and quote a single written total for any work. Sometimes the answer is a new unit, sometimes a better extractor, and sometimes just a different habit.
More on this in Velux Roof Windows & Skylights in Crawley
Two more condensation questions
Does condensation mean my Velux window is leaking?
Not by itself. Condensation forms on the room side and clears as the room warms, while a leak shows as drips, staining or damp around the frame whatever the humidity. If the glass is dry but the plaster is wet, check the flashing and seals. A roofer can tell the two apart in a short visit.
Will a dehumidifier stop the misting?
It can help in a room that is always damp, because it lowers the moisture that reaches the glass. It treats the symptom, though, so fresh air still matters. Open the ventilation bar, use extractors and avoid drying laundry in the room, and then judge whether the window itself is the weak point.