The short answer
A roof window gets condensation when warm, humid air from inside the room touches a surface colder than the air can tolerate, so the water vapour turns to droplets. Cooking, bathing, drying washing and too little ventilation are the usual drivers. The fix is to reduce moisture, ventilate, and keep the glass and frame warmer. It is a comfort and maintenance issue, not a leak.
What condensation actually is
Air holds water vapour, and warm air holds more than cold air. When humid air touches a cold surface, it cools there and cannot keep all its vapour, so the excess comes out as water. That is condensation. A roof window is a likely place because it is a patch of glass and frame in the coldest part of a room, and warm air rises to meet it.
It forms mostly on the inside face, and it is worst on cold mornings, in bathrooms, in kitchens and in loft bedrooms where people breathe overnight. If it clears as the day warms, it is condensation. If you see a drip from the top of the frame during rain, that is a different problem, covered in our page on a leaking roof window.
Misting between the panes is a separate fault. It happens inside a sealed double-glazed unit when the edge seal has failed, and it cannot be wiped off.
Where the damp air comes from
A household produces a surprising amount of moisture every day. Cooking, showers and baths, kettles, drying clothes indoors, and even breathing all add to it. In a modern home with tight windows and little air exchange, that moisture has nowhere to go.
Loft conversions are prone to the effect. The bedroom sits under the roof, with a sloping ceiling and often a roof window above the bed. The room is warm, the people in it add moisture, and the glass above is the coldest surface. Bathrooms and en-suites tucked into roof spaces, with a roof window as the only opening, are the same.
Unvented gas heaters and paraffin heaters add to the load. Furniture pushed hard against walls, and wardrobes on cold external walls, stop air from circulating and leave pockets where moisture settles.
Why a roof window is worse than a wall window
Glass is a poor insulator compared with a wall, and a roof window sits high in the room, where warm air gathers. Cold air also drops straight off the glass, which keeps the surface temperature low.
The frame matters as much as the glass. The edges of a double-glazed unit, particularly where the sealed unit is held in a frame, are colder than the middle. Condensation tends to start at those edges and run down to the bottom corners. A modern unit with a warm edge spacer resists it better than an old one with a metal spacer.
Condensation matters because water that sits on timber lets rot start. It darkens the lining, lifts paint and encourages black mould on the plaster beside the frame. Over years it can rot the bottom corners of a timber frame, which is why it deserves attention rather than a daily wipe. The life of the frame is covered in how long roof windows last.
What actually fixes it
The cure has three parts. Reduce the moisture at its source: use an extractor fan in the bathroom and above the hob, close the door while showering, and keep washing out of the bedroom. Add ventilation: open the window a little each day, use the trickle vent at the top of the sash if it has one, and make sure the room can breathe. Warm the surfaces: heat the room gently and consistently rather than in bursts, because cold surfaces collect moisture first.
Wiping the sill and lining each morning stops water soaking in. A dehumidifier in a problem room helps for a season while you find the underlying cause.
The window itself may need work. A failed glazing unit, damaged seals or poor insulation around the frame in the lining can leave cold bridges. Upgrading to a current unit with better glazing makes a real difference. If you are replacing the window, Approved Document L expects a roof window to meet 1.4 W/m²K or Band B for Window Energy Rating, and a better glazed unit will resist condensation more effectively.
When it is more than condensation
Condensation can mask a real problem. If the stains are on the outside edge of the lining, if the rafters in the loft are damp, or if the dampness gets worse after rain, there may be a leak as well. A survey tells the two apart. We look at the glazing, the seals, the lining and the insulation, and the flashing outside.
The remedy sometimes involves a change to the window itself, such as a different glazing specification. In a conservation area or on a listed home, any change to the outside look of a roof window needs a check first. Our page on a rooflight in a conservation area sets out how that works in Bath, and roof window planning covers the permission side generally.
Do not fit a vent or drill the frame yourself to "let the air out". Holes cut into the glazing bars or the sash weaken the weather seal and may void the maker's guarantee. If ventilation is the missing piece, ask a roofer or the maker about a trickle vent that the window was designed to take.
Where the answer is ventilation and habit, we will say so, and our roof windows and skylights hub has more on each topic. Where the window needs work, we price it as a fixed quote in writing after the survey.
