You wake up, look at the roof window above the bed, and the glass is streaming with water. A few mornings later there is a dark line of mould along the lining. The first thought is a leak, and sometimes it is. More often the window is doing exactly what cold glass always does when it meets warm, damp air: it is collecting the moisture. Roof windows are good at showing it because they are cold, tilted and close to where people sleep, and the air at ceiling height is the warmest and dampest in the house. This guide sorts condensation into its separate causes, explains how to tell which one you have, and sets out what we do about each. It applies to centre-pivot, top-hung and conservation-style windows alike.
What condensation is, and why a roof window gets it
Air holds water vapour, and warm air holds more than cold. Cooking, showers, drying clothes indoors and breathing all add vapour. When that air touches a surface cooler than its dew point, the vapour turns into droplets. A roof window is a patch of glass and frame set into an insulated roof, and even a good modern unit is colder on its inner face than the plastered wall beside it. So the first place the vapour condenses is the glass, then the frame, then the cold corners of the lining where insulation is thin.
Roof windows face a second disadvantage: they sit at the top of the room. Warm air rises, and moist air from the bathroom or kitchen drifts up the stairs into the loft room, where it is trapped. This is why a loft bedroom can mist up while the rooms below stay dry.
Cause one: damp air with nowhere to go
This is the commonest cause, and it is about the house, not the window. If the glass is wet on the room side in the morning, clears by afternoon and returns on cold days, the air in the room is carrying too much moisture. Winter makes it worse, because the glass is colder and windows are kept shut.
Things that help are practical and mostly free:
- Run extractor fans in the bathroom and kitchen during use and for a while afterwards, and check they are venting outdoors and not into the loft.
- Keep the door of a bathroom closed while showering so steam does not travel up the stairs.
- Dry washing outdoors or in a ventilated space, not on radiators in the bedroom.
- Open the roof window a short way for a few minutes each morning. Most designs have a ventilation flap or a vent position that opens the sash a fraction without letting rain in.
- Heat the room steadily. Cold rooms that are warmed in bursts mist up more than rooms held at a modest temperature.
If the condensation vanishes when you ventilate more, the window is fine. A roof window cannot fix a house that makes more moisture than it removes.
Cause two: a failed double-glazed unit
A sealed unit is two panes of glass separated by a spacer bar, with an inert gas or air between them and a sealant around the edge. When the seal fails, moist air gets into the cavity. The test is where the moisture sits. If it forms on the inside face of the room-side pane, or the outside face of the outer pane, it is surface condensation. If you see a misty film or droplets trapped between the panes, which you cannot wipe off from either side, the seal has failed. The cavity then fogs on cold days and may clear on warm ones, and over time the glass shows a permanent white haze or mineral deposits.
A failed unit also loses most of its insulating value, so the inner pane runs colder and surface condensation gets worse too. The fix is to replace the glass unit, not the whole window, if the frame is sound. That is covered in our page on replacing roof window glass. If the window is old, a new roof window may be better value, and replacing an old roof window compares the choices.
Cause three: cold bridges at the frame and lining
If the glass is clear but the frame, the sill or the plasterboard around the opening runs wet, the problem is thermal bridging. A cold bridge is a path through the roof where heat escapes faster than through the rest, so the surface on the warm side stays cold and collects water. In a roof window the usual causes are a gap in the insulation around the frame, an insulation collar that was never fitted or was trimmed to fit, and a lining that touches cold timber.
Manufacturers supply insulating collars and lining kits designed to wrap the frame and link it to the roof insulation. When a window is fitted without them, or when the surrounding rafters are insulated poorly, the reveal is cold. The telltale sign is black mould along the angled lining at the top or sides, which has little to do with the glass. The fix involves access: the lining is opened up, the gap is filled with insulation or a proper collar is fitted, and the reveal is re-lined. This is much easier when a window is being replaced, and the work is covered in our guide to roof window flashing kits, which explains how frame, flashing and insulation are meant to go together.
Cause four: moisture from the roof itself
Sometimes the water is not from the room at all. A leak at the flashing, a gap in the seal between frame and flashing, or a tile joint that lets wind-blown rain through can run along the underlay, down the frame and appear as droplets at the head of the window. A leak usually shows after rain and drips from a point, while condensation shows on cold dry mornings and covers a whole surface. If you are unsure, our guide to leaking roof windows explains how to separate the two.
A roof with trapped moisture can also feed condensation. A cold loft with poor ventilation, damp insulation or a spray foam layer that has sealed the roof void can raise humidity around the window. Spray foam deserves a special mention. The RICS consumer guide of March 2023 notes that it can hold damp against rafters and leave the roof hard to inspect. If your roof has foam and your window runs wet, get it looked at, because the problem may be much bigger than the glass.
How to tell which cause you have
A few simple checks help before anyone climbs a ladder. Wipe the wet glass on the room side. If the water wipes off, it is surface condensation, so ventilation or thermal bridging is the likely cause. If the mist is trapped inside the glass, the sealed unit has failed. Look at the position of the wettest area: the lower edge of the glass points to cold air falling and pooling, a patch near the frame points to a cold bridge, and the very top corners point to a collar or lining gap. Note the time of day and the weather. Wet glass on dry cold mornings is condensation, and wet glass during or after rain is more likely a leak.
Next, check the room. A bedroom with the door closed all night, a drying rack, no trickle vents or extractor, and two sleepers produces a lot of moisture. Then look at the window age. A window from the 1990s may have a tired unit, and an older roof window may lack the thermal performance of current ones. Current rules for replacement roof windows in existing homes, under Part L, call for a U-value of 1.4 W/m²K or Window Energy Rating Band B, which a modern double-glazed unit should meet. An old unit may be far behind. Our page on roof windows and Building Regulations gives the details.
What we do about it
The fix depends on the diagnosis, so the first step is looking. We check the glass, the seals, the frame, the lining and the roof around the window from outside and inside, and we photograph what we find. If the cause is household moisture, we tell you that and suggest ventilation, which costs nothing from us. If the sealed unit has failed, we offer a new glass unit or a new window with a fixed quote. If there is a cold bridge, we open the lining, fit insulation or a collar and re-line. If the flashing or tiles around the window are letting water in, we repair them, and where the window is old and the flashing worn we recommend renewing both together.
New roof windows with good seals, integrated vents and a correct collar are far less prone to misting, and trickle vents or a ventilation flap give a controlled route for stale air. If you are planning a new window, roof window types compares opening styles, and the reasons for a bathroom or kitchen to have a window that opens easily are covered there. If you are in the first stages of a loft conversion, bear in mind that new bedrooms need proper ventilation as part of the design.
What not to do
A few cures make things worse. Sealing up the vent flap to stop draughts removes the window's built-in ventilation. Taping bubble wrap on the glass looks like insulation but traps moisture, hides the problem and can crack the glass through heat build-up in sun. Sealant smeared over the outside of the frame can block a drainage path that the window needs, and drilling holes in a sealed unit to let the mist out will ruin it. Painting over mould with ordinary paint seals the spores in. Clean mould off with a suitable fungicidal wash, find the cause and then redecorate.
For more on how we approach roof windows in Basingstoke, see our roof windows page, or phone 01256 213956 and we will come out, look at the window from both sides and send you pictures of what we find.
