Condensation on roof windows in winter

Air in a warm room carries water vapour from breathing, cooking, showering and drying clothes. When that air touches a surface colder than its dew point, the vapour turns to droplets. A roof window is often the coldest surface in a loft room, because the glass is exposed to the sky and has less thermal mass behind it than a wall.

That is why bedrooms and bathrooms in lofts show it first, and why it peaks on still, cold mornings. It is also why a window that lets in a lot of light can mist while an opaque wall does not: it is simply colder on the inside.

Condensation on the room side is a symptom of how the room is used, not of the window failing. Modern windows with good glazing resist it better, but they do not eliminate it.

On the inside face of the glass is the commonest and least worrying. On the outside face, it appears on very cold, clear nights with high-performance glazing and is a sign the glass is doing its job, because little heat escapes through it to warm the pane. And between the two panes it signals a failed edge seal, letting damp air into the gap.

Mist between panes cannot be wiped off and does not clear. The sealed unit has to be replaced, and sometimes the whole window is the better answer; our page on broken roof window glass explains the options for panes.

A quick check helps: press a dry tissue to the area between the panes and to the inner face, and see where the dampness sits. Moisture that wipes away from the room side is ordinary condensation. Moisture that stays is inside the unit.

Staining on the timber or paint, or dampness in the lining below the window, points to condensation running down the glass and soaking the frame. That is worth acting on early.

Most inside-glass condensation responds to a few habits. Open the window slightly, using its trickle or vent setting, so moist air leaves the room. Run an extractor fan while showering and cooking and for a time afterwards. Keep doors to bathrooms shut so steam does not drift into cooler rooms, and dry laundry elsewhere.

Bathrooms deserve special attention, because a single shower can load the air with moisture within minutes. An extractor sized for the room and ducted outside, not into the loft, is far more effective than opening a window a crack. Moisture pushed into the loft space can return as condensation on the timbers and on the felt or boarding beneath the tiles or slates, which is a larger problem than wet glass.

Heat helps too. A room kept at a steady background temperature has warmer glass than one that is heated in short bursts and left to cool. A dehumidifier is a useful temporary measure in a new loft room whose plaster is still drying out.

Blinds can make it worse, as the closed fabric holds warm air against cold glass; see which blinds suit roof windows. Open the blind after waking, and wipe the glass.

Sometimes the cause is not the room but the way the window is built into the roof. If the insulation around the frame is thin, missing or gappy, the lining and frame become cold bridges where moisture collects. Dampness around the frame, black mould on the lining, or water marks on the ceiling below can follow.

The roof space also needs ventilation. Under BS 5250, cold roofs rely on eaves ventilation, and blocked eaves can raise humidity in the loft. A window sealed into that damp space may suffer too.

Fresh air also matters for the roof itself. A loft that is closed up tight and warm will hold moisture for longer, and that moisture can show first at the coldest points, and the window is often one of them.

We check these when we survey: the frame insulation, the lining, the underlay and the roof ventilation. Our page on the rooflight upstand covers the equivalent details for flat roofs.

A leak depends on rain: marks appear during or after a shower, often at the lower corners or the flashing, and can drip onto the same spot every time. Condensation depends on temperature and humidity: it is worst on cold, dry mornings and on days after heavy use of the room. It runs down the glass and collects on the lower rail.

Another clue is timing. Condensation tends to clear once the room warms and ventilates, while a true leak keeps showing for a day or more after the rain has stopped. Looking at the same spot at the same hour for a few days gives you a pattern worth reporting.

A simple test helps: dry the glass and frame, then note where moisture returns first and on which day. Still unsure? Take dated photographs and send them to us. Should the cause prove to be rainwater getting in, see roof window replacement for when a repair is no longer enough.

We start with the simplest explanation. If the room is humid and the window sound, we tell you so and suggest better ventilation. If the pane has failed, we price a replacement unit or window. If the installation is at fault, we repair or reset the lining and flashing.

After a roof survey you receive pictures of your own window and a written quote, with the work we carry out covered by a 10-year workmanship guarantee in writing. See our skylights and roof windows page.