Roof window condensation appears when warm, damp room air meets a surface colder than its dew point. That surface is usually the inner pane, the frame, or the lining around the opening. The glass is rarely the only fault. More often the air is too wet, the frame is a cold bridge, or the sealed unit has failed and is showing moisture between the panes.
Three different kinds of wet
Before changing anything, work out where the water sits, because each place has a different cause.
On the room side of the glass. This is ordinary skylight condensation. Moist air from cooking, washing, drying clothes and breathing hits a cold pane overnight. It clears once the room warms and the air moves.
Between the two panes. A misty film you cannot wipe off means moist air has got past the perimeter seal of the glass unit. No amount of airing the room will clear it. That unit needs replacing, and we cover it under misted and failed rooflight units.
On the frame, lining or plaster around the window. Dark spots, flaking paint or a damp patch on the reveal point to a cold bridge. The frame is colder than the ceiling beside it, so water forms there first, and mould follows.
Why a loft is a hard place for it
A sloping ceiling gives air little room to circulate. Warm air collects at the top of a bedroom or a landing, right where the roof window sits. The glass also faces the open sky, so on a clear night it loses heat quickly and drops well below room temperature. A window in a wall loses less to the sky, which is why a roof window mists first.
In an older loft with thin insulation around the opening, the reveal around the window can run cold too. That makes the lining the second place to look.
What the window itself can do
A newer unit helps. The warm-edge spacer between the panes, a low-emissivity coating and a gas fill all keep the inner pane closer to room temperature, so it takes longer to reach the point where water forms. A replacement that reaches the Approved Document L target of 1.4 W/m²K (or Band B for its Window Energy Rating) will mist less than an old single-glazed or early double-glazed one. Our page on rooflight U-values and Part L explains the figures.
Triple glazing warms the inner pane further, though it adds weight and thickness to the frame. We weigh it fairly in double or triple glazed rooflights.
| Where you see it | Likely cause | Next step |
|---|---|---|
| Inner pane, clears by midday | Damp room air meeting cold glass | Ventilate, and keep the room warm |
| Between the panes | Failed edge seal | Replace the sealed unit |
| Frame or reveal | Cold bridge or missing collar | Check insulation at the opening |
The collar and the vapour barrier
A roof window sits in a hole cut through the insulation. If the insulating collar around the frame is missing, squashed or cut short, the timber frame touches cold air on its outer face. If the vapour control layer in the ceiling is not sealed to the frame, damp room air also slips into the gap and meets that cold timber inside the build-up.
At survey we look at the lining, feel the reveal, and where a window has been marked by damp we plan to open the surround and refit the collar and the membrane when the new window goes in. Building this properly once is cheaper than treating the stain each winter.
Things you can do today
Run the extractor in the bathroom and kitchen, and keep the door shut while it works so steam does not drift up the stairs. Dry washing outdoors or in a room with an open window. Open the roof window for a few minutes in the morning, even in winter, so the damp air leaves. A trickle vent or a sash held slightly open lets air move without losing much heat.
Keep blinds clear of the glass at night where you can. A blind that seals against the frame traps cold air behind it, and the pane behind the blind runs coldest. One that sits a little proud of the glass lets room air move across it.
When the cause is the window
If the same pane mists every cold morning, the frame is damp at the corners, or the unit is cloudy inside, the window is telling you it needs more than ventilation. We replace the sealed unit alone where the frame and flashing are sound, and the whole window where they are not. The life plan in your quote names which route applies to the product we propose. The 10-year guarantee covers our fitting, including the new collar and its seal to the frame, and we look at that seal again at year 5.
A window swap is controlled work. We tell Building Control for you, using the council or a private approver, and you finish the job holding the record of that notice. Two neighbouring pages cover insulation from a new window and summer heat in a loft.
Our patch runs thirty road miles out from Sheffield, taking in Rotherham to the north east and Chesterfield to the south. Message us on WhatsApp with a photo of the window and we will book the survey, which costs nothing, and follow it with a quote in writing at a fixed price.