Why Do Flat Rooflights Get Condensation?

Every room holds water vapour. Cooking, showering, drying washing and simply breathing all add to it. Warm air carries more vapour than cold air, so when it touches a cold surface it cools, can no longer hold the lot, and leaves droplets behind. A flat rooflight faces the open sky on a clear night, which cools it quickly, and it sits above the warmest air in the room. Both facts push in the same direction.

The wet you see on the inside face is therefore about the room, not the roof. That is worth separating from water arriving from outside, which would show as staining on the ceiling or round the kerb. Droplets that clear by late morning are the room breathing out. A stain that grows after rain points somewhere else.

Weather and season shift the picture. On a still, cold night the glass drops furthest below the room temperature, so the first hour after waking is when it looks wettest. On a mild, breezy day the same pane stays dry. That swing is useful to notice, since it shows the cause is the temperature gap and not a defect in the unit.

Several things decide how readily a pane mists, and most can be chosen at the quote stage.

Older single-glazed or poorly sealed units are the likeliest to run wet. Where an old unit has a milky look or moisture trapped between its panes, the sealed unit has failed and replacing it is the answer. The misted and failed rooflights page explains how to tell trapped moisture from the ordinary kind on the room side.

Glass specification is covered further in the flat rooflight glass guide. Part L 2021 sets the energy limit for a rooflight at 2.2 W/m²K, and meeting it means a better insulated unit than many older ones, which is a quiet help against misting.

You can reduce the moisture in the air as well as warming the glass. An opening rooflight lets damp air leave at the highest point of the room, where it gathers. Tilt the sash for a few minutes after a shower or while the kettle boils, and the room clears faster. A vent bar gives a thin gap for slow, overnight air without leaving the roof open.

A rain sensor, available on electric models, closes the sash when drops arrive, so you can leave it ajar while out. The fixed or opening comparison looks at which rooms benefit, and bathrooms and kitchens are the obvious ones. On a landing with little moisture, a fixed unit is usually fine.

• Run the extractor fan in kitchens and bathrooms, and leave it on a little after cooking.
• Keep furniture and curtains a short way from walls so air can circulate.
• Dry washing outside or in a ventilated space rather than a closed room.
• Wipe the glass if droplets build up, so they do not pool at the frame.

Heating plays a part. A room that is kept steadily warm holds its glass a little warmer than one that swings between cold and hot. None of this removes moisture entirely, and none of it needs to. The aim is that the glass clears on its own once the activity stops.

Bathrooms deserve their own thought. Steam from a shower is the heaviest load a household produces in one go, and a rooflight over the shower or bath will catch it first. An opening sash there, used for a short spell after each wash, does more good than any coating. Lofts and landings, by contrast, rarely generate moisture of their own, so any wet glass there often reflects air drifting up from the rooms below.

If moisture persists all day, appears between the panes, or comes with staining and drips after rain, the cause may be the unit or the way it was fitted. A kerb with no insulation, or a flashing that has failed, can send water where it should not go. A new, properly detailed unit is then worth considering, and the flat roof rooflight installation page shows how we approach it. At the free survey we look at the glass, the kerb and the room together, and the written quote shows the glazing and opening options side by side.

For the general causes and the steps that help in any room, see our guide on how to stop rooflight condensation. This page keeps to flat roofs, where the kerb, the fall and the extension below do most of the work.