Can rooflights cause condensation?

Everyday living puts a surprising amount of water into the air: cooking, showers, drying clothes, even breathing overnight. Warm air carries more of it than cold air. When that air touches a cold surface, it cools, can no longer hold the water and leaves it behind as droplets.

The inner pane of a rooflight is exposed to the cold sky and sits at the top of the room, where warm moist air collects. That makes it a likely place for the first mist. The better the glazing insulates, the warmer that inner pane stays, and the less often it falls below the point where droplets form. A low U-value helps, as does a warm-edge spacer, because the edge of the unit is the coldest strip of the glass. Condensation that gathers in a thin line round the border of a pane is the classic sign of a cold edge.

The result is not harmful in small amounts. Droplets on the glass on a cold morning that clear by the afternoon are ordinary. It is the heavy, daily, running kind that needs attention.

Where the water appears tells you what is going on, and the fix differs for each.

On the inside face

This is the usual kind: moisture from the room, settling on the inner pane or the frame. It points to cold glass or humid air, or both. Better glazing, better ventilation, or both together will reduce it.

Between the panes

Fog or droplets inside a sealed unit means the edge seal has failed and humid air has got in. It cannot be wiped away, and it does not clear. The unit has lost its insulating gas along with its clarity and needs replacing. A new sealed glass unit fitted into a sound frame is not notifiable, whereas a whole new rooflight is. We cover this under misted and failed rooflight replacement.

On the outside

Mist or dew on the outer face on a clear, still night is a sign of good insulation, not a fault. The outer pane stays so cold because little heat is escaping through the glass, so the damp outdoor air condenses on it. It usually clears as the sun comes up. It is a little annoying and entirely harmless.

Sometimes the glass is fine and the condensation is on the lining, the plaster or the frame. That points to a thermal bridge, a path along which heat leaves faster than through the rest of the build. A gap in the insulation behind the lining, an uninsulated upstand or kerb, or insulation that stops short of the frame all make cold strips on the surface you see.

Moisture on the plaster is the sign. You may notice a faint damp line, a dark mark where dust sticks to damp, or flaking paint round the opening. Left alone, it can lead to mould. The cure is not a different pane. It is insulation, continuous from the roof build to the frame, and a lining formed so that it does not leave a void behind it.

This is where the finish inside is more than looks. A lining that is boarded, taped and plastered over properly fitted insulation stays warm to the touch. A box knocked together with gaps behind it does not. Our finish schedule records what sits behind the lining, so that part of the job is written down and not left to chance. The principle is covered in how a rooflight is finished inside.

• A bathroom, kitchen or utility room with little ventilation.
• Drying washing indoors without an open window or an extractor.
• Older single glazed or failed units, with a cold inner pane.
• A rooflight with an aluminium spacer or a poorly insulated frame.
• Blinds or curtains fitted close to the glass, trapping a cold pocket against it.
• A very airtight new build or extension, where moisture has nowhere to go unless the ventilation works.

The last point surprises people. Cranbrook new builds and modern extensions are tight, and the moisture that older, draughtier houses lost through gaps now stays in unless the extractor fans and trickle vents do their job. Approved Document F sets expectations for ventilation, and an opening rooflight can contribute to purge ventilation, which helps clear a steamy room quickly.

Start with the air. Run the extractor in a bathroom or kitchen during and after use, and keep trickle vents open. Open a window or the rooflight itself briefly in a steamy room. Keep the heating steady, since a room that swings between cold and hot gives the damp air repeated chances to condense.

Next, look at the glass. If the unit has failed, replace it. If the inner pane is a basic double unit and condensation is a daily nuisance, a unit with a low-e coating, a gas-filled cavity and a warm-edge spacer will keep its surface warmer. Triple glazing warms the inner pane further, which is one reason it appeals over a bathroom or kitchen island, as the guide to double or triple glazed rooflights explains.

A powered opening rooflight, such as the electric opening rooflights we fit, can clear a room quickly and can be closed by a rain sensor on models that have one.

Finally, check the surroundings. A new rooflight is a good moment to make sure the insulation around the opening is continuous, the lining is warm and the room has the ventilation it needs. That is part of the specification, not an extra.

New work contains moisture of its own. Wet plaster, new screed, paint and sealants all release water as they dry, and a freshly finished room can mist for the first weeks. Keep the room ventilated and the heating steady, and give a lime plaster finish the extra time it needs. If the condensation continues well after the room has dried, something in the build or the ventilation needs a look, and under our 10-year workmanship guarantee you can email us with photos so we can arrange a visit.

If you would like us to look at a rooflight that mists, or specify a new one that will not, ask for a quote. For the wider picture of glazing, frames and the build around them, see energy efficient rooflights.