Do flat rooflights cause condensation?

Warm air holds more water vapour than cold air. The air in a kitchen or a bathroom is warm and carries a lot of it, from cooking, kettles, showers, drying washing and the people breathing in the room. When that air drifts up and meets a surface that sits below its dew point, the vapour it can no longer hold turns back into liquid and settles on the cold surface as a film or as beads. That surface is very often a pane of glass, because glass is usually the coldest thing in the room.

The rooflight has added no water to the room. It has simply given the moisture a cold place to land. Swap that same volume of humid air into a room with no glass at all and the water still has to go somewhere: onto a cold wall, into the plaster, behind a wardrobe on an outside wall. A rooflight often gets blamed for condensation that the room was always going to produce, purely because the glass makes the result so visible.

Two figures decide whether it happens. The first is how humid the room is. The second is how cold the inner face of the glass gets, which is governed by the U-value of the unit and by how well the frame and the upstand are insulated. Lower the humidity or warm up the glass and the water stops forming.

Flat rooflights get a reputation for condensation for reasons that are about geometry and airflow rather than any fault in the glass.

Warm, moist air rises and pools at the highest point of a room. On a flat roof extension that highest point is the underside of the rooflight, so the most humid air in the whole room sits directly against the coldest surface for hours at a time. A window in a wall never collects air the same way. A rooflight in a pitched roof at least lets condensation run down the slope to the frame, where it can be caught and drained. A flat pane holds the water where it forms, so a thin film that a pitched window would shed becomes a visible pool of droplets that drips when it gets heavy enough.

Older glazing makes all of this worse. A single skin polycarbonate dome or a tired double glazed unit with a plain aluminium spacer runs cold around the edge, and the edge is exactly where you first see misting. If your flat rooflight streams every winter and it is more than fifteen or twenty years old, the unit itself is usually the thing holding you back, and replacing it is the cleanest way forward. That is genuine replacement work rather than a running battle with the wet glass, and it is the core of what we do on flat roof skylight installation.

You cannot change the weather or stop a family cooking, so the practical lever is the temperature of the inner glass. Keep that surface warm enough and the humid air never reaches its dew point against it. A modern unit does that in several ways at once:

• A low U-value. The U-value measures how fast heat escapes through the unit, and a lower number means a warmer inner pane. A flat rooflight built up to somewhere between about 1.6 and 0.8 W/m2K keeps the inside face far warmer than a single skin dome ever could, and warm glass is dry glass.
• A warm edge spacer. The spacer is the bar around the perimeter that holds the two panes apart. A plain aluminium one conducts cold straight to the inner glass and starts the misting at the border. A warm edge spacer breaks that path, so the edge no longer runs coldest.
• Argon fill and a low emissivity coating. The gas between the panes slows heat loss and the coating reflects warmth back inwards, both of which lift the inner surface temperature.
• An insulated upstand and a proper thermal break in the frame. The kerb the rooflight sits on and the frame itself can be a cold bridge if they are poorly built, and cold metal drips just as readily as cold glass.

None of this is exotic. It is the standard build for a new or replacement flat rooflight installed to current Building Regulations, where Part L already sets a ceiling on the U-value of the unit you fit. The gap opens up when an old unit is left in place, or when a cheap one is dropped in to save money on a rear extension, and then rediscovered every January.

There is one kind of condensation that catches people out. On a clear, still night a really well insulated rooflight can wake up with a film of dew on the outside, on top of the glass rather than underneath it. This looks alarming and is completely harmless.

It happens because the outer pane is so well isolated from the warm room below that it loses heat to the cold night sky and drops below the outdoor dew point, so dew forms on it exactly as it does on a parked car or a lawn. It is proof the glass is doing its job of holding your heat inside, not a fault. It clears as the morning warms up. The only condensation worth acting on is the sort that forms on the inside face or, worse, inside a sealed unit, which is a failed unit and a case for replacement.

Warming the glass is half the answer. The other half is giving the moisture a way out of the room so it never reaches the rooflight in the first place. An openable rooflight vents the most humid air from the highest point of the room, which is the ideal place to lose it, and pairing that with an extractor fan in a kitchen or bathroom deals with the source directly. Drying washing outdoors, using the extractor while cooking and leaving a trickle vent open all cut the humidity the glass has to cope with. If overheating in summer is also on your mind, the same openable unit helps there too, and the glass specification that controls solar gain sits alongside the thermal one on our energy-efficient skylight installation pages.

So work through it in order. If the glass runs wet inside every winter and the unit is old, plan a replacement to a modern warm edge, low U-value build rather than living with it. If the unit is already good, look at the room: add or use extraction, add an opening rooflight or vent, and bring the humidity down. If water is sitting between the panes, the sealed unit has failed and needs swapping out.

If you would like us to look at which of those applies to your roof, tell us the rough size of the rooflight and what the room below is used for. Ring 01702 898232 or request a quote, and we will specify a unit that stays clear rather than one you have to wipe down. Our skylight installation costs page shows where a replacement flat rooflight tends to land.