Do flat rooflights cause condensation?
They do not create moisture. They create a cold surface, and a cold surface is where moisture already present in the air becomes visible. A flat rooflight is the coldest part of most extension ceilings on a January night, so if a room is producing more water vapour than it is losing, the glass is where you find out.
That distinction matters because it changes what to do about it. The answer is rarely the rooflight and usually a combination of ventilation, how the room is used, and the specification of the unit itself. The installation detail behind it is on flat roof rooflight installation.
Dew point, and the temperature the glass sits at
Air holds water vapour in proportion to its temperature. Warm air holds a lot, cold air holds little, and the temperature at which a given parcel of air becomes saturated is its dew point. Any surface below that temperature collects liquid water out of the air touching it.
A room at twenty degrees with a comfortable fifty per cent relative humidity has a dew point around nine degrees. The inner pane of a decent modern double glazed rooflight on a cold night sits above that, so nothing forms. Raise the humidity to seventy per cent, which a kitchen reaches quickly with a pan on the hob, and the dew point climbs to about fourteen degrees, which almost any glass surface will fall below.
Why a horizontal unit shows it first
Two reasons, and they compound on a flat roof specifically.
The first is stratification. Warm, moist air rises and collects in a layer at ceiling level, so the air in contact with a rooflight is the warmest and wettest air in the room. The second is the sky. A horizontal surface radiates heat directly to the night sky, which on a clear still night behaves as a very cold body, and the outer pane can fall below the outside air temperature. That is the same effect that puts frost on a car roof and not on its doors.
Where the water actually forms
Not usually in the middle of the pane. The perimeter is colder than the centre because the spacer bar at the edge of a sealed unit conducts heat around the cavity, and the frame is colder again.
So the first appearance is a band of misting at the bottom edge of the glass, a line of droplets along the frame, or a damp mark where the frame meets the plasterboard reveal. A unit misting at the edges on the coldest mornings of the year and clear by ten o’clock is behaving normally. A unit running with water down the reveal every morning through the winter is telling you something about the room or about the build-up.
The rooms that produce it
Water vapour has sources, and they are countable. Cooking, particularly boiling and steaming without an extractor running. Showers and baths. Drying laundry indoors, which is the largest single source in most houses and the one people forget. Ordinary breathing and perspiration, which is why bedrooms are worse than living rooms per square metre.
Kitchens and bathrooms therefore see it most, and those are exactly the rooms that get rooflights in an extension. A rooflight over a hob without a working extractor above that hob is a predictable outcome rather than a fault.
The frame, and where the thermal break sits
The glass is only half of the surface. Aluminium framed units conduct heat readily, and an aluminium frame without an adequate thermal break runs several degrees colder than the glass it holds, which makes it the first thing to wet.
A properly broken frame carries a polyamide or resin barrier separating the outer and inner profiles, so the inner face stays close to room temperature. This appears on datasheets as a whole unit U-value rather than a centre pane figure, and the two can differ substantially. The whole unit number is the one that predicts behaviour on a cold morning.
The upstand, and the cold line around the opening
The most common cause of persistent winter damp around a flat rooflight is not the unit at all. It is a timber kerb left uninsulated on its outer face.
That kerb is a solid path from the warm build-up to the cold air outside, so its inner face sits well below the surrounding ceiling temperature. Condensation forms on the plasterboard reveal, which absorbs it, and mould appears in the corners within a couple of winters. The detail that prevents it is insulating the kerb over its full height and sealing the vapour control layer to the frame, which is set out on what a rooflight upstand is.
Condensation inside the roof build-up
The less visible version. Warm moist air that gets into the roof construction condenses on the underside of the cold deck, and the first sign is often staining on the ceiling that looks like water from outside.
It matters most on cold decks, where insulation sits between the joists and a ventilated void runs above it. Cutting a rooflight opening into a cold deck interrupts the cross ventilation of that void, and a kerb can create a dead pocket where air stalls. Where a survey finds a cold deck, the ventilation path around the new opening is designed rather than assumed, and the vapour control layer on the warm side is made continuous around the kerb.
Ventilation, and the quickest thing to change
Moisture leaves a house the same way it arrives, in air. Extract at source is the most effective single measure: an extractor over the hob that vents outside rather than recirculating, and one in the bathroom that runs on past the end of the shower.
Beyond that, background ventilation keeps the whole house from sitting at a high humidity, and a rooflight that opens gives the highest and wettest air in the room a route out. Ten minutes of purge ventilation after cooking removes more moisture than an hour of trickle. Options for opening units are on electric rooflight installation.
Misting between the panes is a different matter
Everything above concerns water on the room side of the glass, which is physics and manageable. Misting inside the cavity is a component reaching the end of its life.
The perimeter seal of a sealed unit eventually admits moisture, the desiccant in the spacer bar saturates, and water then condenses between the panes where no ventilation or heating will ever reach it. It comes and goes with the sun and eventually leaves permanent deposit lines on the inner faces. Twenty to twenty five years is a typical working life. Sunspire replaces units that have got to that point, and it is a good moment to upgrade the glass, since the opening already exists.
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