Flat rooflight condensation and thermal bridging

Air holds water as vapour, and the warmer the air the more it can hold. Warm the air in a kitchen with a hob and a kettle, or a bathroom with a shower, and you load it with moisture. That air rises, because it is warm, and collects against the ceiling. If part of that ceiling is a sheet of glass at 4 degrees on a cold night, the air touching the glass cools, its capacity to hold vapour falls, and the surplus comes out as liquid on the pane. That is condensation. It is the same process as breath on a cold window, running at a slower pace across a much larger surface.

The temperature at which the surplus starts to appear is the dew point, and it depends on how warm and how humid the room air is. A flat rooflight makes the problem worse than a wall window for a plain geometric reason: it sits horizontal, or nearly so, at the very top of the room. Warm moist air stratifies, meaning it floats upwards and pools at the ceiling, so the most humid air in the house is pressed against the coldest overhead surface for the longest time. And when water does form, gravity carries it inwards and downwards to the lowest corner of the frame, where it gathers and drips rather than clinging to a vertical face and evaporating away by morning.

None of this is a fault in the roof covering. A roofer checking the felt or the single-ply membrane above will find it sound, because the water never came from outside. The fault, when there is one, is a cold surface inside that should have been warm, and that is a specification and installation question rather than a weatherproofing one.

A thermal bridge is a path through the building fabric where heat escapes faster than it does through the insulation either side of it. Anywhere the insulation is interrupted by a more conductive material, heat pours through that route, the inside surface at that point runs colder than its surroundings, and cold inside surfaces are exactly where condensation lands. On a flat rooflight there are three bridges that matter, and they sit in a ring around the glass.

The first is the spacer bar, the strip that holds the two panes of a sealed unit apart around their perimeter. For decades this was a hollow aluminium section, and aluminium conducts heat at around 160 watts per metre per kelvin. That metal ring connects the cold outer pane directly to the warm inner pane at the very edge of the glass, so the inner pane runs several degrees colder within 50mm of its border than it does in the middle. The centre of the glass can be perfectly warm while a cold frame of condensation forms right around the edge. A warm-edge spacer, made from thermoplastic or stainless steel with a conductivity closer to 0.2 to 17 watts per metre per kelvin, breaks that path and lifts the edge temperature by two to three degrees, which is often the whole difference between a dry pane and a wet one.

The second bridge is the frame itself. An uninsulated aluminium frame is a superb conductor straight from the cold outside air to the warm room. Powder-coated aluminium looks smart and lasts well, but without a polyamide thermal break buried inside the profile it will run cold on its inner face and stream with water on a frosty morning. A thermally broken frame, or a timber or composite frame, keeps the inner face warmer.

The third bridge, and the one most often ignored, is the upstand or kerb, the raised box that lifts the rooflight above the flat roof plane so water drains away from it. This is where most flat-rooflight condensation is actually generated, and it gets its own chapter below.

A flat rooflight cannot sit flush on a flat roof, because water would pond against it and work its way under the seal. So it is raised on an upstand, typically 150 to 300mm tall, and the rooflight frame sits on top of that kerb. The upstand is a short vertical wall poking up out of the insulated roof, and if it is built without insulation it becomes a continuous cold bridge running right around the opening, cooling the plasterboard reveal that lines the inside of it. That reveal then grows condensation and, in time, black mould, in a neat rectangle around the rooflight, and people blame the glass when the glass was never the problem.

There are two common ways to build the kerb, and they behave very differently:

• A site-built timber upstand. Fast to make from softwood, but bare timber and any un-insulated void inside it conduct heat out and run cold on the inside face. Unless it is wrapped in insulation to the same standard as the roof around it, and the insulation is carried up to meet the frame with no gap, it bridges.
• An insulated builder’s upstand. A prefabricated kerb, often GRP or timber with a bonded insulated core, designed so the insulation line of the roof continues up the sides of the kerb and meets the rooflight frame without a break. This is the detail that keeps the reveal warm, and it is what belongs under a flat rooflight in a heated room.

The junction matters as much as the parts. The roof insulation, the upstand insulation and the frame have to meet with the insulation continuous across every joint. Leave a 20mm gap where the loft insulation stops short of the kerb, and you have reopened the bridge no matter how good the components were. This is fitting work, done once, on installation day, and it is the reason a like-for-like replacement of an old rooflight is worth specifying properly rather than dropping a new unit onto a tired kerb.

Condensation is not a matter of opinion. It happens when a surface falls below the dew point of the air touching it, and both of those are measurable. The table below gives the dew point for room air held at 20 degrees at different humidities. Any surface colder than the figure in the right-hand column will grow condensation in those conditions.

Read it against real glass temperatures. On a night at 0 degrees outside and 20 inside, the inner pane of a single-glazed rooflight sits at roughly 6 to 7 degrees, so it drips at anything above about 45 per cent humidity, which is most of the time. A basic double-glazed unit with an aluminium spacer might hold its centre at 13 or 14 degrees but let the edge fall to 9 or 10, so the middle stays clear while the border weeps. A modern double-glazed unit with a warm-edge spacer and a whole-unit U-value around 1.2 keeps even the edge up near 15 degrees, which clears everything except a bathroom mid-shower, and that is a job for the extract fan rather than the glass.

*Indicative inner-pane edge temperature at 0°C outside, 20°C inside. Actual figures depend on the frame, the spacer and the kerb detail.

The pattern is the same one that runs through all rooflight glazing: the centre of the glass is rarely the problem, the edge and the frame are, and the whole-unit U-value, which includes that cold perimeter, is the honest number to ask for. A centre-pane figure flatters the unit by leaving out the very zone where the water forms.

Everything so far is surface condensation, water forming on a face you can see and wipe. There is a second kind that forms inside the construction, out of sight, and it is worth understanding because it changes how a flat roof around a rooflight is built.

Interstitial condensation happens when warm moist air passes into the roof build-up and reaches a cold layer somewhere within it, depositing water inside the insulation or on the underside of the deck. On a warm-deck flat roof, where the insulation sits above the structural deck and below the waterproofing, the detail relies on a vapour control layer on the warm side to stop household moisture getting into the build-up in the first place. Where a rooflight upstand punches through that roof, the vapour control layer has to be carried up the kerb and sealed to the rooflight frame, or moist air finds the gap and condenses inside the kerb where nobody will see it until the timber has been damp for a season.

This is why the kerb detail is not only about warmth but about airtightness and vapour control together. A well-made insulated upstand keeps the reveal warm and keeps household vapour out of the construction at the same time. It is a single detail doing two jobs, and it is decided on the day the rooflight goes in.

A rooflight can only condense the water that is in the air, so the amount of moisture you put into a room is the other lever, and it is the one you hold rather than the installer. A family of four generates a surprising quantity of vapour: cooking, showering, drying washing indoors, even breathing overnight. If that vapour has nowhere to go it drives the room humidity up towards the right-hand end of the dew point table, and then even good glass starts to weep.

Three things keep it down. Extract at source, meaning a working extractor fan in the kitchen and bathroom that vents outside rather than recirculating. Background ventilation, meaning trickle vents or a rooflight that opens, so the moist air has a route out. And a way to clear the warm humid layer that gathers at ceiling height, which is exactly where an opening rooflight earns its place. An opening unit at the top of the room dumps stratified warm moist air straight out through the ceiling, faster than any window at head height can, and on a high or awkward ceiling an electric opening rooflight makes that easy enough to actually do. Glass and kerb control the cold side of the equation. Ventilation controls the wet side. You need both working, and a roof full of good glazing will still stream if the extractor fan has been disconnected and the washing is drying under it.

The condensation question lands hard on the kind of homes across south Essex, because so much of the local work is exactly the situation that generates it. The 1930s semi-detached belt that runs through Southend, Leigh, Westcliff and out towards Rayleigh very often carries a flat or shallow-pitched rear extension over a knocked-through kitchen and dining room. That is a humid room, full of cooking moisture, with a flat rooflight or a run of them set into the roof over it. If the original extension went up decades ago with a bare timber kerb and a single-skin dome, condensation around that opening is close to guaranteed, and replacing the unit is a chance to put the insulated upstand and the warm-edge glazing in that were never there.

The coastal setting adds to it. Air coming off the Thames Estuary carries more moisture than air well inland, so the baseline humidity that a Southend rooflight has to cope with sits a little higher than it would in the middle of the county. Loft conversions in the Victorian and Edwardian terraces along the cliff, and the flat-roofed dormer cheeks that many of them use to gain headroom, add more flat glazing into humid bedroom and bathroom air. None of it is a reason to avoid rooflights. It is a reason to specify the kerb and the glass for the moisture, in the same way we specify the solar-control glass for the summer sun on the south-facing planes. Winter condensation and summer overheating are the two sides of getting a Southend rooflight right, and the same survey settles both.

You can head off almost all of this before anything is ordered, by asking the right things of whoever is quoting. When you are planning flat-roof skylight installation, these are the points that decide whether the finished rooflight runs dry through the winter.

1. Ask for the whole-unit U-value, not the centre-pane figure. The whole-unit number includes the cold edge where condensation actually forms. For a heated room you want it at 1.2 or better.
2. Ask whether the sealed unit has a warm-edge spacer. This single component lifts the glass edge temperature by two to three degrees and is the cheapest condensation insurance there is.
3. Ask whether the frame is thermally broken. An aluminium frame with no polyamide break will run cold on its inner face whatever the glass does.
4. Ask how the upstand is built and insulated. An insulated builder’s upstand, with the roof insulation carried up to meet the frame and no gap in the line, is what keeps the plasterboard reveal warm. This is the detail that most often decides the outcome.
5. Ask how the vapour control layer is sealed to the kerb. On a warm-deck roof this keeps household moisture out of the construction and prevents hidden condensation inside the upstand.
6. Sort the ventilation. Confirm the room has working extract at source and a way to clear ceiling-level humid air, whether trickle vents or an opening unit.

Get those six right and condensation stops being a worry, because there is no longer a surface cold enough for the moisture to reach and no route for it to travel into the fabric. If you are weighing up a flat rooflight and want the U-values, the spacer and the kerb detail set out for your own roof before you commit, ask us for a specification that names each of them, alongside the winter and summer glazing figures for the way your roof faces.