Thermal bridging at a rooflight is a path through the building fabric where heat escapes faster than through the surrounding roof, almost always because the insulation stops or thins at the opening. It runs right around the perimeter of the unit. The cold line it creates drops the internal surface temperature at the reveal, warm humid air condenses on that cold surface, and mould follows within a season or two. The unit itself can be excellent and the problem still appears, because the fault is in the junction, not the glass. The fix is a continuously insulated upstand and a thermally broken frame, so there is no cold path from outside to in.
This is the single most common failure we survey on rooflights that are not actually leaking. People assume water is getting in. It is not. The surface is condensing, and no amount of sealant touches it.
What a thermal bridge is, and why the perimeter is the weak point
Heat takes the easiest path out of a building. Where insulation is continuous, that path is slow. Where the insulation stops, thins, or is replaced by a more conductive material, heat crowds through the gap. That concentrated path is a thermal bridge, and it does two things: it loses heat, and it chills the internal surface next to it.
A rooflight opening creates a perimeter, and a perimeter is where materials meet and insulation is most likely to be interrupted. On a 1.5 x 1.0m unit that is five metres of junction running around the opening. Get the detail wrong and you have five metres of cold line in the ceiling.
The bare timber upstand problem
The upstand, sometimes called the kerb, is the raised box the rooflight sits on. The commonest failure we find is an upstand built as a plain timber box with the roof insulation stopping at its outer face. Timber conducts heat roughly four to six times faster than rigid insulation. So a bare 100mm timber upstand is a continuous conductive fin, warm on the inside, cold on the outside, running around the whole opening.
The result is predictable. The internal surface at the reveal runs cold. Warm humid air from a kitchen or bathroom meets it. Water condenses. Mould grows, because mould does not need liquid water, it needs a surface sitting above roughly 80 per cent relative humidity for long enough. The homeowner reasonably assumes the rooflight leaks. It is condensing, and sealant does nothing.
Cold frames and point bridges
The frame is the other bridge. An aluminium frame with no thermal break conducts heat straight from the cold outer face to the warm inner face and runs cold enough to condense and drip. See what a thermally broken rooflight is for the detail. On top of that, individual fixings that pass from the cold outer structure to the warm inner structure are point bridges. One is minor. Twenty around a perimeter show up as a row of cold dots on a thermal image and a row of faint marks on the ceiling.
The pencil test. A correctly detailed junction lets you draw a line through insulation, from the roof, up and over the upstand, without lifting the pencil off the insulation. If the pencil has to cross bare timber, a cold frame or an air gap, that crossing point is your thermal bridge, and it is where the mould will appear.
How condensation and mould follow the cold line
Air holds a certain amount of moisture depending on its temperature. Cool it and at some point it reaches its dew point and drops that moisture as water. The dew point of comfortable room air in a Suffolk kitchen might be around 12 to 14 degrees when it is busy. If the reveal surface is colder than that, water forms on it. If the surface merely runs damp, sitting above 80 per cent surface humidity for hours a day, you do not even see water but mould still takes hold.
Kitchens and bathrooms are the worst cases because they generate moisture. A kitchen with a recirculating charcoal extractor is adding humidity to the air with nowhere to send it, and a cold reveal gives that moisture a cold surface to land on. So thermal bridging and room ventilation work together. Fix the bridge and improve the ventilation and the mould has nowhere to start. We go into the moisture side on why condensation forms on rooflights.
The details that eliminate the bridge
| Junction | What fails | What we build instead |
|---|---|---|
| Upstand body | Bare timber box, roof insulation stops at its face | Rigid insulation to the outer face, lapped continuously into the roof insulation |
| Frame | Aluminium with no thermal break | Thermally broken frame, polyamide barrier through the profile |
| Fixings | Screws passing cold-to-warm through the section | Fixings kept out of the cold-to-warm path, no straight-through bridges |
| Reveal | Square, cold at the corner | Splayed where ceiling depth allows, insulation carried into the reveal |
| Vapour control | Cut and left open at the opening | Reinstated, lapped and taped, continuous with the roof layer |
Two of those deserve a note. A preformed insulated upstand, PIR or GRP-faced, takes the timber out of the thermal path almost entirely and turns the whole detail into one component. It costs more than a site-built timber upstand and on flat roof work it is frequently worth it. And a splayed reveal does two jobs at once: it spreads daylight into the room, and it raises the surface temperature at the corner where condensation starts, because a splay presents a warmer, more open surface than a square return.
The insulation must be continuous. Not mostly continuous. A 50mm gap in the insulation around a five-metre perimeter is a five-metre-long cold line with a break in it, and the mould will find the break. This is the stage that separates a rooflight that stays clean for its whole life from one that streaks within three winters.
Why this matters more than the unit you choose
You can spend the extra on triple glazing and still get a mouldy reveal if the junction is bare timber. The heat lost through a bridged upstand can undo a large part of what a better unit gained, and the mould it causes costs plasterboard, decoration and eventually structural timber. If a budget is finite, the junction comes before the third pane. That is not a slogan, it is the order in which the heat leaves the building. We rank the whole set of measures on how you reduce heat loss through rooflights, and the hidden version of this failure, damage inside the roof build-up, is on interstitial condensation in the roof build-up.
My rooflight is not leaking but the reveal has gone black. What is it?
That is condensation caused by thermal bridging, not a leak. A bare timber upstand with the roof insulation stopping at its outer face runs cold all the way round the opening. Warm humid room air meets that cold surface, condenses, and mould grows within a season or two. Sealant will not touch it. The fix is rebuilding the upstand with insulation continuous with the roof insulation, reinstating the vapour control layer, and improving how the room is ventilated.
Can a thermal bridge be fixed without replacing the rooflight?
Sometimes the reveal can be improved from inside, but the bridge is usually the upstand itself, which is under the roof covering. Getting insulation continuous around the opening generally means lifting the covering at the perimeter, which is most of the work of a replacement. In practice, if the unit is also old or cold it makes sense to do both together. If the unit is genuinely sound, we tell you, and we scope just the junction.
Get the junction surveyed, not just the glass
We survey the upstand, the frame and the reveal as well as the unit, tell you honestly whether you have a leak or a cold bridge, and price the detail that fixes it. One fixed quote, the whole-unit U-value written on it, a 10-year workmanship guarantee, and installers with 25 years on rooflights.
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