Cold bridging at a flat rooflight is what happens when the kerb, the timber box the unit sits on, runs uninsulated from the warm room straight to the cold outside face of the roof. That unbroken line of relatively conductive timber and plasterboard sits several degrees below room temperature in winter, warm moist air touches it, reaches dew point and condenses. The homeowner sees water on the reveal and black mould in the corners and reasonably assumes a leak. It is not a leak. It is a construction fault, and no amount of sealant will touch it. The fix is to carry insulation up the outer face of the kerb continuous with the roof insulation, so the cold line never reaches the inside face at all.
This page explains the physics, shows how to tell cold-bridge condensation from an actual leak, and sets out the insulated build-up we use as standard rather than as an upgrade. The full kerb specification is on rooflight kerb construction, and the wider job on flat roof skylight installation.
What a cold bridge actually is
Heat moves through a building from warm to cold, and it takes the easiest path. A well-built flat roof puts a thick layer of insulation between the warm room and the cold sky, so the easy path is blocked and the inside surfaces stay close to room temperature. A kerb built without insulation punches a hole straight through that layer. It is a continuous ring of timber, sometimes 150mm or 200mm deep, connecting the warm inside to the cold outside around the entire perimeter of the rooflight, with a strip of plasterboard fixed to its inner face.
In January that inner face runs cold. How cold depends on the timber, the outside temperature and how much warm air is washing over it, but on a bare kerb it commonly sits five to eight degrees below the room air. That is more than enough to matter, because of what air does at those temperatures.
Dew point, in plain terms
Warm air holds moisture. The warmer it is, the more it holds. Kitchen and bathroom air is warm and wet, full of moisture from cooking, kettles, showers and drying washing. When that air touches a surface cold enough, it can no longer hold all its moisture and the surplus condenses out as liquid water on the surface. The temperature at which that happens is the dew point.
A room at 20 degrees and 60 per cent humidity has a dew point around 12 degrees. Any surface below 12 degrees in that room will run with water. A bare kerb inner face at 10 or 11 degrees on a cold night is below that dew point, so it wets. Raise the humidity, by cooking or drying clothes under the rooflight, and the dew point rises with it, so the kerb wets sooner and more heavily. This is why the problem is worst in kitchens and worst in the mornings after a night of cooking and closed windows.
Why it looks exactly like a leak
Water is water. On a ceiling, condensation from a cold kerb and ingress from a failed covering both show as dampness on the plaster reveal, and both can stain over time. The two have completely different causes and completely different fixes, so telling them apart is the whole game. They read differently once you know what to look for.
| Condensation (cold bridge) | Leak (covering failure) | |
|---|---|---|
| When it appears | Cold weather, worst on winter mornings | During and after rain, especially wind-driven |
| Where it appears | Evenly on all four reveals | One corner or one edge |
| What triggers it | Cooking, showering, drying washing, closed windows | A particular wind direction |
| Mild wet weather | Often stops completely | Continues or worsens |
| The water | Clear, beads on the surface | Brown tide marks, tracked through plaster |
| Company it keeps | Black mould in the corners | Blistered paint, spreading stains |
If it appears in cold weather, forms evenly and stops when the weather turns mild, it is condensation, and drainage or sealing work will change nothing. The leak side of this is on leaks at the rooflight kerb.
The black mould in the reveal corners is a giveaway that is worth reading. Mould needs sustained surface moisture and a temperature it likes, both of which a cold kerb provides all winter. A leak that only wets during storms rarely stays damp long enough between events to grow mould that evenly. Uniform mould in all four corners is a cold-bridge signature.
The insulated fix, and why it is not optional
The cure is construction, not sealant. You break the cold line by carrying insulation up the outer face of the kerb so it is continuous with the roof insulation, and by using a frame that has a thermal break of its own. Do both and the inside face never gets cold enough to reach dew point.
Insulating the kerb face
The outer face of the kerb is clad in rigid insulation, usually PIR, of a thickness that brings the insulation line level and continuous with the roof insulation. If the roof carries 100mm of PIR, the kerb face wants insulation that meets that layer and runs up to the kerb head without a gap. A 20mm shortfall at the top of the kerb is a 20mm cold ring around the whole rooflight, and warm air finds it in the first cold snap. The joint where kerb insulation meets roof insulation is taped, because an open joint lets warm moist air move into the construction and condense inside it, out of sight, where it rots timber rather than just staining plaster.
The frame and the thermal break
The rooflight frame is the other half of the perimeter. A thermally broken aluminium frame has a polyamide strip separating the cold outer profile from the warm inner one, so the inner frame stays warm. A frame without that break will condense along its inner edge no matter how well the kerb around it is insulated. If your quote does not say the frame is thermally broken, the frame is a cold bridge waiting to happen.
Splayed reveals
The internal reveals, the plaster returns from the ceiling up into the opening, do more than finish the job. Cut square, they form a cold, still shaft where warm room air cannot circulate, so they run cold and damp. Cut back at 45 degrees, they let warm air wash across the plaster instead of stagnating, which keeps them above dew point, and they throw noticeably more daylight into the room for the same opening. We splay every reveal as standard. It costs a little more in plasterboard and skim and it earns that back in a room that stays dry and bright.
Before you pay anyone to chase a leak. If the water only ever appears in cold weather and never during a summer storm, it is condensation, and resealing, re-flashing or re-bedding will change nothing. It is a cold bridge at the kerb, which is exactly why we insulate every kerb we build rather than offering it as an extra.
Managing the humidity as well
Construction stops the cold surface. It helps to attack the moisture side too, because a very wet room will find any surface that runs even slightly cool. Practical measures, in rough order of usefulness:
- Ventilate at the source. An extract fan in a kitchen or bathroom that vents outside removes moisture before it reaches the rooflight. An opening rooflight vent at ceiling level, where the warm wet air collects, is the most effective ventilation you can add to a room with a flat ceiling. That option is covered on opening and hinged flat rooflights.
- Do not dry washing under the rooflight. A single load of wet laundry puts litres of water into the air, and it rises straight to the coldest surface, which on a poorly built job is the kerb.
- Warm-edge spacers and good glazing. The spacer bar around the edge of a sealed glass unit can itself be a cold line. A warm-edge spacer runs warmer than the old aluminium type and reduces condensation at the very edge of the glass.
Where thermal performance is the main driver for the project, our energy efficient skylight installation page goes into U-values, psi-values and how the whole detail works together.
When it really is the unit, not the kerb
Occasionally the condensation is inside the glass rather than on the reveal, and that is a different fault. A sealed double or triple glazed unit that has lost its seal fogs between the panes, the moisture is trapped in the cavity and no ventilation or insulation will clear it. That unit has failed and needs replacing. So does a triple-skin polycarbonate dome whose flutes have filled with condensation that cannot be cleaned out. Neither is a repair. Both conclude with a replacement unit, covered on yellowed and brittle dome rooflights. The distinction is simple: condensation on the reveal is a kerb problem, condensation inside the glass is a unit problem.
Common questions about cold bridging at flat rooflights
Is the water on my reveal a leak or condensation?
If it appears in cold weather, worsens after cooking or showering, forms evenly on all four reveals and stops in mild weather regardless of rain, it is condensation from a cold bridge at the kerb. A leak follows rain, shows at one corner or edge, and leaves brown tide marks rather than clear water. Condensation is a construction fault, not something sealant can fix.
Will resealing the rooflight stop the condensation?
No. Sealant addresses water coming in from outside. Condensation forms on the inside from warm moist room air meeting a cold surface. The only fix is to stop the surface running cold, by insulating the kerb face continuous with the roof and using a thermally broken frame. We do not offer resealing on any rooflight, and on condensation it would achieve nothing anyway.
Why is there black mould in the corners?
Mould needs sustained surface moisture. A cold, uninsulated kerb runs damp all winter, which is exactly the condition mould grows in, and the corners are the coldest and stillest part of the reveal. Even mould in all four corners is a reliable sign of cold bridging rather than a leak, which tends to wet one area only and dry out between storms.
Can you fix cold bridging without taking the rooflight out?
Usually not properly. The insulation has to go on the outer face of the kerb, continuous with the roof, which means getting to the covering and often lifting the unit. Splaying the reveals and improving ventilation from inside helps at the margin, but a genuinely uninsulated kerb needs the fix built in at the kerb, which we do as part of a replacement or a rebuild.
Get a cold, damp rooflight looked at
Send us photographs of the reveal and the mould, tell us when the water appears and whether it stops in mild weather, and roughly where the property is. We will survey it, confirm whether it is a cold bridge or ingress, and give you one written fixed price for putting it right. Every installation carries a 10-year workmanship guarantee, and every kerb we build is insulated as standard.
Request a fixed quote for a cold or damp flat rooflight in Ipswich, Suffolk or north Essex. If the water appears in storms rather than cold snaps, start with a flat rooflight leaking after a storm instead.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.