Why the edge matters more than the glass
A modern rooflight can have well-insulated glass and still leave a cold ring around it, because the weak point is usually the joint between the unit and the roof, not the pane.
When an opening is cut, the rafters or joists are trimmed and the insulation between them is cut back. The new skylight frame then sits in a hole with a gap around it: timber trimmers, a strip of bare roof deck, and often an air gap where the insulation has been pulled away. Heat finds the shortest route out, and it takes that ring. Building physics calls this a thermal bridge, or cold bridge. The glass may meet the Part L limit on its own, but the room still loses heat through the frame edge and the reveal around it.
You feel the result in winter. The plaster or plasterboard lining in the reveal is colder than the ceiling beside it, and on a frosty Cambridge morning you can feel the difference with the back of your hand. In a loft bedroom in a Romsey or Chesterton terrace, where the roof window sits close to the bed, that cold surface is also the first place moisture settles.
Pitched roof windows: collars and linings
On a pitched roof the job is to close the gap between the window frame and the rafter insulation, then line the reveal with something that is itself insulated.
Several roof window makers supply an insulation collar: a preformed strip that wraps the outside of the frame and fills the space between it and the rafters or battens. Some also supply an underfelt or membrane collar that seals the roof underlay to the frame so wind cannot wash through the insulation. Whether your chosen window has these as standard, as an option, or not at all is specific to the make and model, so we check it at the specification stage rather than assume it.
Inside, the reveal lining matters as much. A plain plasterboard lining over bare timber gives a cold reveal. An insulated plasterboard, or insulation fitted behind the lining and taped to a vapour control layer, keeps the reveal surface closer to room temperature. Some makers offer a prefabricated insulated lining panel for their windows. If you want to shape the reveal to spread the light, the splay has to be insulated too; our page on the skylight light shaft explains how that shaping works. The same thinking runs through our roof window installation work.
Flat rooflights and lanterns: the kerb and upstand
On a flat roof the rooflight or lantern sits on a raised kerb, and the kerb is the part that has to be insulated.
A kerb is typically a timber upstand built up from the deck, at least 150mm above the finished roof so water runs away from the frame. Built from solid timber alone, it becomes a cold wall around the opening, running straight from the heated room below to the outside air. The better detail lines the kerb with rigid insulation, carries the insulation of the roof up to meet it, and finishes the inside face with an insulated lining. Some manufacturers make a factory-insulated upstand to pair with their rooflight, which takes some of the variation out of site work. Which route suits depends on the unit and on whether the roof is a warm deck or a cold deck. Our flat roof rooflight installation page covers the kerb build in more detail.
Condensation, and where it shows up
Condensation forms on any surface colder than the dew point of the room air, and a poorly insulated reveal is often the coldest surface in the room after the glass itself.
The water runs down the lining, stains the paint at the bottom corner of the reveal, and over a few winters can lead to mould in that corner. It is easy to blame the skylight for this, but the frame and glass may be sound. The cause is the missing insulation and the air leakage around the junction. Ventilation matters as well: a bathroom or kitchen skylight that can open, or a trickle vent in the frame, gives damp air a way out. We also look at where the sun lands, because a reveal that never sees direct sun on a north slope stays colder for longer than one on a south slope. Our light planning page explains how orientation affects a room.
Misting between the panes is a different fault. That is a failed sealed unit, not a junction problem, and the answer is a new unit; see misted and failed rooflight replacement.
What Building Regulations expect
Part L of the Building Regulations sets a limiting U-value of 2.2 W/m²K for rooflights in existing homes, and since the 2021 update that figure is assessed with the rooflight in the horizontal plane.
The limit applies to new rooflights and to replacements, and it is a ceiling, not a target: many units perform better. Part L also expects the work to avoid unreasonable thermal bridging and air leakage, which is why the junction detail matters to Building Control and not only to comfort. Where the work is notifiable, we handle the Building Control notification and install to current Building Regulations. Our guide to Building Regulations for rooflights goes through Parts L, K, B, F and A, and energy efficient rooflights covers glass and frame choices.
Questions people also ask
Can I add insulation around an existing skylight?
Sometimes, from inside, by stripping the reveal lining and fitting insulated board. The outside junction is hard to reach without lifting the unit, so it is best done properly when the skylight is replaced. Our rooflight replacement process guide explains the stages.
Why is the wall around my rooflight cold?
Usually because the roof insulation stops short of the frame, or the reveal is lined with plain plasterboard over bare timber. Heat escapes through that ring, and the lining cools.
Is triple glazing enough to stop a cold reveal?
No. Better glass lowers heat loss through the pane, but it does nothing for a gap at the frame edge. The collar or kerb insulation and an insulated lining deal with the reveal.
Does an insulation collar come with every roof window?
It depends on the make and model. Some include it, some sell it as an option, and some rely on site-fitted insulation. We confirm which applies when we write the specification. To have it planned for your room, book a light survey, or read more about our skylight and rooflight installation in Cambridge.
