The short answer: A modern rooflight, also called a skylight, should not make a room cold. An old single glazed or tired double glazed unit can, because the glass and the frame lose heat faster than the roof around them. A replacement that meets current Part L, fitted with an insulated kerb and sealed properly, behaves like the rest of the roof. If a room under a rooflight feels cold, the cause is usually the unit, the kerb or the air leaking round it, not the idea of having a rooflight.
This is one of the questions we hear most on the energy efficient rooflights hub, so it gets a full answer here.
Why an opening in the roof can feel cold
Warm air rises. In a room with a high ceiling, the warmest air collects at the top, right where a rooflight sits. Any weak spot in the glazing then pulls that heat out faster than a wall or an insulated ceiling would.
Glass is also a poorer insulator than a built-up roof. A pane has a thin layer of material between the room and the weather, while the roof around it has insulation, boards and a covering. Even the best glazing cannot match a fully insulated roof, but a good unit gets close enough that you stop noticing it.
There is a second effect that has nothing to do with the air temperature. A cold pane takes heat from your body by radiation. Sit under a cold rooflight and you feel chilled even if the thermostat says the room is warm. That is the sensation people describe when they say the room "feels cold".
The usual culprits
When we survey a cold room, we look for these in turn:
- Old glazing. Single glazing, or an early double glazed unit that has lost its gas fill, loses heat quickly.
- A failed seal. Once a sealed unit has misted, its insulating gap is no longer doing its job. See misted and failed rooflights for what that means.
- A bare kerb. On a flat roof, the upstand under the rooflight needs insulation. A glazed unit sitting on an uninsulated kerb leaves a cold ring round the opening.
- A plain metal spacer. The edge of a sealed unit is a weak point unless it has a warm edge spacer.
- Air leaks. Gaps between the frame and the roof let cold air in, whatever the glass is made of.
What the regulations expect of a new unit
Part L sets two ceilings for how much heat a replacement may lose. For a flat-roof unit or a lantern it is 2.2 W/m²K, and for a window in a pitched roof it is Window Energy Rating band B or 1.4 W/m²K. How that applies when a whole unit is replaced is covered in the Building Regulations guide.
Both values are set so that new units sit comfortably alongside a modern insulated roof. Units that meet them are a clear step up from the glazing fitted a generation ago. Our page on U-values explained shows what the figures mean in practice.
Things that make a real difference
If you want a rooflight that does not leave the room cold, the glass is only one part of the choice.
Glass specification
Low-emissivity coating reflects heat back into the room. A gas fill between the panes slows heat transfer. Whether a third pane is worth it depends on the room, which we cover in is triple glazing worth it for a rooflight.
Frame and kerb
A frame with a thermal break stops cold travelling through the metal. An insulated kerb, built to match the roof, keeps the opening from becoming a cold bridge.
Fit and seal
A tight fit counts for a great deal. A perfectly rated unit that is badly sealed will still draught. That is a workmanship point, and it is why our work carries a 10-year workmanship guarantee.
Cold rooms in tall Brighton and Hove houses
Many of the homes we survey have tall rooms, with a stair well or a landing rising to a rooflight at the top. In that shape the warm air climbs and meets the glass first. A cold pane at the top of a stair well can pull a draught down the stairs, so the whole hallway feels cooler than the thermostat suggests.
Flat roofs on terraces and extensions have their own pattern. The rooflight sits in a kerb on a thin roof build-up, and any gap in the insulation round the kerb shows as a cold line on the ceiling. We check for that when we look at the unit from inside and from the roof.
In converted buildings, the roof may belong to the freeholder, and a replacement may need the freeholder's consent. What to check before you commit is listed under flat and leasehold rooflights.
Whatever the building, the question is the same: is the cold coming through the glass, round the edge, or through the air gaps? The answer decides whether a new unit will cure it, and we would rather tell you that at the survey than sell you glass that cannot.
Cold in winter, hot in summer
The same glass that loses heat in January can gain too much of it in July. The fix for one is not the fix for the other, so it is worth thinking about both seasons together. Our page on rooflights and summer overheating looks at the other side, and solar control glass explains the coatings used.
What to do if your room feels cold
Look at the glass on a frosty morning. Heavy condensation on the room side, or moisture trapped between the panes, tells you something. Run a hand around the frame on a windy day and see whether you feel a draught. Note whether the cold ring is round the edge or across the whole pane.
Then book a free survey. We inspect the unit, the kerb and the roof build-up, and tell you plainly whether the answer is a replacement unit or something else. The quote is fixed and in writing, and it includes an access plan that shows how we will reach the roof, who arranges any scaffold licence, and how Building Control is handled where the work is notifiable.
For the wider picture on keeping heat in, return to the energy efficient rooflights hub.
