Short answer
Yes, a skylight loses more heat per square metre than the insulated roof around it, because glass and a frame cannot match a thick layer of insulation. Whether a room feels colder depends on the unit's declared U-value, how the opening is insulated and sealed, and the sun that comes in through the glass. A current, well-fitted unit on an insulated kerb or a sealed lining keeps the loss small, and sunshine can offset some of it.
Why a rooflight loses more heat than the roof beside it
Heat moves from warm to cold through whatever is in its way. An insulated pitched or flat roof is thick, with a layer of mineral wool or rigid board that traps still air and slows that movement down. A rooflight swaps part of that thickness for a thin sandwich of glass, gas and a frame. Even the best unit is a thinner barrier than the build-up around it.
So the straight answer to whether do skylights lose heat is yes, area for area. That comparison is not the whole story, though. A rooflight is usually a small part of a roof, and its losses are only one part of the heat balance of a room. What matters is how large the loss is next to the rest of the room's envelope, and what the unit gives back.
How a modern unit holds on to warmth
Today's sealed units are built to slow heat flow at several points. Two or three panes make a still-air cavity, a low-emissivity coating on one surface reflects heat back into the room, and a gas fill in the cavity conducts less than air. A warm-edge spacer deals with the rim, and a thermally broken frame deals with the metal around the glass. Each is explained on its own page: see warm edge spacers, gas fills and thermally broken frames.
All of that is summed up in one number, the whole-unit U-value in W/m²K, where a lower figure means less rooflight heat loss. Part L of the Building Regulations sets a ceiling on that figure for new and replacement units in existing homes, with one limit for roof windows and another for flat rooflights. Meeting it is the minimum, and some triple-glazed units declare figures well under it. The caps themselves are listed in our rooflight U-values guide, alongside a plain reading of a data sheet.
The installation details that matter as much as the glass
A good unit in a poor opening still leaks heat. The weak points are not in the glass: they are the edges where the unit meets the roof. Over a flat roof, the unit is carried by a kerb, a raised upstand of timber or insulated board. If that kerb is bare timber, or is wrapped in membrane but not insulated, heat bypasses the glazing through the structure. An insulated kerb brings the upstand up to the standard of the roof around it.
Inside, the lining or light well between the ceiling and the glass has a similar job. If it is uninsulated and has gaps at the top, warm room air reaches the cold underside of the unit, and the lining surface can feel cold or collect condensation. A sealed, insulated lining holds the warm air in the room, with a continuous air barrier joining it to the ceiling finish.
On pitched roofs the same idea applies to the insulation collar or flashing kit supplied with a roof window: it is there to carry the roof insulation up to the frame without a gap. These are the details we note at survey and write into the job plan, so they are settled before the fitting day, not found on it.
- Insulated kerb or upstand on a flat roof
- Insulation collar or flashing kit fitted to the manufacturer's instructions on a pitched roof
- Lining or light well insulated and sealed to the ceiling finish
- Air barrier continuous round the opening
Solar gain: the heat that comes in the other way
Glass lets the sun's energy in. A south or west facing rooflight on a clear day can warm a room considerably, and in a cold month that is useful free heat. The measure of how much of the sun's energy a pane lets through is its g-value. A higher g-value passes more sun, a lower one passes less.
That cuts both ways. In summer, the same gain can overheat a room, which is why some units are specified with a solar control coating; our guide to solar control rooflight glass covers that choice. In winter, a low sun and short days mean gain is small and uneven, and at night there is none, so the loss through the unit carries on while the benefit stops. Heat loss is steady; solar gain comes and goes.
So a rooflight is not simply a cold spot or a heater. It is a window with both effects, and the specification lets you choose where the balance lies.
Will the room feel colder, then?
In a room with an old single-glazed or failed unit, probably yes: a cold downdraught from the glass is a common complaint, and misted double glazing that has lost its seal insulates poorly. Replacing it with a current unit usually makes a clear difference. In a new extension, a well-specified rooflight over a well-heated room should not feel colder, particularly where the kerb and lining are insulated.
Where a room does feel cold under a rooflight, the cause is often the opening around the glass or the lining, not the glass itself. Condensation at the frame is a related symptom, and our answer on rooflight condensation explains the causes.
What we put in the plan before you commit
The free survey looks at the roof construction, the kerb or flashing and the lining. From it we write a job plan naming the unit, its declared whole-unit figure and the insulation round the opening, and the quote follows in writing. Notifiable jobs go to Building Control through us, and ten years of workmanship guarantee stand behind the fitting.
Back on the hub, energy efficient rooflights lays out the choices from double to triple glazing. Ridgeway Skylights maps your opening on paper first, and nothing is ordered until you agree the plan.