Short answer: Do rooflights lose heat? Yes: a rooflight loses more per square metre than the insulated roof around it, so a poorly chosen one can make a room feel cooler on a winter night. A well-specified unit, set in a properly insulated kerb or frame, narrows that gap a good deal. Skylight heat loss is usually worst at the edges, not in the middle of the glass.
Why glass loses more heat than a roof
If you are asking whether a skylight makes a room colder, you are really asking how it compares with the roof it replaced. A pitched roof with insulation between and under the rafters is a thick, layered build-up. Glass is a thin sheet, however many panes you stack, and heat passes through it far more easily.
That is the honest starting point for any rooflight. Every unit is a hole in the insulation filled with something that insulates less well than the construction around it. The question is how big that difference is and what you do about it. Our energy efficient rooflights page covers the range of units we fit, and this page deals only with winter heat loss.
It also feels worse than it measures. Glass on a ceiling is cooler to the touch than plasterboard, and you notice radiant cold even when the air is warm.
The frame and the kerb let more heat out than the glass
Ask a surveyor where a rooflight leaks heat and the answer is rarely the centre of the pane. A modern sealed unit has a coating and a gas fill that keep the middle of the glass reasonably warm. The trouble is round the perimeter.
A flat-roof rooflight stands on a kerb, which is either a small timber upstand or a manufactured insulated box. If that kerb is bare timber with a thin skin of felt, heat runs straight through it, and the inside face of the upstand stays cold. Pitched roofs work differently, because the roof window is let into the slope instead of being raised on a kerb. The weak points there are the gap between frame and rafters, and how thick the insulation collar around the frame is.
Edges matter for a second reason. The spacer bar that holds the panes apart in the sealed unit can act as a conductor. An aluminium spacer carries cold round the edge of the glass, while a warm-edge spacer made of a composite or stainless material breaks that path. It is a small part that makes a visible difference to how the edge of the glass feels.
- Insulated kerb or frame, not bare timber and felt
- Warm-edge spacer in the sealed unit
- Low-emissivity coating and a gas fill between the panes
- Insulation packed round the frame with no gaps to the rafters
- Vapour control layer lapped to the frame on the room side
What a good specification changes
A rooflight is rated by its U-value, which measures how fast heat passes through it. A lower number means less heat lost. Under the 2021 edition of Part L (volume 1, Table 4.2), a new or replacement unit in an existing English home is limited to 1.4 W/m²K if it is a roof window and 2.2 W/m²K if it is a rooflight. Our guide to rooflight U-values and Part L explains why there are two figures and how a quoted number should be read.
What matters here is what you gain by choosing well. A unit made with a low-emissivity coating, gas fill and warm-edge spacer loses far less heat than a single-glazed or early double-glazed one. If your existing rooflight is decades old, the replacement will often make the room warmer than before, even though glass is still glass.
Triple glazing narrows the gap further, but it adds weight and cost and lets less light through. Whether it earns its place depends on the room, and the double or triple glazed rooflights guide goes through that choice properly. For most pitched roofs a good double-glazed unit with the right coating is a sensible place to start.
Size, position and how you use the room
Even the best glass loses more heat than an insulated roof, so the area you put in matters. One well-placed rooflight over a dark landing adds very little to the heating load. A wall of glazing over a kitchen extension is a different matter, and it changes how you heat the space.
North-facing rooflights lose heat without much compensating sun in winter. South-facing and west-facing ones can bring in useful low sun on a clear winter day, which offsets part of what the glass loses. Merseyside gets plenty of grey weather off the Irish Sea, so do not count on that gain, but it is worth knowing when you decide where to put the glass. July brings the opposite problem, and the guide to keeping a sunny room from overheating deals with it.
A cellular blind pulled across at night traps still air against the glass and softens the radiant chill.
Getting the detail right at the survey
Whether a rooflight ends up making your room colder comes down to details no catalogue shows. We survey the roof before any price is given, and for heat loss we look at four things:
- the build-up of the roof around the opening, so the insulation can be carried right up to the frame
- the height and construction of any kerb on a flat roof
- whether the existing opening is square and sound enough to take a new unit without gaps
- what the room is used for, since a bedroom and a home office have different needs
Our quote follows the survey as a fixed, written figure, and a written guarantee covers the workmanship for 10 years. Wherever the job is notifiable we notify Building Control, since a rooflight, new or replacement, is controlled work under the Building Regulations 2010. Send us a WhatsApp message or request a free quote to get a unit that meets the limits and keeps the room comfortable.
Winter heat loss, answered briefly
Will replacing my old skylight reduce my heating bill?
It can help. An old single-glazed or early double-glazed unit loses a lot of heat, and a modern sealed unit with a coating and warm-edge spacer loses much less. The saving depends on the size of the opening and how well the surrounding insulation is detailed, so we cannot promise a figure.
Can a rooflight feel cold even if it meets Part L?
Yes. Meeting the limit means the unit loses heat at an acceptable rate, but glass still feels cooler than a plastered ceiling. A good blind and a warm-edge spacer soften that feeling.