The honest answer: glass is not insulation
A roof is built to hold heat in. Thick insulation between the rafters, a vapour layer, plasterboard and a covering on top all slow the escape. A rooflight replaces a patch of that with glass and a frame, and glass lets heat through faster than any of those layers. So on a cold night, a square metre of rooflight loses more than a square metre of roof.
That is true of every window in the house, not just the ones in the ceiling. Heat leaves through a window by three routes. It conducts straight through the glass and frame. It leaks out as warm air through gaps. And it radiates away: your warm room sends heat towards the cold sky, and clear glass lets that happen.
The roof position adds one more factor. Warm air rises, so the air just under a skylight is the warmest in the room. The bigger the difference between inside and out, the faster heat moves, and that is at its largest at the top of a heated room on a clear winter night.
None of this is a reason to avoid rooflights. It is the reason to choose the glass carefully and size the opening sensibly.
How much is a lot
The measure for heat loss through glass and frame is the U-value, written in watts per square metre per kelvin (W/m²K). Lower is better. If the unit is unfamiliar, our guide to rooflight U-values and Part L explains it in plain English.
The Building Regulations set a ceiling for new work. For an existing home, Part L (2021 edition) sets the roof window limit at 1.4 W/m²K (or band B if the unit carries a Window Energy Rating) and the rooflight limit at 2.2 W/m²K. A new unit has to hit those, so your quote should show the figure. You can read the document itself on GOV.UK.
The two figures differ because a roof window and a rooflight are different products. A roof window is a manufactured window for a pitched roof, tested as a whole. A rooflight covers the other kinds, such as flat-roof glazing and lanterns, which are built up from parts. The ceiling is looser for the second group, and a good installer still aims to beat it.
An old unit is another matter. A single-glazed skylight, or a double-glazed one whose seal has failed, sits well above those figures. Swapping it for a current unit usually makes the biggest difference of anything in this article.
What decides how much a rooflight loses
Five things matter, and only the first is about glass type.
- The glazing build. Double glazing traps a layer of gas between two panes. A low-emissivity coating, a thin film on one pane, sends heat back into the room rather than letting it radiate away. Triple glazing adds a third pane. Triple or double glazing is its own question, answered separately.
- The spacer. The strip round the edge of the sealed unit holds the panes apart. A warm-edge spacer, made of material that conducts less heat than a metal one, cuts the loss round the perimeter, where it is worst.
- The frame. Frames conduct too. A frame with a thermal break, a barrier of low-conducting material between its inside and outside faces, keeps the cold from creeping in. Wide, chunky frames matter on small units, because the frame is a bigger share of the whole.
- The kerb and the junction. On a flat roof the unit stands on a raised kerb, and if that kerb is uninsulated, heat slips around the glazed unit through the structure instead. This is called a cold bridge, and it also makes condensation on the plaster below.
- The airtightness of the fit. A unit that is perfect on paper and drafty on the roof is a poor unit. Sealing, insulating foam and tapes all count.
You can fix the first three by choosing well. The last two are down to the person doing the work.
Size and position count as much as spec
Two small skylights lose less than one big one, if you only count the area of glass. But the room does not want the same amount of light everywhere, and a glazed area sized to the floor below works better than one sized to the budget.
Position matters in a quieter way. A south-facing unit collects the sun and gives some of its heat back to the room in winter. A north-facing one gets no help. A skylight over a cold stairwell loses heat into a space you may not be heating. A rooflight in a bedroom loses it into the space where you want the warmth.
Then there is summer. Large areas of glass admit heat as well as light, and the answer is a different glass coating, not more insulation. The solar control glass guide covers it. It is worth knowing that the glass that keeps the winter heat in and the glass that keeps the summer heat out are tuned differently, and a good quote says which way it leans.
If you cannot tell from a quote what the glass is, ask. The coating, the gas and the spacer should all be named.
Light is worth heat, up to a point
A rooflight pays back in daylight. A dark room with a skylight that brings in a lot of light can run without the lamps on for much of the day, and people simply use the room more. A skylight also tends to deliver more light into a room than a wall window of equal area, since it looks at open sky and not at the building opposite.
So the question is not whether a rooflight loses heat, because it does. The question is whether it loses more than the benefit it earns, and whether you chose the spec that keeps the loss modest. For most rooms, a current double-glazed unit with a good coating and a warm edge, fitted on an insulated kerb or flashing, is a sensible balance. Where a room is very cold, very exposed or kept at a high temperature, such as a sunroom heated all winter, a higher specification can earn its place.
Treat any number you are quoted for annual savings with care unless it comes with its workings. Heat loss depends on the house, the heating system and how you live in it.
How we help you keep it in
The free survey covers the roof, the room underneath and any old unit. The fixed written quote that follows names the glazing build, the frame and how the kerb is treated. Glazing is picked to meet the Part L figure for that type of unit, we notify Building Control where the job requires it, and ten years of workmanship guarantee sit behind the fit.
Everything we fit for warm rooms is gathered under energy efficient rooflights. For figures on your own roof, ask for a quote, or send photos by WhatsApp and we will tell you what we would check first.