01The number
Start with the legal ceiling, then aim lower
There is a limit you must not exceed, and there is a figure worth wanting. They are not the same number.
A U-value tells you how fast heat escapes through a material. It is measured in watts for every square metre, for every degree of difference between inside and outside (W/m²K). A lower number means less heat gets out, so lower is warmer.
If you are replacing a rooflight in an existing dwelling, the Part L limiting value is 2.2 W/m²K. A new unit that is worse than that does not meet the Building Regulations. Think of it as the floor of acceptable, not a target to be proud of.
Good modern units do far better. A well made double glazed rooflight with low-e coating, argon and a warm-edge spacer sits well below the limit, and triple glazed versions go lower again. So when someone asks what U-value a rooflight should have, the plain answer is: at least meet 2.2, and choose one that beats it comfortably.
- 2.2W/m²K, the most a replacement rooflight may allow in an existing home under Part L
- 1.4W/m²K or better, a sensible aim for a modern double glazed unit
- 1.0W/m²K or lower, where good triple glazed units can reach
02Reading a quote
Which U-value are you being shown?
Two rooflights with the same glass can carry different figures, because the label may describe only part of the unit.
This is where people get caught out. A brochure may quote a centre-pane value, which describes just the middle of the glass. That is the best-looking number, because it ignores the edge and the frame. The edge of the glass is where the spacer bar sits, and the frame is often the weakest link of the lot.
The figure that tells you how the finished unit behaves is the whole-unit value, usually written Uw on a roof window or rooflight datasheet. It includes the glass, the spacer, the frame and the way they join. A glass-only 1.0 can turn into a whole-unit 1.6 once a poor frame is added. Always ask which one you are looking at before you compare two quotes.
| Figure quoted | What it covers | Use it to |
|---|---|---|
| Centre-pane | Middle of the glass only | Compare glass types |
| Whole-unit | Glass, spacer and frame together | Compare finished rooflights |
| Part L limit | Maximum allowed for a replacement unit | Check you are legal |
03What moves it
What pulls the number down
Four parts of a rooflight set its U-value, and the glass is only one of them.
Glass comes first. A low-e coating (a thin layer that returns warmth to the room) and a gas fill such as argon cut the heat escaping through the panes. Adding a third pane gives a further drop, which we weigh up in is triple glazing worth it on a rooflight.
The spacer bar around the glass edge comes next. A warm-edge spacer is made of plastic or composite instead of aluminium, so the rim of the glass is warmer. Our glazing always includes warm-edge spacers. Frames matter too: timber, PVC or a thermally broken aluminium frame (one with an insulating strip inside it) all beat a plain metal section.
Last is how the rooflight is joined to the roof. On a flat roof the kerb, the raised collar the unit sits on, should be insulated and tied into the roof insulation. A great unit on a cold kerb loses the benefit. Our guide to warm-edge spacers and thermal breaks goes deeper, and what is a thermally broken frame explains the frame side.
What we typically specify for warmth
- Glass
- Low-e coated, argon filled, double or triple
- Spacer
- Warm-edge as standard
- Frame
- Timber, PVC or thermally broken aluminium
- Kerb
- Insulated and tied into the roof insulation
04Choosing
How low is low enough for your room
You do not need the lowest figure on the market for every roof. Match the unit to how the room is used.
For a landing or a bathroom that is warm for short spells, a good double glazed unit that clears the limit comfortably is plenty. For a kitchen-diner, a family room or a loft bedroom where you sit under the glass for hours, a lower whole-unit figure pays you back in comfort, because the glass above you stays closer to room temperature.
Size matters as well. A roof is insulated far better than any glass, so a bigger rooflight lets out more heat than a small one, whatever the figure. The right answer is the lowest U-value you can sensibly afford on a unit sized for the room. We look at how glass in a roof compares with glass in a wall in do rooflights lose more heat than windows, and a fuller guide sits at rooflight U-values explained.
If the whole rooflight, frame and glass comes out, that is a controlled fitting under Part L. Building Control is notified and the new unit must meet the limit. We handle the notification, and our page on whether rooflights need Building Regulations explains the rules. For the wider picture, see energy efficient rooflights.
The limit tells you what is allowed. The whole-unit figure tells you what you will feel.
U-value worries before you buy
Does a lower U-value always mean a warmer room?
It means less heat loss through that unit, which helps. But the kerb, the lining and the size of the opening also decide how the room feels. A low figure on a poorly joined unit will not deliver what it promises.
Is the 2.2 limit the same for a skylight in a new extension?
No. New extensions and new dwellings are assessed differently, and the figures can be tighter. That 2.2 W/m²K figure applies to replacing a rooflight in a home that already exists. Ask Building Control or us about your project, and see the Building Regulations approved documents on GOV.UK.
Where do I find the U-value of a rooflight?
On the manufacturer's data sheet or in your written quote. Ask for the whole-unit figure. If only a centre-pane value is given, ask for the other one.
