What a U-value actually measures
A U-value is the rate at which heat passes through one square metre of a building element when the air on one side is a degree warmer than the air on the other. It is written in watts per square metre per degree of temperature difference, or W/m²K. The lower the figure, the better the element holds heat in. A rooflight or skylight with a low U-value costs less to keep warm underneath than one with a high U-value, all else being equal.
Glass is the weak point in any insulated roof, so the figure matters more on a rooflight than on most things in your house. A solid insulated roof build-up lets very little heat through. A pane of glass lets far more through, which is why a large rooflight can be felt on a cold night as an area of cooler air above you.
The number you see on a data sheet is a figure for the whole unit in most cases: glass, spacer, frame and the way they join. That is a different thing from the figure for the glass alone, and the gap between the two is the source of most confusion when two quotes seem to disagree. We come back to that later in this guide.
A U-value is a laboratory or calculated figure for a unit as the maker built and tested it. It does not account for how well the rooflight is fitted into your roof, how it faces, or how you heat the room. It is a fair way to compare one unit against another, and a poor way to predict a heating bill.
A lower U-value means less heat escapes: the number rates the unit, not your roof.
The Part L limits for an existing home
Part L of the Building Regulations covers the conservation of fuel and power. The 2021 edition, in Table 4.2 for existing dwellings in England, sets the limits that apply when you fit new glazing to a home that is already built. For a rooflight the limit is 2.2 W/m²K. A roof window, held to the window standard instead, aims for 1.4 W/m²K or for Band B under the Window Energy Rating scheme, whichever its maker declares.
The two sit apart because the regulations treat them as different products. A roof window is set into the pitch of the roof, usually opens, and is treated in the regulations like any other window. A rooflight is the wider family: flat rooflights on flat roofs, roof lanterns, fixed glass in a pitched roof and similar rooflights that are not roof windows. The rules give those a more generous limit.
| Item | Rooflight | Roof window |
|---|---|---|
| Where it sits | Flat roofs, lanterns, fixed rooflights in a pitched roof | Pitched roof, in the plane of the slope |
| Limit that applies | 2.2 W/m²K | 1.4 W/m²K as a U-value, or WER Band B |
| Usual products | Flat glass, domes, lanterns, walk-on glass | Opening or fixed roof windows from the main makers |
| What we show on a quote | The maker's stated figure next to the limit | The maker's figure or rating next to the limit |
The limit is a ceiling you must stay under, so a unit that sits well inside it is allowed and a unit above it is not. A conservation-style unit is held to the limit for its type too, and the maker's data shows the figure it achieves.
When a quote does not state the figure for the unit it prices, ask for it. Every option on our written quotes carries the maker's figure alongside the Part L limit it has to meet, so you can see the gap without a calculator.
Centre-pane, whole-unit and the figure on the data sheet
Makers publish more than one figure, and the difference matters when you set two quotes next to each other. There are three you are likely to meet.
- Centre-pane figure: the glass alone, measured away from the edges. It is the most flattering number, because it leaves out the spacer, the frame and the join between them.
- Whole-unit figure: the glass, the spacer and the frame together, for a unit of a stated size. This is the one that matters for a Part L comparison.
- Window Energy Rating: a banded rating for roof windows that balances heat lost against useful solar heat gained. Band B is the Part L benchmark for a roof window.
If one quote gives a centre-pane figure and another gives a whole-unit figure, the first will usually look better and is not comparable. We put whole-unit figures on every written quote, and say when a maker only publishes the centre-pane value, so you are comparing like with like.
Size changes the whole-unit figure too. A small unit has proportionally more frame and edge for its area, and the frame and spacer lose more heat than the middle of the glass. That is why a maker may quote a slightly different figure for the same glazing build in a small size and a large one. Ask which size the figure refers to.
Check the size and the build
A figure is only meaningful with its unit size, its glass build-up and its frame. If a data sheet leaves any of them out, we ask the maker before it goes on your quote.
What moves the figure down
Four things change how much heat a rooflight lets through, and each one trades against something else. Seeing them side by side makes the choices easier.
What improves the U-value, and what it costs you
Extra pane
- Effect on heat loss
- Lower
- Effect on weight
- Heavier
- Effect on light
- Slightly less
- Effect on price
- Higher
Triple glazing is covered in our double or triple glazed comparison.
Low-emissivity coating
- Effect on heat loss
- Lower
- Effect on weight
- None
- Effect on light
- Slight tint possible
- Effect on price
- Small step up
Standard on most modern sealed units.
Gas-filled cavity
- Effect on heat loss
- Lower
- Effect on weight
- None
- Effect on light
- None
- Effect on price
- Small step up
Argon is the common fill.
Warm-edge spacer
- Effect on heat loss
- Lower at the edges
- Effect on weight
- None
- Effect on light
- None
- Effect on price
- Small step up
Matters most on small units.
Frame performance counts for just as much as the glass does. An aluminium frame with a thermal break, a polyurethane core or a well-insulated kerb will do better than a thin metal frame with no break. On a flat roof the upstand or kerb the rooflight sits on is part of the picture too, because an uninsulated kerb lets heat bypass the glass entirely.
Our hub page on energy efficient rooflights sets these options out across the whole range, and the comparison of how much heat skylights lose looks at what the figures mean in a lived-in room.
Comparing two quotes fairly
Two quotes for the same room can differ by a long way in price and in the figures they print. Before you decide the cheaper one is a bargain or the dearer one is a rip-off, lay out the same few lines for each. Most disagreements dissolve once the same measure is used on both.
| Item | What to find | Why it matters |
|---|---|---|
| Unit size | Outside and glass sizes | The figure depends on size |
| Figure type | Centre-pane, whole-unit or rating | Only whole-unit figures compare fairly |
| Glass build | Panes, coating, gas, spacer | Explains most of the gap between figures |
| Frame | Material and thermal break | Sets the edge losses |
| Limit it must meet | Rooflight or roof window | Decides which Part L number applies |
| Kerb or upstand | Insulated or not | Heat can bypass the glass |
A quote that prints a very low figure without saying what it refers to deserves a follow-up question, not an assumption. In the other direction, a figure that sits just under the limit is not a fault: the limit exists precisely so that anything under it is acceptable, and a unit that meets it may be the right choice for a room you do not heat much.
Wherever we price several options, the makers sit next to each other on exactly these lines, drawn from different manufacturers if your roof can take them, so the comparison is finished before you open the quote.
Does a lower figure always pay back?
Not automatically, and it depends on the room. A rooflight in a bathroom you use for short stretches is a different case from a large glass roof over a kitchen and dining space that is warm all day. Where the area of glass is large and the room is heated for long hours, every step down in the figure is worth more. Where the glass is small, the saving from a better unit is small too.
Orientation and comfort come into it as well. A rooflight that faces south gains useful sun in winter, which offsets some of the heat it loses, while a north-facing one does the opposite. Summer is the other side of the same coin: a unit that holds heat in well can also hold unwanted solar heat in, which is where a solar control coating comes in, covered in our guide to solar control glass for rooflights.
A saving figure depends on how you heat and use the room, so any number quoted without seeing your house would be a guess. What we do is show you the Part L limit, each option's figure, and the practical differences in weight, light and lead time, so you can decide how far down the scale suits your room and budget.
Replacing a rooflight and the rules that follow
If you are replacing an old rooflight, the new one has to meet the limit above. A whole-unit swap, frame and glass together, counts as fitting a controlled fitting and is notifiable work, so the Building Control notification forms part of the job we carry out. A new sealed unit fitted into a frame that is still sound is not notifiable, though the glass is still worth choosing with the figure in mind.
That distinction shapes a surprising number of decisions. An old aluminium frame with no thermal break may hold a modern sealed unit, but the frame will still lose heat at its edges, and the whole-unit figure of the combination may not match a fresh unit. How a glass-only job differs from a full replacement is set out on our misted and failed rooflight replacement page, and the quote shows both routes where that helps.
A brand new opening cut into the roof counts as building work, and the Building Regulations expect the glazing in it to meet the same figures. Whether the opening needs planning permission is a separate question that depends on the address; the conditions are in our rooflight planning guide, and your address gets checked by us while we are on site.
Other glazing already in the roof
If the existing rooflight is old polycarbonate, wired glass or a single sheet, the improvement from a modern sealed unit is usually the biggest step available on that roof.
What we do at the survey
The figures are straightforward once you know where to look, and we do that part for you. This is the order we follow when a customer asks about energy performance:
Identify the product type
Rooflight or roof window, which decides which Part L limit the unit has to meet.
Note the room and its use
How much of the day it is heated, which way it faces and how large the glass will be.
Collect whole-unit figures
From the makers, for the exact size, glass build and frame in question.
Set the options side by side
Thermal figures, weight, light, lead time and price on matching lines, with several makers set against each other whenever the roof has room for them.
Check the kerb and fit
Insulation around the opening, because a good unit on a poor upstand gives less than it should.
Our written quote then prints each option's declared figure beside the Part L limit, and adds the 10-year workmanship guarantee together with the warranty the manufacturer gives on the glass unit. To see the figures for your own roof, book a free survey and we will measure up and lay the options out on one page.
Common questions
What U-value does a new rooflight need to meet?
Part L 2021 requires a rooflight installed in an existing English home to achieve 2.2 W/m²K or better. For a roof window the benchmark is that of an ordinary window: 1.4 W/m²K as a U-value, or Band B on the energy label that rates windows. We show the relevant limit beside each option on the quote.
Is the centre-pane figure the one to trust?
It describes the glass alone, away from the edges, so it flatters the unit. The whole-unit figure includes the spacer and frame and is the fair one to compare, so we ask makers for it and print it on the quote.
Why do two rooflights with similar glass have different figures?
Frame material, thermal breaks, spacer type and the size of the unit all change the result. A small unit loses proportionally more at its edges than a large one with the same glazing.
Does a lower U-value mean the rooflight will not overheat?
No, heat kept in and sun let in are separate matters. A well-insulated unit can still warm a south-facing room in summer, so we look at the orientation and may suggest a solar control coating alongside.
Do I need to notify Building Control when I change a rooflight?
Changing the entire rooflight, frame and glass, is notifiable, and the notification is something we deal with. A frame that is still sound, with only its sealed glass unit changed, is not notifiable.