Question answered
What U-value should a roof window have?
The short answer
A new or replacement roof window in an existing home should reach a U-value of 1.4 W/(m2.K) or lower, or Window Energy Rating Band B or better, under Table 4.2 of Approved Document L. A rooflight on a kerb has a separate figure, 2.2. Ask the supplier for the rating of the complete window, glass and frame together, and keep that paperwork.
Building Regulations set one energy limit for a roof window in an existing home and a different one for a glass unit raised on a kerb. Knowing which applies to your project is the first step.
What a U-value actually measures
A U-value tells you how quickly heat leaks through a building element, counted per square metre for every degree of difference between inside and outside. A smaller number is better. Glass, frame, spacer bars and seals all add to the total, so a figure worth trusting describes the finished window, not the pane on its own.
Roof windows are among the weaker spots in an insulated roof. They are glass set into a sloping surface that loses heat in winter and gathers sun in summer, which is why the regulations treat them as an element in their own right, separate from walls and the roof itself.
The Window Energy Rating is a different scale: a lettered band instead of a U-value. Building Regulations accept Band B as the minimum on an alternative route in their table, so a window can satisfy the standard by either measure. Both are printed on a good data sheet.
The figure in Table 4.2
Approved Document L Volume 1 (2021 edition incorporating 2023 amendments) lists limiting values for new or replacement parts of an existing home. The window row carries a note that it includes roof windows, and sets 1.4 W/(m2.K) or Band B at the least. Another note says the full 1.4 or Band B standard has applied since 15 June 2023.
Paragraph 4.8 sets a baseline too: a replacement should do no worse than the element coming out. In practice the table figure is the target, because a modern unit comfortably clears an old single-glazed one.
The same table lists the roof itself at 0.15 W/(m2.K). If the window is one piece of a bigger re-roof, the roof thresholds in that document may govern the whole job. Questions about approval go to your building control body, and our page on roof window security helps if you are comparing specifications side by side.
Roof window or rooflight: two limits
The two terms are not interchangeable. Approved Document L describes a roof window as flush with the slope around it, and a rooflight as a unit that rises from the roof, mounted on a raised frame. A unit set into a pitched tile slope is the first kind. A unit on a flat roof with a raised frame is the second.
The rooflight limit is 2.2 W/(m2.K), and 0.35 is the ceiling for a kerb that is built on site. Treat the two figures as separate: the tighter roof window number is not the one a kerbed rooflight is held to, and the looser rooflight number does not excuse a poor roof window.
That is worth knowing when you are choosing between the two types, because the better thermal performance of a roof window is one of the trade-offs. Our answer on rooflight or roof window compares them more fully, and the skylights and roof windows page gives the wider picture.
How to check the rating you are given
Start with the data sheet or the label on the frame. Confirm that the figure belongs to the whole product, that it matches the glazing specification you are buying, and that the model is the roof window version rather than a vertical window with a similar name.
Look for the energy rating band as well, and keep the paperwork. If a building control body or a buyer's solicitor asks later what was fitted, a document is better than a memory.
Glazing choices change the number: how many panes, what coatings and what gas fills the gap between them. Heavier glazing also changes how rain sounds on the glass, which our answer on roof window rain noise explains. Ask what you gain and lose before you simply chase the lowest figure.
If the supplier quotes a centre-pane figure, ask for the whole-window one. The centre of the glass always looks better than the unit with its frame included.
Old roofs, new glass
A window in an old slate or stone roof faces the same thermal limits as one in a new tile roof, so the glass specification does not relax because the building is old. Where the roof is in a sensitive setting, the question is more about appearance and consent, and our page on fitting a roof window in a slate roof deals with that.
At the survey we find out whether the opening is new or a replacement, read the specification of the product you plan to use and note its rating beside the quote paperwork. Photographs of your own slope go with the written quote, which gives one total price.
Where the existing window is single-glazed and draughty, the improvement from a modern unit can be noticeable in the room below, but the size of the gain depends on the old window, the loft insulation round it and how well the new frame is sealed.
Roof window ratings: two more questions
Is a lower U-value always better for a loft room?
A lower figure means less heat lost in winter, which is a clear gain. In summer, how hot the room gets depends more on the sun reaching the glass, the aspect and blinds than on the U-value, so look at both when you choose.
Where do I find the rating for a window that is already fitted?
Look for a label on the frame or sash, the installer's paperwork or the maker's data for the model. Failing that, the model name from the frame lets you find the sheet online.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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