The short answer
Low enough to meet the limit in Approved Document L for the type of rooflight you are fitting, and ideally better. A U-value measures how much heat passes through the glazing: the lower the number, the better it holds heat in. The legal ceiling comes from the 2021 edition of Approved Document L, Volume 1, and it differs between a roof window, a rooflight and a roof lantern, so the rooflight U-value requirement for your job is the figure for that type, checked against the product data sheet, not a general rule.
What you should aim for is a sensible step beyond the limit, because the limit is a minimum. For most homes, that points to good double glazing at least, and triple glazing where the room is used all year.
What the limit is, and where to find it
Both a newly cut rooflight and a replacement of frame and glass are building work. The new unit has to meet the thermal standards in Approved Document L, Volume 1, which covers dwellings. The document sets a maximum U-value for each kind of glazed element, and some products are assessed by their window energy rating instead.
We do not quote a single figure here, because it changes with the type of element, the kind of work and the edition of the document, and a figure on a web page can go out of date. The current numbers are in the approved document on gov.uk, and your installer should state which one applies to your unit. We handle the Building Control notification, and the U-value is part of what is checked.
A glass unit renewed on its own, frame retained, is a different case. It is not notifiable, but a better unit is still a sound choice, and it should be chosen with the existing frame in mind.
Typical figures by glazing type
As a rough guide to what the market offers, higher numbers are poorer and lower numbers are better.
| Glazing | Typical range | Suits |
|---|---|---|
| Older single or early double | Well above 2.0 | Nothing new: replace |
| Modern double, low-e, argon | About 1.2 to 1.6 | Most rooms in most homes |
| Triple glazed | About 0.8 to 1.1 | Bedrooms, kitchens, rooms used daily |
These are indicative ranges for the whole unit and they vary with size, frame and spacer. A single figure on a data sheet may describe the glass alone, the centre of the pane, or the whole window with its frame, and the three can differ by a good margin. Ask which one you are being given.
What pulls a U-value up or down
Several parts of the unit decide the number, not only the number of panes.
- A low-emissivity coating on one or more panes reflects heat back into the room.
- An argon or similar gas fill between the panes slows heat flow compared with air.
- The spacer between the panes: a warm-edge spacer cuts heat loss at the perimeter.
- The frame: timber, aluminium with a thermal break and uPVC conduct heat differently.
- Size and shape: a small unit has proportionally more edge, which tends to raise the whole-window figure.
The frame and spacer matter more than people expect. Our answer on warm edge spacers explains the edge effect, and the guide to double or triple glazed rooflights compares the two options in detail.
Picking a figure for your room
The right target depends on how the room is used, which way the roof faces and how the surrounding construction performs. A rooflight on a well-insulated loft room is worth matching to its surroundings, because a poor window sits in a good roof like a gap in a coat. A utility room or a seldom-used landing can be served by a good double unit.
Also think about comfort, not only bills. A lower U-value means the inner pane stays warmer, so you feel less cold air falling off the glass on a winter night and are less likely to see condensation on it. A rooflight sits over your head, so that falling cold air is easy to notice. Our answer on whether rooflights cause condensation goes into it.
Reading a U-value on a quote
Three traps catch people comparing figures. First, the figure may be for the glass at the centre of the pane, which flatters the unit, while the figure that matters for Part L is for the whole window including its frame. Second, a quote may state a number for one size and fit another. Third, a rooflight may be described as "A-rated" or "energy efficient" with no number at all, which is a label, not a measurement.
A sound quote names the product, the glazing build-up (for example the number of panes, the coating, the gas fill and the spacer), the whole-unit U-value in W/m²K and the standard it is measured to. If any of those is missing, ask for it in writing. A written specification is also easier to put in front of Building Control if the notification needs the evidence.
Replacing an old failed unit is a good moment to improve the figure. A misted double glazed rooflight from some time ago may well have a poorer U-value than a modern one, and a new unit of the same size can be a clear step up in warmth.
What to do next
When you compare quotes, ask for the U-value of the whole unit and what it is measured on. Check that the product meets the current limit in Approved Document L and that the figure appears in the written quote. For the wider background on replacements, see Part L and replacement rooflights and our energy efficient rooflights page.
Every quote we write comes with a finish schedule, and the glazing specification sits beside the lining and finish. To have a U-value chosen for your room, ask for a quote.