Open any rooflight data sheet and one figure leads the page: the U-value. It is quoted in surveys, written into building control paperwork and used by salespeople as if it settled the matter. It is a useful number, but only if you know what was measured and what was left out.
A U-value is a single figure for how fast heat leaks through a window or rooflight, and a lower figure means less leaks. Below you will find what the unit means, the limits for altering an existing home in England, and how to compare two products without being misled by a headline number.
What the number measures
A U-value describes how quickly heat travels through a building element. The unit is watts per square metre per kelvin, written W/m²K. Read it as the heat that escapes through one square metre when the air on one side is one degree warmer than the air on the other.
The lower the figure, the slower the heat leaves. A rooflight with a U-value of 1.2 holds warmth better than one at 1.8. Nothing about the figure rewards size or thickness for its own sake; it is simply a rate of loss, measured under set conditions in a laboratory or worked out by calculation.
Centre-pane, window and installed figures
Three different numbers get called "the U-value", and they are not interchangeable. The centre-pane figure describes the middle of the glass only. The whole-unit figure, sometimes called Uw, includes the frame and the spacer round the glass edge. The installed figure would include the junction with the roof, though that is rarely published.
Frames and edges are where glass loses most ground, so the centre-pane number is always the flattering one. When a leaflet shows a very low figure in large type, check the small print for what it covers. For a rooflight on a house, you want the figure for the whole unit.
| Figure | What it covers | How much weight to give it |
|---|---|---|
| Centre-pane | The middle of the glass only | Useful for comparing coatings, not whole products |
| Whole unit (Uw) | Glass, spacer and frame together | The figure to compare and the one regulations refer to |
| Installed | The unit plus its junction with the roof | Rarely published, so the quality of the fit has to fill the gap |
The limits for an existing home
Where you alter an existing home in England, Part L of the Building Regulations (the 2021 edition) sets the thermal standard for new glazing. Two figures matter here. A roof window must achieve 1.4 W/m²K, or reach Band B on the Window Energy Rating scale. A rooflight, meaning the glazed unit on a flat roof or low pitch, must achieve 2.2 W/m²K.
A roof lantern follows the rooflight limit. The difference between the two figures is not a loophole. Rooflights are bigger, they are built differently and they sit on a raised kerb, so the table allows for that. The practical result is that a modern rooflight with sensible glazing will comfortably meet its limit, and many go well past it.
Which limit applies to you
- Glazing set into a sloping roof, openable on a hinge or pivot: roof window, 1.4 W/m²K or Band B
- Glazing on a flat roof or low pitch, sitting on an upstand: rooflight, 2.2 W/m²K
- A lantern with a ridge and glazed sides: rooflight limit
- Only the sealed glass unit being swapped: not notifiable, though a better unit still helps
What brings the number down
Four things in the glazing build pull a U-value lower. A low-emissivity coating on one pane reflects heat back into the room. A gas fill, usually argon, slows the transfer of heat across the cavity. A warm-edge spacer cuts the cold line round the rim of the glass. And an extra pane adds a second cavity.
The frame matters just as much. A thermally broken aluminium frame or a well-chambered uPVC frame avoids passing cold straight from outside to in. Our guides to low-E glass on a rooflight and warm-edge spacers take two of those parts one at a time.
The number can change with the size
A published U-value is calculated for a particular size of unit. Because the frame and the spacer occupy a fixed width, a small unit has proportionally more of its area taken up by the weaker parts. A large pane has more of its area as clean glass, so the same build usually returns a better whole-unit figure.
That is why a maker may show one figure for a small roof window and another for a large one in the same range. When you read a data sheet, find the figure for your size or the size nearest to it. A headline number for the best-performing size in the range tells you little about the one going on your roof.
Window Energy Rating bands
Roof windows can also be judged by their Window Energy Rating, a letter band that takes in more than heat loss. It looks at how much useful solar warmth the window lets in during the heating season as well as how much heat escapes. Band B satisfies the regulations for an existing home as an alternative to the 1.4 figure.
The rating is handy when a window has a slightly higher U-value but admits a good deal of winter sun. It is less common on flat rooflights and lanterns, where the U-value remains the working measure. If a maker quotes a band, ask for the underlying U-value too, so that you can set it against another product on the same terms.
Why the U-value is not the whole story
A low figure describes the glazing in the factory. What reaches your room also depends on what happens round the edge of the opening. If the insulation between the frame and the roof structure is thin, patchy or missing, heat finds the shortest path out round the unit instead of through it. The best glass in the world cannot make up for that.
This is why we treat the hidden work as part of the energy performance. The insulated upstand or collar, the membrane and the flashings are done carefully and photographed, and you get the photographs at handover. Our are rooflights cold in winter answer explains how that shows up in a room.
Condensation is the other thing a number cannot predict. A cold frame edge, damp room air and poor ventilation produce it whatever the centre-pane figure says. We look at that in the guide to condensation on rooflights.
Older glass and what it recorded
Many extensions built a generation ago carry rooflights that were never tested against today's tables. Plain double glazing in a metal frame without a thermal break can lose heat several times faster than a modern unit. Single-glazed rooflights and polycarbonate sheets are further behind still. The result is a room that is hard to heat in winter and sometimes stuffy under the same glass in summer.
You can tell a good deal by looking. Hold a lit torch or a phone light near the glass at night and count the reflections: one bright image per surface. A low-E coating shows as a faint tint in one of the images. A frame that feels cold to the touch on a winter morning, with water running down the inside, is telling you its profile has no break between inside and out.
Reading a data sheet line by line
Data sheets differ, but the lines worth finding are the same. Look for the build of the glazing, written as the thickness of each pane and cavity. Look for the gas fill and the coating. Look for the spacer type. Then look for the U-value and its footnote, which should say which size or standard it relates to.
| Glazing build | Panes and cavities, in millimetres, in order from outside to inside |
|---|---|
| Coating and fill | Low-E on which surface, and the gas between the panes |
| Spacer | Warm-edge or aluminium |
| Frame | Material, thermal break and colour |
| U-value note | Whole unit or centre-pane, and for which size |
If a sheet leaves any of those lines out, ask the maker. A reliable product has nothing to hide behind a missing footnote.
What the number feels like in a room
People rarely feel a U-value directly. What they notice is the temperature of the inner surface of the glass, the movement of air beside it and the mist that forms on it on a cold morning. A lower U-value keeps the inner pane closer to room temperature, so you feel less chill when you sit under it and condensation has less cold surface to land on.
Take a kitchen extension with a flat rooflight over the table. On a still, cold night with the heating on, an old unit leaves a pool of cooler air above the chairs. A modern one, fitted on an insulated upstand, leaves the area under it much closer to the rest of the room. The change comes from the glass, the frame and the upstand working together rather than from the number on the sheet.
Comparing two rooflights fairly
To compare products without being steered, put them side by side on the same terms. Use the whole-unit figure for each, calculated for the size you actually need. Check the number of panes and what the spacer is made from. Ask whether the figure was measured or calculated, and whether it refers to the same frame material and colour.
- Is the quoted figure for the whole unit or the middle of the glass?
- Is it for my size, or the best size in the range?
- Does the unit meet the Part L limit for its type, roof window or rooflight?
- What is the spacer, and is it warm-edge?
- How is the gap between the frame and the roof insulated?
Our written quote sets the glazing out pane by pane so that you can ask those questions of any other quote you hold. A related guide, double or triple glazed rooflight, shows how one extra pane shifts the figure.
Where U-values meet the survey
The glazing specification starts with a visit. Someone measures the opening, reads the roof build-up and asks how the room is used, and the written quote that follows names the glazing pane by pane. If the job is notifiable, the Building Control notification is ours to arrange, and the certificate joins the other handover paperwork.
The thermal performance of an existing home is judged as a whole, so a rooflight is rarely the single deciding factor in an energy certificate. If you wonder what a new unit does for your energy certificate, can a rooflight improve an EPC rating is the place to start. For the whole picture of glazing, frames and the opening, go back to energy efficient rooflights.