What a U-value actually measures
A U-value is a rate of heat loss. It says how much heat passes through one square metre of a building element for every degree of difference between the air on either side. The unit is watts per square metre per kelvin, written W/m²K. A rooflight with a U-value of 1.5 loses less heat than one rated 2.2, given the same size and the same weather.
Two things follow from that. First, the figure is a property of the whole assembly: glass, spacer, frame and the way they are joined. Second, it is a rate, so the size of the opening matters as much as the rating. A large rooflight with a good U-value can still lose more heat in total than a small one with a slightly worse figure, simply because there is more of it.
It helps to hold the number against something familiar. A solid insulated roof is designed to lose very little heat per square metre, and glass will always lose more than the roof around it. A rooflight is a deliberate weak spot in the thermal envelope, traded for daylight. The point of a good U-value is to keep that trade fair: you get the brightness, and the room does not pay for it all winter.
This page sits under our guide to energy-efficient rooflights, which covers the wider picture of glazing, frames and fitting. Here we stay with the one number and what it can and cannot tell you.
Centre-pane, whole-unit and why the figures differ
The most common reason two quotes seem to disagree is that they quote different kinds of U-value. There are three you will see.
- Centre-pane (Ug). The heat loss through the middle of the glass only, away from the edges. It is the most flattering figure, and manufacturers quote it prominently.
- Frame (Uf). The heat loss through the frame on its own.
- Whole window or whole rooflight (Uw, or Ud for the unit including its kerb). The figure for the finished product, with glass, spacer bar, frame and edge effects all counted. This is the one that tells you what the room will feel.
The gap between them is real. The edge of a sealed unit is colder than its centre, because the spacer bar and the frame conduct heat more readily than the gas fill. On a small rooflight, edges make up a larger share of the total area, so the whole-unit figure drifts further from the centre-pane figure. On a large one the two sit closer together.
So when a leaflet says a glazing unit achieves a certain figure, ask whether that is the centre-pane value. If so, the finished rooflight will perform a little worse than the headline, and that is normal, not a fault. What you want to compare across products is the whole-unit number, for a stated size, measured to a recognised standard.
If a supplier cannot tell you which of the three they are quoting, treat the figure as unconfirmed until they can.
The limits Building Regulations set
For an existing house in England, Approved Document L Volume 1 (Part L, 2021 edition) sets a limiting figure in Table 4.2 for replacement and new elements. For rooflights the limit is 2.2 W/m²K. For roof windows, the opening kind set in the slope of a pitched roof, the limit is tighter: 1.4 W/m²K, or alternatively a Window Energy Rating of band B or better.
These are limits, not targets. A rooflight right on 2.2 meets the rules and little more. There is no reason to aim for the exact limit when better glazing is available for a modest step in cost, and in practice the better-performing units sit comfortably under it.
The distinction between a roof window and a rooflight matters here, and it catches people out. If you are fitting an opening, pivoting window into a pitched roof, the stricter 1.4 applies. A fixed pane over a flat roof, or a lantern, falls into the rooflight category at 2.2. We explain the naming in our page on roof windows.
Cutting a new opening into a roof, or replacing an existing rooflight, is work that normally needs Building Regulations approval. Where the job is notifiable we make the Building Control submission for you, and the fitting follows current Building Regulations. The U-value on the product datasheet is part of what that process checks, so it should be written down before the work starts, not guessed at afterwards.
One more point, because it is often asked: the regulations set no minimum amount of daylight for a house. The thermal limits are about heat loss. How bright the room becomes is a design question, not a compliance one.
What moves the number
Several design choices decide where a rooflight lands on the scale. Knowing them lets you see what a quote is really offering.
Number of panes. Each extra sealed layer adds a gas-filled gap that slows heat flow. Double glazing is the usual starting point and triple glazing improves the figure further. We look at when that extra layer pays off in double or triple glazed rooflights.
Low-emissivity coating. A thin, almost invisible metallic layer on one surface of the glass reflects heat back towards the room. It makes a large difference to the U-value for very little loss of light, which is why it is standard in decent glazing.
Gas fill. The cavity between panes is filled with an inert gas such as argon, which conducts heat more slowly than air. The gain is real but modest next to the coating.
Spacer bar. The strip that holds the panes apart is a weak point at the edge. Insulating, so-called warm-edge spacers reduce the cold band around the glass, which also helps against condensation.
Frame material and thermal break. Aluminium conducts heat quickly, so aluminium frames rely on a thermal break, a strip of low-conductivity material splitting the inner and outer parts of the profile. Timber and PVC conduct less to begin with.
The kerb or upstand. On a flat roof, the raised kerb that carries the rooflight is part of the heat path. An uninsulated kerb can undo much of the glazing's good work.
Reading a datasheet or a quote
You do not need to be a technician to ask the right questions. When a quote arrives for a rooflight, check these things in order.
- Does it state a whole-unit U-value, and does it say for which size? A figure for a standard test size will not exactly match your opening.
- Is it the glazing-only figure or the finished rooflight, kerb included?
- Does the stated figure meet the Part L limit for the type of unit: 2.2 for a rooflight, 1.4 or WER band B for a roof window?
- Is the specification written down: number of panes, coating, gas, spacer, frame material?
That last check protects you most. A verbal promise that the glass is energy-efficient means nothing; a written specification can be checked on the day. The free survey ends in a written quote with a fixed figure, so the glazing it describes is the glazing that arrives.
Be a little wary of comparing two figures that differ by a tenth or two. Test conditions, rounding and the size used can account for that much on their own. A difference of that scale should not decide the purchase. A difference between a unit near the limit and one comfortably below it is worth weighing, especially for a large opening or a room you heat for long hours.
If you have an existing rooflight, the data plate or the glazing spacer sometimes carries markings, but the number is rarely printed on it. Photographs sent to us before the survey help us work out what is already there.
What a U-value cannot tell you
A single heat-loss figure leaves several things out, and for a house lived in through a Nottingham winter some of them matter.
How much light gets through. Glass has a light transmission value. Extra panes and coatings trim it slightly. In summer that hardly registers. From October to March, under low cloud, every percentage point of brightness is worth having, which is one reason we do not push every rooflight to the highest pane count. A well-placed double-glazed unit can brighten a room more than a poorly placed triple one.
How much solar heat it admits. That is a separate figure, the g-value, and it works against the U-value in some respects: coatings that block unwanted summer heat can also cut the free warmth you get in winter. We cover this in what solar control glass is.
Condensation. The inner surface temperature of the glass decides whether moisture forms on it, and a lower U-value generally keeps that surface warmer. But humidity, ventilation and the fit of the reveal play their part. Our guide to stopping condensation on rooflights goes through the causes.
Air leakage and workmanship. A rooflight with an excellent rating, badly sealed into the roof, will let cold air round the frame. The datasheet describes the product, not the installation.
In short, treat the U-value as a necessary check and not as the whole verdict.
Fitting it so the number holds
The figure on the datasheet is measured in a laboratory, with the unit sitting in a test frame. On your roof it depends on what happens at the joints.
The first concern is continuity of insulation. Around a new opening, the roof insulation has to meet the rooflight frame or kerb without a gap. A void between the two lets heat through a path the glazing figure knows nothing about, and it can chill the plasterboard around the opening, which then collects moisture. Good detail here means insulation carried up the kerb and tight to the frame, a sealed membrane that links the roof's waterproofing to the rooflight, and an airtight seal on the room side.
The second is the lining, the reveal between the ceiling and the glass. A deep reveal in a thick roof can be a cold shaft. Insulating it, and angling it outward, helps both warmth and light.
The third is correct fixing and flashing for the roof type, because a rooflight that leaks will lose its insulation value to damp long before the glass wears out.
That is the reason a survey comes first. We look at the roof build-up, the structure and the position, because those decide how the opening is detailed and therefore how the rooflight will really behave. The fitting is covered by a written workmanship guarantee of ten years, and the terms are set out on the page about our guarantee. To have your own roof looked at, use the quote form below or send a WhatsApp message to 07880 284133.