Rooflight U-values explained

The unit is watts per square metre per degree of temperature difference, written W/m²K. It describes how much heat passes through one square metre of a material when the air on one side is one degree warmer than the other. A lower number loses less.

Roofs are an exposed place to apply the idea. Warm air rises to meet the glass from underneath, and the cold sky sits on the other side of it, so the figure for a roof glazing unit is worth reading closely.

A U-value belongs to a construction, not to a material in the abstract. Glass, spacer, frame and kerb each contribute, and the number you are given should say which of those it includes. A figure with no description attached is only half an answer.

A U-value is a number on a sheet, but you feel it as a surface. On a frosty morning the inner face of a roof window is the coldest thing in a warm bathroom, and steam settles on it first. A better figure raises the temperature of that surface, which is one reason efficient glazing and condensation are linked. Our page on how much heat a rooflight loses looks at the same question from the heating bill's side.

Glass type is the usual lever. VELUX lists double and triple glazing among the options on its glass rooflights, and ties both to better energy efficiency and reduced sound. Which glazing a given unit offers depends on the product, and the manufacturer's literature for that product is the place to check.

The frame and the kerb matter too. A fine figure at the centre of the glass counts for less if the edge or the upstand lets heat out at the margins, and that is the argument for reading the whole-unit number. A well-fitted unit should also be reasonably draught-proof, which Approved Document L (Wales) asks of installed windows, roof windows and rooflights.

Centre-pane describes the glass in the middle of the pane. The edges of a sealed unit and the frame around it let heat through faster, so the figure for the whole window usually comes out worse than the figure for the glass alone. A brochure that quotes the centre-pane number looks better than a brochure that quotes the whole unit, and both can be true of the same product.

For rooflights, Approved Document L (Wales) goes a step further. It takes the unit's whole developed surface as the basis for a rooflight's U-value, an area often larger than the opening cut in the roof. Where the kerb is delivered together with the rooflight in one piece, expect the limit to apply to the kerb too.

So when you compare two quotes, check what is being measured. A whole-unit figure and a centre-pane figure are different numbers, and putting one beside the other tells you nothing. Ask for the same basis on both and the comparison starts to mean something.

Welsh guidance separates two products that people often call by the same name. A roof window sits in the plane of the roof. A rooflight sits out of plane, on a kerb or upstand. Approved Document L (Wales) defines both, and gives each its own limit.

For a new or replacement roof window in an existing home, the figure to meet is 1.4 W/m²K, or Window Energy Rating band B. For a new or replacement rooflight, it is 2.2 W/m²K. It is easy to swap the two, so check the limit being applied against the unit you are buying.

These limits are guidance, and the wording matters: 1.4 is what a roof window should reach, 2.2 what a rooflight should. Fitting a new rooflight will generally also bring Building Regulations approval into play, and where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver.

Roof lanterns are a separate conversation. The 2.2 figure is the one for a rooflight as the document defines it, and we do not apply it to a lantern by name. A lantern built into a fresh extension is a case where we ask the council which rules apply.

A house of architectural or historic interest, or one whose street face has a character to keep, can struggle to hit 1.4 with a replacement roof window. The guidance allows a fallback for that case: 1.2 W/m²K at the centre of the pane, or low-emissivity secondary glazing. It covers roof windows and other windows. For rooflights, there is no such fallback.

That detail matters on a terrace of slate roofs where a conservation-style roof window is the natural choice. A figure still applies, only a different one, and which route suits your roof is a conversation for the survey. Bring photographs of the existing windows and of the slope from the garden if you have them, because the character of the old roof is half of that decision.

Listed houses carry a separate layer. Listed building consent is applied for through the local planning authority, and Cadw is consulted. Our pages on listed building rooflights and on conservation rooflights go through the planning side.

Each new square metre of glazing is another place for heat to leave, so Welsh guidance looks at area as well. Approved Document L (Wales) says that where an opening for a window or door is made bigger, or a new one added, the combined area of windows and doors should stay within 25% of the home's total floor area. The edition that takes effect on 4 March 2027 adds roof windows and rooflights to the list counted in that total. It also lets a home go past the quarter if its energy efficiency is improved to compensate.

What you replace changes the picture. Swapping a window or roof window for the same kind is building work as the Building Regulations see it; repairing one is not. Keep the existing frame and change only the glass, and the energy rules leave that out of building work as well. How that choice plays out on a misted unit is in our guide to sealed units against whole new rooflights.

Ventilation should not slip backwards either. A replacement for a window that had trickle vents should carry vents at least as large, under the control of the occupant or of an automatic mechanism, as Approved Document F (Wales) puts it.

When a figure arrives, work through four checks. What does it describe: the pane, the unit, or the unit with its kerb? What product does it belong to: a roof window, a rooflight or something else? Which Welsh limit does that product face? And does the figure come from the manufacturer's own literature for that exact unit?

Keep the units straight. A WER band, a centre-pane figure and a whole-unit figure all live on spec sheets, and quoting one as though it were another leads to a wrong decision. If something does not add up, ask. A supplier who can explain the number is worth more than one who simply offers the lowest.

Keep the written figures with your house papers. If the roof is ever sold on, or a building control body asks what went in, a note of the product, its figure and the basis it was measured on saves a search through old emails. It also gives you a yardstick for the next quote, which may arrive in a different format and with a different basis, and a yardstick makes the difference plain at a glance. A note of the date, the supplier and the product name helps as well, and takes a minute to write.

The survey is where the product, the roof and the rules sit side by side, and a fixed written quote follows it. To start, ask for a survey, or read the short answer on the figure to expect for a Welsh rooflight.