Roof window U-values
What U-value does a new roof window need?
The short answer
Approved Document L asks for a U-value of 1.4 W/m²K or lower, or a Window Energy Rating of at least Band B, when you fit a roof window to an existing house. A rooflight on a kerb gets a looser 2.2. Those limits hold until 24 March 2027, when the 2026 edition takes over with matching values. A like-for-like swap need only match the unit coming out.
What a U-value tells you, which of the two glazed figures applies, and how to check a unit before you order it.
What the number tells you
A U-value says how readily heat passes through part of a building. The unit is watts for each square metre of the element, for every degree separating indoor and outdoor air, written W/m²K. A smaller number means slower heat loss, which is what you want from glazing. A roof window at 1.4 holds on to more warmth than one at 2.0.
The label does not decide how the room feels on its own. The way the frame meets the roof, the insulation packed around it and the airtightness of the fit all play a part, and a good unit badly set can lose more heat than its data sheet promises. Condensation tells the same story. The figure on the sheet is a test result for the product, and it is the one a building control body asks to see, so treat it as the starting point and the fitting as the other half.
Two glazed limits: roof windows and rooflights
Approved Document L covers the saving of fuel and power, and it sets a cap on heat loss for every new or swapped element in an existing home. In the 2021 edition, Table 4.2 allows 1.4 W/m²K for windows, with a note that roof windows are included, or Band B as a minimum on the Window Energy Rating scale. The same table allows 2.2 for a rooflight and 0.15 for the roof itself.
Two figures exist because the products sit differently. The document describes a roof window as one lying in the plane of the slope, facing the way the roof faces. A rooflight is glazing that stands up from the roof on a raised kerb or upstand, and any upstand built on site is limited to 0.35. So a window lying in a sloping roof has to reach 1.4, while a unit on a raised kerb above a flat roof has to reach 2.2. Our answer on rooflights and roof windows shows how to tell yours.
Which edition governs the job, and the 2027 changeover
Work started today is judged against the 2021 edition, including its 2023 amendments. A newer 2026 edition is already published. It begins to apply on 24 March 2027, for building work other than the higher-risk kind. Its table of limits for existing homes is numbered 3.2 and carries the same figures for roof windows, rooflights and upstands.
For you, that means a unit chosen to the present limit will still comply under the next edition, so there is no reason to hold off ordering. How jobs that begin before the date and finish after it are treated is set out in a government circular letter, so ask your building control body if the dates overlap. Timber windows once had a slightly looser allowance of 1.6 or Band C, but that ended in June 2023, and every material now has to meet the full standard. If a supplier quotes the older figure, ask why.
Swapping a unit that is already there
A swap faces an easier test than a new window. The new unit must not perform worse than the one it takes out, so an old single-glazed roof window replaced with a modern double-glazed one passes with room to spare. Even so, it is sensible to choose a unit that reaches 1.4, because you gain comfort and avoid a second job if standards move.
Bigger projects can pull in more of the roof. The document names the renewal of a flat roof's waterproofing as renovation, and renovating over half of a thermal element's surface means the element should be upgraded. If more than a quarter of the external envelope is renovated, the project is a major renovation. One window barely registers against that measure, whereas a window fitted during a complete re-cover might not. Our guide to roof windows and the Building Regulations shows how the thresholds are used.
How to check a unit before it is ordered
Ask the supplier to state the declared U-value or the energy rating for the exact model and size you are buying, and to put it in writing. Make sure the figure covers the entire window with its frame, not just the glass, because the middle of a pane always does better than the whole unit. Then match the figure to the product: 1.4 for a roof window on a slope, 2.2 for a rooflight on a kerb.
File the data sheet and the delivery note together with the building control paperwork. An inspector may want to see the evidence, and so may whoever buys the house later. We choose units to the limit that applies, name the choice in the written quote, which gives a single price, and fit the insulation and flashing around it to suit. The rest of our work is described on the skylights and roof windows page.
Related questions
U-value questions
Is a lower U-value always worth paying more for?
Below the limit, a lower figure means less heat lost, but each step brings smaller gains and a higher price. The required value is 1.4, so meeting it comes first. Beyond that, weigh the extra cost against the size of the window and how often the room is used.
Is a dormer window held to the roof window limit?
A dormer has vertical glazing in its own small structure, so the glass is treated as a window and the dormer's roof as roof, with the cheeks and top of the dormer counted as roof. How your own dormer is classed is something to confirm with building control before ordering.