The short answer
A roof window loses more heat than the insulated roof around it, because glass is a poorer insulator than a thick layer of insulation. How much more depends on the glazing and the fit. Approved Document L sets 1.4 W/(m²·K) or a rating of Band B for a roof window, and a well-fitted insulation collar limits the loss around the frame.
What Part L asks of a roof window
Part L deals with saving energy in buildings. In Approved Document L Volume 1 (2021 edition with 2023 amendments), Table 4.2 is the table for fresh or swapped-in parts of an older home. Its window row asks for 1.4 or Band B, a note confirms roof windows are included, and the rating is that of the Window Energy Rating scheme. Rooflights, the flat-roof product, are held to 2.2 instead.
That figure is a U-value, which measures how quickly heat flows through a surface and is written W/(m²·K). A lower number is better. The roof itself is held to a tougher number: 0.15 for a new roof element, so a window is allowed to be a weaker point than the roof around it.
Windows also have a band scheme of their own, the Window Energy Rating, which Part L accepts as an alternative to the U-value. The two scales should not be confused. The page on roof window U-values explains how to read a data sheet.
The edition changes on 24 March 2027, when AD L 2026 takes effect for most building work in England, and the table numbers move with it.
Insulation collars and the fit
Most of the heat that escapes at a roof window does not go through the glass. It goes around the frame, where the opening meets the roof. The frame sits in a gap that has to be filled with insulation, sealed against draughts and lined inside, and a small mistake there leaves a cold bridge, a path that carries heat past the insulation.
Makers supply an insulation collar, a pre-cut surround that fills the gap between the frame and the rafters and sits against the roof insulation. Used as designed, it keeps the layer continuous. Skipped, or squeezed in by hand, it leaves a void that cools the reveal and invites condensation.
The vapour seal matters as well. Warm air from the room must not reach the cold side of the insulation, so the lining is sealed to the frame and to the ceiling or slope around it. If that seal fails, moist air passes into the insulation and cools there.
The quality of the fit is not visible once it is covered, so we photograph the collar and the sealed lining before the plasterboard goes up. You see what is behind the finish, and the photos are yours to keep.
Glazing, size and where the window faces
Heat loss follows the product you choose. Double glazing is the usual base, and the better products add a low-emissivity coating, a gas filling and an insulated edge. Triple glazing exists too, at higher cost and weight. The data sheet gives the U-value for the whole window, which is the number to compare, not the glass alone.
Size is the second factor. A larger window loses more in total, simply because it has more area, however good its rating. Two smaller windows can light a room as well as one large one, with a different balance of loss and gain, and the choice is part of the design.
Direction matters in both directions. A window that faces the sun gains heat in daylight, and one that faces away from it loses more than it gains in winter. In summer a south-facing roof window can overheat a room, which is where blinds and shading help.
None of this makes a roof window a bad idea. Daylight in a loft room is valuable, and the heat loss is a manageable part of a design that also has good insulation elsewhere. Our page on escape roof windows covers a case where size is set by safety rather than energy.
Upgrading the roof when you fit one
Fitting a window in a loft conversion often means renovating the roof around it. For existing elements, Table 4.3 gives a threshold of 0.35 and an improved U-value of 0.16 for a roof, and paragraph 4.13 accepts a lesser improvement if the full target cannot be reached in practice or needs over 15 years to pay for itself. The renovation and loft conversion figure is 0.16, never 0.15.
A rooflight upstand made on site is capped at 0.35 W/(m²·K) in the same document. That is a flat-roof detail, and our flat roof rooflight guide covers it.
Regulation 2(3) of the Building Regulations 2010 keeps windows, roof windows and roof-lights out of its list of thermal elements, which names only walls, floors and roofs. The roof structure around a window is still covered when it is renovated.
At the survey we check the insulation you have, the depth available in the rafters and the window you want, then explain what is needed. A single written total follows. See roof windows and skylights for the other work.
