A U-value is the measure of how quickly heat escapes through a building element, and Part L of the Building Regulations sets targets for it. For a roof window, the target is a single number that most buyers never see on the quote, though it decides whether a window can be fitted lawfully in an existing house.
The rules are not hard, but they are easy to mix up. Roof windows have one figure and rooflights another. A roof window is not counted as part of the roof for the roof renovation rules, so a roof upgrade and a window upgrade are two different questions. And the document that holds the figures is being replaced in March 2027, with different table numbers.
This guide explains what a U-value is, the figure for roof windows and the alternative energy rating test, how rooflights differ, why thermal elements exclude roof windows, what changes with the 2026 edition, how to read a data sheet and what to ask before buying, as it applies to roof windows around Farnborough and nearby.
What a U-value tells you
A U-value is expressed as W/(m²·K), meaning the number of watts of heat that leak through every square metre when the inside and outside air differ by one degree.
Lower is better
A window with a U-value of 1.4 loses less heat than one at 2.2. A roof with 0.16 loses less than one at 0.35. When comparing products, the lowest figure holds heat best, though cost and other factors also matter.
Whole window, not just glass
A window's U-value covers the glass, the frame, the spacer between the panes and the seals. A glass-only figure can look better than the whole unit. Ask for the figure for the complete window as it will be installed, which is what Part L means.
What affects it
- The number of panes and the coatings on them.
- The gas between the panes.
- The frame material and whether it is thermally broken.
- The edge spacer.
- The quality of fixing and sealing in the opening.
Why roof windows are harder
A roof window sits in a sloping surface, exposed to the sky, and it faces more weather than a vertical window. Heat loss and condensation are therefore a bigger concern, and the insulation collar around the frame matters.
Comfort as well as compliance
A window with a low U-value keeps the room side of the glass warmer, so the area near it feels less cold in winter and the window is less likely to mist. That is useful in a bedroom or study under the roof, where the window sits close to where people sit or sleep.
The target is set for each type of element, and the next sections give the figure for each, including the one most often quoted wrongly.
The target for roof windows
Volume 1 of Approved Document L is the Building Regulations guidance on energy performance in houses and flats. The 2021 edition, with its 2023 amendments, has applied in England since 15 June 2022.
Table 4.2
Table 4.2 is the one for new or replacement elements in a home that already exists. Its window row, which the table's note says takes in roof windows, asks for 1.4 W/(m²·K) or an energy rating in Band B at the least. Passing either test is enough, so a window in Band B complies even if its U-value is slightly higher.
What Window Energy Rating means
A Window Energy Rating is a labelled score that takes account of both heat loss and the heat that comes in through the glass from the sun. It gives a fuller picture than the U-value alone, and a window can pass on one measure and fall short on the other. The label on the product shows the band.
Which window to specify
When ordering, ask the supplier whether the window meets 1.4 or Band B and for the document that shows it. Do not accept a verbal assurance. The data sheet or the label is the evidence, and building control may ask to see it.
When it applies
It applies when a window is replaced or newly installed in an existing house. Our guide to replacing a Velux window explains how that sits with the structural and planning side.
The figure is a standard to meet, not a ceiling to beat, and a better window costs more but loses less heat.
Rooflights have a different figure
A rooflight is the glazed unit set in a flat or low-pitched roof, usually on an upstand. Table 4.2 gives it its own row, with a maximum U-value of 2.2 W/(m²·K).
Why the numbers differ
Rooflights are classed apart from windows, because they are built differently and used differently. The figure of 2.2 is more relaxed than 1.4 because of how these units are built and used. It does not make a rooflight a poor choice, only a differently classed one.
The common mistake
Mistakes come from applying 2.2 to a window in a pitched slope, or 1.4 to a unit on a flat roof. A quote that cites the wrong figure suggests the supplier has not looked at the roof. The figure of 2.2 is reserved for rooflights, and roof windows keep the window figure.
Upstands
Table 4.2 also caps an on-site upstand at 0.35 W/(m²·K), which is a figure for the kerb and not for the glass above it. Our guide to rooflights for flat roofs goes into the upstand, falls and waterproofing.
Condensation risk
A unit that is poorly insulated at the edges can run cold enough to mist or drip. That is a thermal comfort problem more than a leak. A proper insulation collar and a thermally broken frame reduce it, and good ventilation in the room helps.
Pitch decides the class
If you are unsure which a particular window is, look at the roof it sits in. A glazed unit in a pitched slope is a roof window. One in a flat or low-pitched roof, standing on an upstand, is a rooflight. A roofer or the manufacturer's literature will confirm the classification for the product.
Roof windows sit outside thermal elements
Part L has separate rules for renovating the main parts of a building, such as the roof. Roof windows fall outside those rules, and the reason is a definition.
The definition
The Building Regulations 2010, regulation 2(3), treat a wall, a floor or a roof as a thermal element, while windows, doors, roof windows and rooflights are excluded. To renovate a thermal element is to add a layer to it or replace one of its layers.
What follows
When a roof is renovated, Table 4.3 applies: the roof's existing U-value is compared with a threshold of 0.35 W/(m²·K), and an upgrade aims at 0.16, subject to the payback relaxation in paragraph 4.13. A roof window is not part of that calculation. It is judged against the window figure in Table 4.2 when it is replaced or added.
Two questions in one job
On a re-roof with new roof windows, there are two separate questions. Does the roof renovation need insulation to be improved? And do the new windows meet 1.4 or Band B? A quote should cover both. Our guide to insulation when re-roofing sets out the roof side.
Dormer windows
Table 4.3 carries a note about dormers: the word roof there takes in the roof parts of dormer windows. A dormer's roof and cheeks therefore belong to the roof, while its glazing stays a window.
A worked example
A house is re-roofed and two roof windows are replaced at the same time. The roof layers are replaced, so the renovation test is asked of the roof, and the roof's existing U-value is compared with 0.35. The windows are replaced, so each is tested against 1.4 or Band B. The two answers are independent, and a window that passes says nothing about whether the roof insulation needs to improve.
Practical effect
Treat the window and the roof as separate items on the specification, and ask each supplier to confirm which figure applies to which.
AD L 2026 and the March 2027 change
The figures above are those in force today. A new edition of the document is coming, and it is worth knowing the date.
The change
Approved Document L 2026 begins on 24 March 2027 in England, for work outside the higher-risk building category. Tables are renumbered so that new elements move to 3.2 and existing ones to 3.3, and the payback rule becomes paragraph 3.14.
Before the date
Up to 24 March 2027 the 2021 edition governs, with Tables 4.2 and 4.3. The 2026 edition is not yet the rule, and the old numbering will not outlast it. Check the transitional arrangements with building control if your work spans the date.
Why it matters for planning your work
If you are deciding when to buy, the date is a reason to confirm which edition will govern your job. A window installed under the old rules, with notice given to building control before the change, normally follows those rules.
Keeping up
We refer to the figures in the edition in force on the day of the work, and we say which on the specification. For a bigger project, a loft conversion for instance, ask building control which edition applies to your application.
Building control for Farnborough and Aldershot addresses is the Hart and Rushmoor Building Control Partnership, on 01252 398715.
Reading a data sheet and what to ask
A window's data sheet carries the evidence of its performance, and knowing what to look for turns it from a page of numbers into a check.
What to find
- The U-value of the complete window, in W/(m²·K).
- The Window Energy Rating band, if given.
- The glazing build-up and any coatings.
- Whether the frame is thermally broken.
- Whether the figure is for the window alone or as installed with its kit.
Questions for the supplier
- Does this window meet 1.4 or Band B, and where does it say so?
- Is it a roof window or a rooflight, and which row of Table 4.2 applies?
- Is the figure for the whole window?
- Which edition of Approved Document L is the specification written against?
Condensation and comfort
Warmer glass and frame resist condensation better, which is a benefit beyond the regulations. Where a window mists up, our guide to why a Velux window leaks explains how a roofer tells moisture on the glass from water getting in.
Keeping the evidence
File the data sheet, the delivery note that shows the model, and a photograph of the label on the frame before it is hidden by the lining. Years later, a buyer or a building control officer may ask which window went in and what its rating was, and the answer is on those three items.
Cost
For a standard mid-range Velux, MyJobQuote (Sep 2026) gives £1,100 as a typical UK range, and about £100 less for a straightforward swap. Glazing with a better rating costs more, and the survey settles the figure. Our roof windows and skylights service gives one written total, VAT shown.
