A U-value is a number on a data sheet, but it decides how much heat leaves a room through the glass above it. Part L of the Building Regulations sets limits for that number when a rooflight or roof window goes into an existing home. Those limits are different for a roof window and for a rooflight, and many people meet the two figures mixed up. This guide sets out what the number means, which limit applies to which product, and what else has to be right before the number on paper turns into a warm room.
What a U-value measures
A U-value tells you how fast heat passes through a building element for each degree of difference between inside and out. The unit is watts per square metre per kelvin, written W/m²K. The lower the number, the less heat escapes, so a low U-value is the good one.
It is worth being clear about what the number covers. A U-value for a whole rooflight, sometimes written Uw, includes the glass, the spacer between the panes and the frame. A figure for the glass alone, written Ug, is usually better than the figure for the whole unit, because frames and edges lose more heat than the middle of the pane. When two quotes give different numbers, check whether they describe the whole window or only its glass.
Standard test conditions are set for the measurement, so a number from a manufacturer is a fair way to compare two products. It is not a prediction of what you will save, because that also depends on how the room is used, how well the roof is insulated and how the unit is fitted.
The Part L limits for an existing home
Approved Document L, volume 1, covers dwellings, and Table 4.2 gives the limits that apply when you fit new or replacement elements to an existing house. For glazing in a roof, there are two figures, and they depend on what the product is.
| Product | Limit | Typical example |
|---|---|---|
| Roof window | 1.4 W/m²K, or WER Band B | A top-hung or centre-pivot window in a pitched slope |
| Rooflight, including roof lanterns | 2.2 W/m²K | A flat rooflight on a kerb, or a lantern on a flat roof |
The two limits are not an oversight. A roof window sits in the plane of the roof, with a frame built to be part of it. A rooflight is a different type of product, usually larger and set on a kerb, and the regulations allow it a higher figure. We specify to the right limit for each product and state the figure in the quote.
Window Energy Rating as an alternative
A roof window can meet Part L either by its U-value or by its Window Energy Rating. WER is a scheme that grades a window from A++ down to E, taking into account heat loss and the heat gained from the sun. Anything at Band B or better is accepted for a replacement roof window on an existing house.
Two roof windows with the same U-value can sit in different bands, because the energy rating also credits the useful solar gain through the glass. For that reason a window with a rating can pass where a quick look at the U-value would be marginal. Rooflights are held to a U-value alone.
When you read a quote, look for either figure for a roof window and for the U-value in the case of a rooflight. If neither is stated, ask. A figure that is not written down cannot be checked by Building Control.
Why a replacement counts as controlled work
Putting a new glazed unit into a house roof counts as controlled work. In practice it must be notified to Building Control, except where the fitter is a registered scheme member. We make the notification as part of the job. The inspection is a check on the figures we have given, and you have nothing to chase.
The notification is where the U-value is checked. The product's declared figure and the size of the opening go on record, and an inspector may ask to see the data sheet. We keep that sheet with the life plan, so the proof that the rooflight met Part L sits alongside the note on how it will be looked after.
The inspector may be from the council or a private firm, which has been called a registered building control approver since April 2024. Either takes the same data. Our guide to rooflight planning and building regulations covers the whole picture, including the planning side.
What sits behind a low number
A U-value comes from several parts working together, and each is worth knowing when you compare products.
The glazing. Double glazing, with a low-emissivity coating on one pane, is the standard. Triple glazing adds a pane and improves the figure, at a cost in weight and money. We look at that decision in our page on double or triple glazed rooflights.
The gas fill. The gap between panes is filled with argon or a similar gas, which conducts heat less well than air.
The spacer. The strip that holds the panes apart is a weak point for heat. A warm-edge spacer in a composite material loses less than a metal one.
The frame. Timber, PVC and aluminium conduct heat differently, and a frame with a thermal break does better than one without.
Together these make the figure on the data sheet. None of them shows from outside, which is why we ask for the manufacturer's data and quote its number.
Where the number is lost after fitting
A good U-value can be undermined by how the unit is fitted. The gap between frame and roof structure has to be insulated, and the lining on the inside has to be airtight. Where it is not, warm damp air reaches the cold edge of the frame, and the result is condensation and a cold patch at the reveal.
The same problem is described in more detail in why a roof window gets condensation. For now the point is that the declared figure is for the product, tested alone. In a real roof, the installation decides how close you get to it.
We follow the maker's fitting instructions on insulation collars and sealing, and the life plan records what was used. If a collar was supplied with the window, the record says so.
Efficiency changes with time
The figure on the data sheet is a figure for a new unit. A sealed glass unit relies on an edge seal that holds the gas in and the moisture out. Over the years that seal can weaken, the gas fill drifts and the unit performs less well than when it was new. We do not give a lifespan for this, because it depends on the unit, its exposure and its orientation.
What we do is plan for it. The life plan says that the glass unit is the part that carries the efficiency, and it notes how it can be replaced. At year 5 we look at the seals and the inside of the unit for any sign of misting between the panes, and again at year 10. Where a unit can be swapped without lifting the surrounding roof, the plan states it.
If you already have an older roof window and wonder whether replacing it would improve things, our answer on whether a new roof window improves insulation deals with that.
Heat in summer and the other half of the story
U-values describe heat loss in winter. They say nothing about heat coming in on a hot afternoon. A south-facing loft room with a large roof window can overheat even if its U-value is excellent, and that is a question of solar control, blinds and glass coatings. We cover it on will a rooflight overheat a loft in summer.
Part L is the starting point, not the end. It sets the least a product must achieve, and the quote can go further where the room calls for it. On a Sheffield stone terrace with solid walls, a loft room already loses heat through the roof, so a better glazing figure may be worth asking about. Our hub page on energy-efficient rooflights shows how those choices fit together.
Reading a quote for the numbers
A quote that names a rooflight should give you three things you can check: the product, its declared figure and the limit it has to meet. For a roof window that is a U-value of 1.4 W/m²K or lower, or WER Band B. For a rooflight it is 2.2 W/m²K or lower. If a quote says only "energy efficient glass", it has told you nothing a Building Control officer could use.
Ask too whether the figure is for the glass or for the whole window. Mixed up, the two make a product look better than it is. Ours state the whole-window number, and the sheet behind it is kept in your file. Cost factors in this area are the glazing build, the frame material and the size, not the paperwork, which is part of the job.
Sheffield roofs and the figure that matters
Loft rooms in solid-wall terraces and 1930s semis raise the energy question sharply, because the sloping roof is all that stands between the bedroom and the weather. A roof window fitted there loses or keeps heat in proportion to its U-value and its surround, so the framing and collars matter as much as the glass.
On Peak-edge cottages, the planning authority inside the National Park is the Peak District National Park Authority, and a conservation-style window may have a higher figure than a standard one. We look at that trade at survey and say what is possible. We work across Sheffield, Rotherham, Chesterfield and Barnsley, and the quote states what each window achieves.
Questions to put to any installer
Before you sign anything, ask three things. Which limit applies to this product, and where is the figure written? Is the number for the whole window or only the glass? Who notifies Building Control, and does the quote say so? A clear answer to each tells you the work has been planned. A vague answer tells you the figure will be found out later, when it is harder to change.
U-values and Part L, answered
What U-value must a replacement rooflight meet?
For an existing home, 2.2 W/m²K is the limit for a rooflight, including a roof lantern. A roof window has a tighter limit of 1.4 W/m²K, or WER Band B.
Is a lower U-value always better?
For heat loss, yes. But the figure only describes the product on its own. How it is fitted, how it is shaded and how it is used decide how much difference you notice.
Do I have to tell Building Control about a replacement?
The work is notifiable unless it is done by a member of a competent person scheme. We handle the notification as part of the job, so you do not have to arrange it.
Does the 10-year guarantee cover the U-value?
Our 10-year guarantee covers the quality of our workmanship. The performance figure belongs to the product, and the manufacturer's terms cover it.
Where can I find the U-value for my quote?
It is written on the quote and in the life plan, with the manufacturer's data sheet kept alongside. If you cannot see it, ask us and we will point you to it.