Do rooflights carry an energy rating like windows?

Go looking for an energy label on a rooflight and you will meet two different habits. They grew up separately, and they answer slightly different questions.

The first is the window energy rating. A rating body tests or calculates a complete window and gives it a band on a lettered scale, with A at the top. The scale is built on a whole-window calculation that weighs two things against each other: how well the unit keeps heat in, and how much useful heat from the sun it lets through. A product that loses a little heat but gathers a lot of sun can score as well as one that loses less and gathers less. The label you see on a window sample or a brochure page is the output of that sum.

The second is the plain U-value. Here the maker states how quickly heat passes through the unit, in watts per square metre per degree of difference. Lower is better. There is no band and no sun in the sum. It is one number about heat loss.

Roof windows, the tilted units set into a pitched roof, are made in factories in large runs, tested as standard products, and often carry the lettered label. Flat rooflights, lanterns and walk-on units are frequently made to the size of the opening, so each one is a one-off, and the maker quotes a U-value calculated for that build. Both are describing real performance. They just are not the same measuring stick.

People sometimes ask which is the "better" figure. They measure different things, so there is no direct conversion between them.

A rating band rewards solar gain. That suits a wall window on a south face, where the winter sun is welcome. On a rooflight it needs care. Sun through the roof is strong in summer, and the same gain that lifts a rating in an annual sum can overheat a loft room in July. Our page on winter warmth from a rooflight looks at that trade, and the answer on what a g-value is explains the sun part of the sum.

A U-value ignores the sun altogether. A unit with a low U-value and heavy solar control glass can be excellent at keeping heat in and poor at gathering any. That is fine for a bedroom that overheats, and a waste in a north-lit kitchen that could use the light.

There is also the matter of where the figure was measured. The centre of a pane is always better than the whole unit, because the edge and the frame leak more than the middle of the glass. A maker who quotes the centre-pane number for a product is telling the truth and flattering it. The piece on whole-unit and centre-pane U-values shows what to look for.

Building Regulations Part L sets the floor, and it speaks in U-values and in ratings as alternatives.

For new work and for replacements, Approved Document L gives limits on how much heat a window or rooflight may pass, stated as a U-value. For ordinary windows it also lets you show compliance by rating band instead. Roof windows and rooflights have their own entries in the tables, and the limits differ from the ones for wall windows, so a figure that suits one product type may not be the one that governs another. The limits are revised from time to time, and we work to the current edition at the date of your job. Our guide to what Part L asks of rooflights goes through the requirements, and the answer on replacement rooflights and current U-value rules covers the replacement case.

When the work is notifiable, we handle the Building Control notification, so the figure you choose is also the figure that gets signed off. That matters, because it is the whole-unit number for the unit actually installed that counts, not a value taken from the best product in a catalogue.

You can compare almost any two rooflights on equal terms by asking for the same information from each.

1. The glazing build in words. Pane thicknesses, coating, cavity width, gas fill and spacer. Two products with the same headline figure can have very different builds underneath.
2. A whole-unit U-value for the exact size quoted, not the centre of the glass.
3. The g-value and the light transmittance, because a low heat loss bought with dark glass costs you daylight. See what light transmittance means on a rooflight.
4. The frame and the kerb. A thermally broken frame on a well-insulated upstand keeps the glass figure honest. The page on cold bridging around a rooflight explains what goes wrong when they are ignored.

Lined up this way, a product with a label and a product with only a U-value become comparable. You are no longer weighing a letter against a number, because each has been reduced to the same few facts.

Every quote of ours carries the glazing build in the quiet spec, with the thermal figure beside the sound choices. The same laminated inner pane that calms rain on a lantern is part of that build, so the warmth and the quiet are written together and can be checked against each other. For an overview of the subject, the energy efficient rooflights hub is the place to start.

Does a better band mean a lower heating bill?

Not by itself. The band includes solar gain, and your bill depends on how your room is heated, which way the roof faces and how much of it is glass. Treat the band as a guide and the whole-unit U-value as the heat-loss figure.

Will an energy label help when I sell the house?

A written specification and the Building Control paperwork help a buyer or a surveyor. The effect on the property's energy certificate is covered in whether an efficient rooflight improves an EPC.

Why does my flat rooflight have no label at all?

Because it was built to size as a one-off, and its maker quoted a U-value for the build. It may still perform well. Ask the supplier or the installer for the calculation.

Can a polycarbonate dome be compared with a glass unit?

Yes, by U-value, and by the same four steps. The comparison of glass and polycarbonate rooflights sets out how they differ. To have a product compared for your roof, get a quote.