What U-Value Must a New Rooflight Meet?

Part L of the Building Regulations sets thermal performance for the new windows and rooflights that go into existing homes. There are two limits, and the unit's type picks the one that applies.

A lower U-value means less heat escapes through the unit. The rooflight limit is the higher figure, and Approved Document L measures a rooflight's U-value over its developed surface area, which takes in the raised frame on its kerb.

Which category applies is a matter of the unit's design, not the slope. A roof window is a window made to be set in the plane of a pitched roof, and it is a factory-built unit with its own frame and flashing. A rooflight sits on a kerb or upstand, out of the plane of the roof; flat glass units and lantern lights are the usual examples.

On our own installations that means a pitched-roof window such as a VELUX is held to the roof window limit. A flat rooflight or a roof lantern on a kerb is held to the rooflight limit. A walk-on unit set flush with the roof is classed by its maker, so check its data sheet.

If a unit's category is unclear, the data sheet and the maker say how it is classed. Ask for that before you order. The energy efficient rooflights hub covers the glazing options that reach these figures.

Mixed cases come up. A pair of roof windows with a flat rooflight between them on an extension roof is three units under two limits. The survey sheet lists each unit with its category, so the order matches the figure.

A U-value is measured in watts per square metre per degree of temperature difference, written W/m²K. It tells you how many watts leak through each square metre of the unit when the air on one side is one degree warmer than on the other.

Makers quote more than one figure. The one on the front of a brochure may be for the centre of the glass alone. The regulation figure is for the whole unit: frame, glass and spacer together. A good pane in a poor frame gives a weaker whole-unit figure than the pane alone suggests.

• Look for "Uw" or "whole window", not "Ug", which is the glass alone.
• Check that the figure is for the size and glazing on your quote.
• Keep the data sheet with your survey sheet.

The guide to rooflight U-values goes into the figures in more depth.

A worked example makes the difference clear. On a data sheet a pane might be listed at a glass-only figure well below the limit, while the whole-unit figure sits close to it because the frame conducts more heat. The regulations judge the second.

Roof windows have a second route. Reaching Band B on the Window Energy Rating scale meets the requirement without a U-value of 1.4. The rating counts the heat a window gains from the sun as well as the heat it loses and the air that leaks through it, and that is how a unit with a slightly weaker U-value can still qualify.

The rating is printed on the manufacturer's label or data sheet for the exact model and glazing. Either route is accepted, so you do not need to meet both.

A Band B rating also gives you a single letter to compare between roof windows, which is easier than comparing three or four decimals on different data sheets.

The limits apply when a whole rooflight or roof window, frame and glass, is replaced, and when a new opening is made. Replacing only the sealed glass unit in an existing frame is not notifiable, but a modern glass unit is still worth specifying for its performance.

Listed buildings and some buildings in conservation areas are treated with more flexibility, since the regulations balance energy savings against the character of the building. Whether that applies to your home is a question for the conservation officer and the building control body. We note it at the survey.

Doing the sums is the maker's job. Our job is to confirm the figure on the data sheet before the unit is ordered, and to put it in writing.

Where the work is part of an extension, Approved Document L also limits the total area of new windows, roof windows and doors, unless the extension is assessed as a whole against an equivalent design. The building control body states what applies to your project.

A low-emissivity coating on the glass and a warm-edge spacer round its perimeter bring many modern double glazed units within the limits. Triple glazing improves the figure again, and is worth a look in a bedroom or a loft room that is hard to heat. The guide to double and triple glazed rooflights compares the two.

If you want to know how much a new unit might cut heat loss compared with the old one, the answer on heat loss looks at that.

Whatever the glazing, the unit is fitted to the opening with the right flashing and a sealed joint at the frame. A good U-value on paper depends on the unit sitting tight in the roof, with no gaps round the frame.