Rooflight U-values explained

U-value is written as W/m²K. Read it as a rate of heat loss: how many watts leak through each square metre for every degree Celsius between the warm side and the cold side. If the room is at 20 degrees and the air outside is at 0, the difference is 20 degrees, and a rooflight with a U-value of 1.5 loses roughly 30 watts through each square metre of it.

Two features make it more useful than it first looks. It is a property of the product, so you can compare one rooflight with another without knowing the house. And it covers all three routes by which heat moves, through conduction in the materials, convection in the gas between the panes and radiation across the gap.

It does not tell you how much solar heat comes through the glass, or how much daylight, and it says nothing about condensation by itself. Those are separate measures, which is why a rooflight specification has more than one number on it.

Glass is a poor insulator compared with a wall. A solid wall or an insulated roof is built to slow heat loss across a thick layer of material. A pane of glass is a few millimetres thick, so even a well made sealed unit passes much more heat than the construction around it.

Rooflights also work in a harsher position than windows in a wall. They face the sky, so they lose heat by radiation to a cold night sky as well as by conduction. Warm air from the room collects against them, so the inner pane is exposed to the warmest air in the house. And a roof glazing unit is tilted or horizontal, which changes how the gas inside moves and how much heat it carries.

None of this makes a rooflight a bad idea. A roof opening gives daylight to rooms that walls cannot reach, and a modern high performing unit loses far less heat than one from a few decades ago. It does mean the specification is worth reading carefully, because the difference between a basic and a good unit is large relative to its small area.

Several parts of the construction add up to the final figure. Each is a choice, and each has a cost and a trade-off.

Number of panes

Double glazing traps a layer of gas between two panes. Triple glazing adds a third pane and a second cavity, which cuts heat loss further but adds weight and thickness. Our guide to double or triple glazed rooflights looks at when the extra pane pays off.

Low emissivity coating

A thin, nearly invisible metallic coating on one glass surface reflects heat back into the room instead of letting it radiate out. It is the biggest single improvement to a sealed unit, and standard on any energy efficient rooflight.

Gas fill

The cavity between the panes is often filled with argon rather than air. Argon conducts heat a little more slowly, so the unit insulates a little better. Some makers use krypton in thin cavities, mainly on triple units.

Spacer bar

The strip around the edge that holds the panes apart. A traditional aluminium spacer conducts heat around the edge of the glass; a warm-edge spacer made of lower conductivity material reduces that. It also warms the inner edge of the glass, which helps against condensation. See are warm edge spacers worth it in a rooflight.

Frame

Timber, PVC and aluminium frames conduct heat at different rates, and a frame with a thermal break or insulated core does better than a plain one. On a small rooflight, the frame is a larger share of the whole unit than on a big window, so its quality has more weight.

When someone quotes a figure, it helps to ask which one it is. There are three common versions and they do not give the same number.

The Building Regulations standard for a rooflight refers to the whole unit, so the whole window figure is the one that matters. A brochure that quotes only the centre pane figure is describing the best part of the product. Ask for the whole window value, ideally from the manufacturer's declared performance for the exact size and type you are buying.

Size changes the answer too. A small rooflight has more edge and frame in proportion to its glass, so its whole window figure is usually a little worse than the same construction in a larger size. Manufacturers often publish a figure for a standard test size, so look at how the quoted number was arrived at.

Rooflights come in different forms, and their U-values are not comparable without allowing for that.

A roof window in a pitched roof is a complete factory made unit with its own frame, and its declared U-value usually covers the unit as made. The flashing and insulation collar around it are fitted on site, and they are part of what makes the installed result work.

A flat rooflight, a lantern or a walk-on unit sits on a kerb or upstand built on the roof deck. The glass may have an excellent figure, but the kerb is a gap in the roof insulation and a path for heat unless it is insulated properly. A well built upstand is insulated inside and out, and the roof insulation runs up it to meet the frame. A poorly insulated kerb can lose more heat than the glass above it.

Plastic domes and some older polycarbonate units have higher U-values than modern glass ones. If you are replacing a basic dome, the improvement in a well specified glass unit is large, and the opportunity to fit the lining and insulation properly goes with it.

New and replacement rooflights have to meet limiting U-values set out in Approved Document L Volume 1, the part of the Building Regulations on the conservation of fuel and power. The 2021 edition is the current one, and it sets a limit for the unit, in terms of the whole window U-value described above. A rooflight that falls short of it does not comply, however good the glass in the middle may look.

The rules apply in two ways. Cutting a fresh opening into an existing roof is building work, and the new unit must meet the standard. Renewing a complete rooflight or roof window is notifiable as well, since windows are controlled fittings: the new unit must meet Part L and be reported to Building Control. A sealed glass unit changed on its own, in a frame that is sound, needs no notification. Where one applies, we send it.

There are a few cases where the standard can be relaxed, such as some listed buildings and historic properties, where the need to protect the character of the building is weighed against energy performance. That is a matter to agree with the local planning authority and Building Control, not to assume. Our guide to Part L and replacement rooflights covers the replacement rules, and a short answer on what U-value a rooflight should have sets out the target in practice.

The U-value is only useful if the figures you compare are alike. A few questions help.

• Is this the whole window value or the centre pane value?
• Is it for the size and type you are buying, or a standard test size?
• Does the figure include the frame, and is it for a kerb mounted unit or a roof window?
• Where does the figure come from: the manufacturer's declared performance, a test report or a calculation?

Be wary of comparing a low number from one product with a different kind of number from another. A triple glazed unit with a centre pane figure of 0.5 may have a whole window figure much closer to a good double glazed unit. And be sceptical of a precise sounding figure with no source behind it: a declared value from the maker, for the product as it will be supplied, is worth more than a rounded number in a leaflet.

A lower U-value is not always worth the extra cost or weight. For a small rooflight over a landing, the gain from a better specification is small in absolute terms because the area is small. For a large flat rooflight or lantern in a kitchen extension, the glass area is large, the heat loss is real and the specification deserves more attention.

A better U-value does more than reduce the heating bill. The inner pane stays warmer, so you feel less cold radiating down from the glass, and the glass is less likely to mist with condensation in winter. A warm-edge spacer and a well insulated frame help in the same way, because condensation shows first where the surface is coldest. If you are curious about it, our answer on whether rooflights can cause condensation explains why.

The same logic applies to the lining. A rooflight with an excellent U-value set in a cold, uninsulated lining still creates a cold spot, and the sill is where staining and damp appear. The insulation has to wrap the frame and carry on down the lining, with a vapour control layer lapped onto the frame behind it. Only then is the lining board, plaster and paint applied. We explain each layer in our guide to how a rooflight is finished inside, and our written quote records the roof build and how the lining is insulated and finished.

The glass number is one part of the performance you live with. The whole result depends on the glass, the frame, the kerb, the insulation and the lining working as one. That is why we look at the roof build and the ceiling at survey, before we talk about a particular unit, and why the finish schedule names the layers rather than only the glass.

The right level depends on the room, the roof and what you want from it. A few cases illustrate this.

For a roof window in a loft bedroom, a good double glazed unit with a low emissivity coating, argon fill and a warm-edge spacer is the usual starting point. Triple glazing becomes worth considering for a north facing slope, a very exposed position or a room that is heated all day.

For a kitchen extension with a large lantern or flat rooflight, the glass area is large enough that the specification matters. Glazing choice, solar control and the insulation of the upstand should be decided together, because a low U-value and a sun-facing aspect pull in opposite directions, and our guide to solar control glass for rooflights covers that balance.

For an older house, such as a Georgian terrace in Southernhay or a cob cottage in the Culm valley, the aim is the best specification the building can take without harming it. A conservation rooflight may have a different profile and a slightly different performance from a standard unit, and the choice is made with the planning and Building Control position in mind.

The full range of options, from specification to controls, is on our energy efficient rooflights page. When you want a figure for your own room, ask for a quote, and the survey and written finish schedule will set out the specification and the build behind it.