Walk-on rooflight U-value: is walk-on glass energy efficient?

A U-value measures how quickly heat passes through a building element. The lower the number, the better the element holds heat in. It is written in watts per square metre per degree of temperature difference, or W/m²K.

For a wall or a roof, a single figure is enough. For a rooflight it is not, because a rooflight is made of different parts that lose heat at different rates. Glass loses heat one way. The frame loses it another way. The edge of the glass, where the spacer sits, has its own behaviour. A walk-on unit adds something more: the structural laminated sheets that carry the load are thick, and every layer affects how the stack performs.

So a number quoted in a brochure needs a second question. Is it the centre of the glass, or the complete unit including frame and edge? The centre-pane figure is always the better-looking one. The whole-unit figure is what the room actually experiences, and it is the one a building inspector will want to see. Our page on energy efficient rooflights explains how to read these figures for any type.

A walk-on rooflight for a roof is not a single sheet. It is built from several layers, and each one affects the heat loss.

• The outer walking surface. Toughened, laminated sheets, sometimes with a slip-resistant finish. This is the part that carries the load.
• A sealed cavity. A gap filled with gas, usually argon, which conducts heat less readily than air.
• A low-emissivity coating. A thin invisible layer on one glass surface, which reflects heat back into the room.
• An inner laminated pane. It completes the sealed unit and is laminated so that, if it breaks, it stays in place overhead.
• A warm-edge spacer. The strip holding the panes apart. A poor spacer is a cold line round the edge of the glass.

Triple glazing adds another pane and another cavity. It lowers the U-value further, but it also adds weight and thickness, and the frame has to be deep enough to hold it. For a panel that people will stand on, that extra weight has to be carried by the same bearing, so the stack is designed as one. See how thick walk-on glass is for a fuller account of the build-up.

The glass is only part of the story. On a flat roof, heat often leaves around the glass as well as through it.

A metal frame conducts heat faster than glass does, so a frame without a thermal break can be colder on the inside than the pane next to it. That is where condensation tends to show first, on the inner edge near the frame. A thermally broken frame has an insulating strip in the middle of the metal, so the inside face stays closer to room temperature.

The upstand or kerb is the second route. A kerb that is thin, hollow or uninsulated acts as a thermal bridge between the cold roof and the warm ceiling. A solid kerb, fully insulated and lined inside, closes that route. The same details that improve the U-value also help the sound side, because a dense, sealed upstand is quieter than a hollow one. Our page on flush and upstand walk-on rooflights shows how the two types differ here.

Energy performance is covered by Part L of the Building Regulations, in England the conservation of fuel and power. It sets limits for the thermal performance of new and replacement glazing, and the limit that applies depends on what the work is and which route is used to show compliance.

We do not quote a limit here, because the figure differs between a new opening, a replacement and an extension, and the published values are revised from time to time. What we do is confirm the applicable figure at the survey, choose a unit whose whole-unit value meets it, and install to current Building Regulations. Where the work is notifiable, we handle the Building Control notification. Our guide to walk-on rooflights and Building Regulations covers the wider rules.

A walk-on panel is also a specialist product. Manufacturers publish U-values for their own systems, and those published values are the ones we work from. We do not rely on a general figure for glass of a given thickness. Laminated sheets, interlayers and the cavity depth all change the result.

A well-insulated walk-on rooflight is usually a quiet one too. The two qualities overlap more than people expect.

Thick laminated glass, a gas-filled cavity, a firmly seated unit and a solid insulated upstand help with heat loss, and they help with rain, aircraft and road noise as well. Around Crawley, where the Gatwick routes and the A23 and M23 carry sound across many neighbourhoods, a room below a terrace benefits from both. We write the glass build, the seals and the upstand finish into the quiet spec with every quote, so that warmth and sound are decided together.

There is a trade-off on light. Extra layers reduce the daylight that passes through a little, which is covered in our answer on whether walk-on rooflights let in less light. The hub page for walk-on rooflight installation shows how the pieces fit together, and you can get a quote when you are ready to compare.

Does the glass feel cold to stand on in winter?

The top surface takes on the temperature of the air outside, so it can feel cold to a bare foot. That is separate from how much heat the unit lets through, which is what the U-value measures. Our answer on frost and snow on walk-on glass deals with the winter side.

Will it get hot in summer?

Glass in full sun warms up, and a solar control coating can limit the heat entering the room. See whether walk-on glass gets hot underfoot for the detail.

Is triple glazing always worth it?

Not always. It lowers the U-value, but it adds weight, cost and depth, and a well-specified double unit with a low-emissivity coating can meet the requirement. We compare the options for your roof at the survey.

Why does the U-value of my unit look worse than the brochure?

Brochures often quote the centre of the glass. The whole unit includes the frame and the spacer, and those lose more heat, so the complete figure is higher. It is the honest one to compare.