Do skylights lose a lot of heat?

Why glass loses more than the roof around it

A pitched or flat roof built to current standards has a deep layer of insulation, often finished with a vapour control layer and a plasterboard ceiling. A rooflight takes the place of a section of that build-up with a pane, or two or three panes, with a thin gap between them. Even the best glazing is a much thinner barrier than the roof beside it, so heat leaves faster through the glass than through the same area of roof.

The loss goes two ways. Heat conducts through the glass and frame, and warm room air also moves past the cold inner surface and cools. On a winter night that is why the area under a rooflight can feel a little cooler than the rest of the room. It is a normal effect of the material, and the choices below can reduce it a good deal.

The limits Part L sets

Building Regulations do not leave this to chance. Under Part L (2021), Table 4.2, for existing dwellings in England, a roof window takes the same limit as a window: 1.4 W/m²K, with Band B on the Window Energy Rating scale as the alternative test. A rooflight, meaning a flat or lantern style unit, is allowed 2.2 W/m²K. A lower figure means less heat lost.

Those numbers describe the whole unit, glass and frame together, not the glass alone. That is the reason to ask for the figure for the unit named in your quote, not for a general description of the glazing. The detail is set out in our guide to rooflight U-values and Part L, including how the two limits are applied and what the figure on a data sheet does and does not cover.

A full rooflight swap, frame included, falls in the controlled-fitting category. Where that applies we deal with the Building Control notification, so the limit is tested against the real product and not assumed.

What changes the loss in a real unit

Several parts of the specification move the figure up or down, and they are worth asking about one at a time.

• Number of panes. Double glazing is the usual starting point. A third pane lowers the loss further and adds weight, as covered in our answer on triple glazing.
• Low-emissivity coating. A thin coating on one pane reflects heat back into the room. It is nearly invisible and does a great deal of the work.
• Gas fill. The gap between the panes is commonly filled with an inert gas, which conducts less heat than air.
• The spacer bar. A metal bar at the edge of the glass is a route for heat, and a warm edge spacer cuts that route.
• The frame. Thermally broken aluminium, timber and PVC frames each conduct differently, and the frame is a large share of a small window.

Solar control coatings are a separate matter. They reduce the heat that enters in summer and are discussed in our guide to solar control glass. Some coatings reduce winter solar gain a little, so the glass should be chosen with the room's aspect in mind.

The kerb and the insulation around the glass

A good pane can still sit in a poor opening. On a flat roof, the kerb is the upstand the rooflight sits on, and it is a hollow box unless it is insulated. An uninsulated kerb lets heat go straight round the glass, and it is also the coldest surface in the room, which invites condensation on the reveal.

On a pitched roof the equivalent is the gap between the window frame and the rafters. It is filled and sealed to the maker's instructions, with the insulation carried right up to the frame. The reveals inside are lined and insulated so there is no bare, cold edge.

This is where planning the rooflight into the roof helps. The kerb or opening size goes to your builder in writing, the roofer finishes the covering up to it, and the insulation and the plasterer's reveals follow in the right order. Done together, the opening performs as the data sheet says. Cut into a finished roof, the joins are harder to get right.

How much it matters for the heating bill

A single rooflight in a heated room adds a modest loss next to the walls, floor and the rest of the roof. The effect grows with the glazed area, so a roof lantern, a run of flat rooflights or a glazed extension roof needs more thought than one roof window on a landing.

What we can do is put the specification in writing. Each quote lists the unit, its glazing and spacer, and the delivery time the manufacturer gives, so it can be set beside another quote like for like. Our page on energy efficient rooflights sets out the options in more detail.

Signs that a unit is losing more than it should

A rooflight that is cold to the touch on a winter morning is not by itself a fault, because glass is always the coolest part of the room. A few other signs are worth noting.

• A persistent fog or droplets between the panes, which means the sealed unit has failed and needs replacing.
• A strip of damp or mould on the reveal, which points to a cold edge or an uninsulated kerb. Our guide to condensation on rooflights covers the causes.
• A draught at the frame, which suggests a seal or a fixing that was not finished properly.
• Frost that forms on the glass much earlier than on the windows nearby.

Related questions

Is a flat rooflight worse for heat loss than a roof window?

Not by nature. Part L gives the two types different limits, 2.2 W/m²K for a rooflight and 1.4 W/m²K for a roof window in an existing dwelling, because they are built differently. What matters is the figure for the named unit and how well the kerb is insulated.