Do Roof Lanterns Make a Room Cold?

Heat leaves a room through a lantern in three places: through the glass, through the frame that holds the glass, and through the kerb the lantern sits on. A flat roof is built up in layers of deck, insulation and covering, so it is the glass that makes a lantern a thinner barrier than the roof around it.

That is simply physics, and it applies to every rooflight, a lantern included. A lantern has more glass than a flat rooflight of the same footprint because its faces slope up from the kerb, so the glass area is larger than the opening it covers. What decides whether you feel it is how well each of the three parts is made.

• The glass. Double glazing with a low-emissivity coating and a warm-edge spacer loses far less heat than plain double glazing. Triple glazing loses less again.
• The frame. An aluminium frame with a thermal break (a strip of low-conductivity material, commonly polyamide, that separates the inside and outside faces of the metal) greatly reduces the frame's role as a bridge for heat.
• The kerb. The upstand under the lantern is insulated like the rest of the roof, with the insulation running up the kerb and meeting the frame.

A lantern made well in all three is a different thing from the single-glazed or poorly framed lanterns that gave the type its reputation for being chilly.

The idea that lanterns are cold comes from older ones. Lanterns fitted a generation ago were often single or basic double glazed, in frames without a thermal break, on kerbs with little or no insulation. Warm air from the room rose to the glass, cooled on its surface, and dropped back as a draught of cold air.

Two symptoms give an old lantern away. The inner glass surface feels cold to the hand on a winter morning, and the frame shows condensation or dampness. Both point to a unit that is losing more heat than a current one would. If you see a failed sealed unit (misting between the panes), the edge seal has let moisture into the gap, any gas fill is likely to have escaped, and the glass will perform worse than when it was new. Our page on why a rooflight mists covers that case.

Replacing a whole lantern is where the improvement is easiest to make, because the new unit has to meet the current standard. In an existing home, a new rooflight must reach 2.2 W/m²K under Part L of the Building Regulations. Makers publish a whole-unit figure for each lantern, so the replacement can be checked against that standard before it is ordered.

If you are choosing a new lantern, these are the specification points that change how warm the room feels. The maker's data sheet states each one.

Triple glazing is worth discussing when the lantern is large or sits over a room you use in the evening. It adds weight, so the kerb and the roof structure must carry it, and that is checked at the survey. The choice between the glazing types is covered in our guide to double and triple glazed rooflights.

Ask for the U-value of the whole lantern, not only the glass. A centre-pane figure ignores the frame and the edge, so it flatters the unit. The whole-unit figure is the one that matches how the room feels.

A lantern also lets sunshine in, which in winter helps. The sun warming a floor through a lantern is free heat, and a south-facing lantern can offset a share of its own loss in daylight hours. At night the loss continues, which is why the insulating specification matters more than orientation.

The reverse problem is summer overheating. A glazed lantern over a sunny room can trap heat, so glass is often specified with a solar control coating that reflects part of the sun's energy. Opening vents in the lantern let hot air leave at the highest point of the room. Options for vents are shown on the roof lantern installation page.

Blinds fitted inside the lantern cut glare and heat in summer, and in winter a closed blind adds a layer of still air between the room and the glass. A lantern is not a fixed thermal condition: how it behaves also depends on how it is used.

The way to be sure a new lantern will not leave the room cold is to see the numbers before you commit. At the survey we measure the opening and the existing kerb, check how the roof is insulated, and note the room below and the way it faces. The written quote then names the glazing specification, so you can compare it with what the maker publishes.

If the existing kerb is uninsulated, the cheapest time to correct that is while the old lantern is off. A new kerb built with insulation as part of it is far easier to put in during a replacement than to add later.

Where a lantern is going onto a roof that has never had one, the point to settle first is the roof build-up under it. Fitting a lantern to an existing flat roof explains how the kerb and the insulation are brought together.