Are roof lanterns energy efficient? Read the whole-unit U-value

Every lantern has a glass figure and a whole-unit figure. They describe different things, and only one of them is the warmth of the room.

A U-value measures how quickly heat passes through a material. A lower figure means slower heat loss. The figure printed large on many data sheets is for the centre of the glass unit alone, measured well away from the edges. It flatters the lantern, because the edges are where a glazed unit loses more heat, through the spacer that separates the panes and the frame that holds them.

The whole-lantern figure includes all of that: the glass, the spacer, the aluminium bars, the ridge, the eaves and the junction with the roof. Because a lantern has far more frame for its glass area than a single large pane, its whole-unit figure sits noticeably above its centre-pane figure. That is physics, not a fault. A long bar layout, with many small panes, loses more through the frame than a simple layout with fewer, larger ones.

When you compare quotes, ask which figure is being given. If two lanterns quote the same centre-pane number and one gives a whole-unit figure as well, the second is the one telling you the truth about the roof. Our page on roof lantern glazing bars shows how bar count follows size.

The glass build has the largest single effect, but the frame and the base complete the picture.

In the glass unit, a low-emissivity coating reflects heat back into the room, and argon in the cavity slows the movement of heat across it. Triple glazing adds a third pane and a second cavity, which lowers the figure again but adds weight and cost, and it is not always needed. Our answer on the best glass for a roof lantern goes through the trade-offs.

The frame is aluminium, which conducts heat well, so a good lantern splits each bar into an inner and outer section with an insulating break between them. Without that break the bar would carry cold straight into the room, and condensation tends to follow. Warm-edge spacers do the same job for the glass unit's perimeter, in place of older metal ones. Frame finishes and profiles are covered under aluminium roof lanterns.

Solar control glass is a separate choice from insulation. It cuts the heat the glass lets in on a sunny day, which matters in a south-facing kitchen, and you can read about that side in whether a lantern makes a kitchen too hot.

A lantern never sits directly on a ceiling. It sits on a raised upstand, and that kerb is part of the thermal envelope.

The upstand is a short wall around the opening, usually timber, with the roof membrane dressed up its outer face. If it is left uninsulated, or insulated only on one side, heat leaks around the whole perimeter of the lantern however good the glass is. The fix is continuous insulation: from the roof insulation, up the kerb, and closing tight against the lantern base, with no gaps where the two meet. Gaps also let warm, damp room air reach cold surfaces, which is how hidden condensation starts.

That junction is also where airtightness matters. A draught around the base of a lantern is a path for heat loss that no U-value figure shows. The installer seals the base to the kerb on the inside and lines the kerb with a material that holds the seal over time. The detail is set out in our guide to roof lantern kerbs and upstands, and the same care that keeps heat in also keeps the room quiet.

Part L sets limits on how much heat an extension may lose, and a lantern is one of the elements it judges.

Part L of the Building Regulations covers the conservation of fuel and power. For an extension to a home, the Approved Document sets maximum U-values for new elements and also limits the proportion of the floor area that may be glazed. A lantern counts as a rooflight, which the document treats with its own limiting value, separate from the figure for a vertical window. Because lanterns are large glazed roofs, they can push an extension over its glazing allowance unless the rest of the building is specified to compensate. The designer can trade insulation in the walls and floor against glazing area.

We install to current Building Regulations, and we handle the Building Control notification where the work is notifiable, so the certificate of compliance for the lantern is part of the job. The quote states the whole-unit U-value of the lantern we propose, so your designer can check it against the extension's calculations before anything is ordered. Our energy efficient rooflights hub explains the wider range of high-performing glazing, and a flat glass unit is compared against a lantern in our page on lantern or flat glass rooflight.

Is a lantern colder than a flat rooflight?

A lantern has more frame and more edge for the same floor area, so its whole-unit figure is usually a little higher than a simple flat unit of the same glass. A good build closes most of that gap.

Does triple glazing always pay for itself?

Not always. It adds weight and raises the price, and for a small lantern on a warm, well-insulated extension a good double unit with a low-e coating may be enough. The whole-unit figure helps you decide.

Will an old lantern fail a Building Control check if I replace it?

A replacement lantern is expected to meet the current limiting value for the new element. The old figure does not carry over, which is one reason a tired lantern is worth renewing in full.

Can the kerb be improved on an existing lantern?

Often, but the work is done when the lantern is lifted off, so the two are best replaced together.

If you want to see what a proposed lantern would achieve for your extension, get a quote and we will put the whole-unit figure in writing. The full service is on our roof lantern installation page.