Are roof lanterns cold in winter?

The word “lantern” carries a lot of Victorian baggage. The originals were single sheets of glass in slim timber or cast-iron bars, no seal worth the name, and they streamed with condensation and leaked warmth all winter. That is the mental picture most people are working from when they ask the question. A modern aluminium or composite lantern with a sealed double or triple-glazed unit is a different object entirely, and it behaves nothing like the one your grandmother had over the scullery.

When a recent lantern still feels cold, it is usually one of three things, and none of them is the glass on its own:

• Air leakage. A gap where the lantern meets the upstand, or where the upstand meets the roof, lets a slow draught into the room. Cold air moving across your skin reads as much colder than still air at the same temperature, which is why a room can feel draughty while the thermostat says it is warm.
• A cold frame. The bars and perimeter of a cheap lantern can conduct heat straight through to the outside. That chills the inner surface, and a cold surface radiating down at you feels cold even when the air is fine.
• Condensation. Warm, moist air from a kitchen or bathroom meets the coldest surface in the room, which on a poor unit is the frame, and beads up. People read the water as a leak or as proof the lantern is freezing. It is neither, but it is a sign the frame is running colder than it should.

Fix the sealing and specify a frame that does not conduct, and the cold complaint disappears. The glass is rarely the culprit.

Heat loss through glazing is measured by its U-value, in watts per square metre per degree, and lower is better. It is the figure that governs how a lantern performs on a cold night, and it is the one Building Regulations control. Under Part L, a new or replacement rooflight has to meet a ceiling on its U-value, so anything installed to current standards already has to clear a defined bar for winter heat loss.

Rough figures worth knowing:

• Old single glazing sits up around 5.0 and above. That is the leaky original everyone remembers.
• Standard sealed double glazing lands somewhere near 1.6 to 1.4.
• A better double-glazed build-up, with a low-emissivity coating and argon fill, gets down towards 1.2 to 1.0.
• Triple glazing, or a high-specification double, can reach around 0.8.

For comparison, a well-insulated flat roof deck is not far off those better glazed figures once you account for the difference in area. In other words, a properly specified lantern is not the weak point in the room’s fabric that people assume it to be. It is the frame and the seal that decide whether the unit lives up to the glass, which is why the whole-unit U-value matters more than the centre-pane figure. Centre-pane describes the glass alone. Whole-unit averages in the frame and bars, so it reads higher, and it is the honest number for judging winter performance.

Condensation forms when warm indoor air touches a surface below its dew point. In a poorly built lantern the coldest surface is the metal frame, because a plain aluminium profile conducts heat straight out to the winter air. The cure is a thermal break, an insulating layer set inside the frame that separates the inner face from the outer one. It keeps the inside of the frame warm enough that moisture stays as vapour instead of running down the bars in the morning.

A kitchen or bathroom throws a lot of moisture into the air, and those are exactly the rooms lanterns tend to sit over. So a lantern above a hot, humid room needs a warm-edge spacer bar around the glazing and a thermally broken frame as a matter of course, not as an upgrade. Get that right and the glass stays clear on a cold morning. Get it wrong and no amount of expensive glazing will stop the frame sweating.

Here is the part that turns the question on its head. On a bright winter day, a south-facing lantern is a net gain, not a loss. The sun sits low, and a lantern presents a large area of glass to it, so it collects free solar heat straight into the room through the middle of the day. In Southend the estuary makes this stronger than it is inland: the horizon to the south is open across the water, so there is nothing casting the roof into shadow, and the glass catches the sun from mid-morning until it swings west.

This is the flip side of the summer overheating problem this business is built around. The same glass that needs a low G-value to keep July bearable is the glass that hands you warmth in January. The trick is to specify for both seasons at once, which usually means a solar-control unit that trims the peak summer gain while a good U-value holds the winter warmth in. Our energy-efficient skylight installation work is exactly this balancing act, and on a north-facing lantern the sums come out differently again, because there is little winter sun to collect and the priority swings fully onto keeping heat in.

If your existing lantern feels cold, work out which of the three causes it is before spending anything. Hold the back of your hand near the perimeter on a cold day: a draught means the seal or the upstand, not the glass. Water on the frame in the morning means a cold, unbroken frame. If the whole thing is single glazed or the seals have gone milky, the unit itself has reached the end of its life, and replacing it is the moment to fix all three at once.

When you are specifying a new one, ask for the whole-unit U-value in writing, confirm the frame is thermally broken, and check the glazing carries a warm-edge spacer. Then think about orientation, because a south-facing lantern wants glass that works in both summer and winter rather than one season only. Our roof lantern installation page walks through the full specification, and the skylight installation costs page shows where the glazing choices land on price. If you would rather we looked at it, tell us the orientation and the rough size and we will come back with a specification. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the glass.