01Straight answer
Cold lanterns and warm lanterns are different things
A lantern is a big area of glass, so it has to be built warm to feel warm. Built that way, it is comfortable all winter.
Are roof lanterns cold in winter? An old one can be. We have all sat in a conservatory-style room with a plastic or single glazed roof, where you wear a jumper in January and cook in July. Modern lanterns are a different product. They use sealed double glazed units with low-e coatings, gas-filled cavities, insulated edges and frames that do not carry cold straight through from outside to in.
The lantern is still a hole in an insulated roof, and glass loses more heat than a solid, insulated roof does. That is plain physics, and a good fitter will not pretend otherwise. What matters is how big the difference is and whether you feel it. With a decent specification you do not sit under a cold spot. You sit under a bright one, and the room around it stays at a steady temperature.
- 2.2W/m²K, the Part L limit for a replacement rooflight, and a ceiling not a target
- 1.0 to 1.4W/m²K, a typical good double glazed rooflight unit
- 0.15 to 0.18W/m²K, a well insulated flat roof for comparison
02Where heat goes
Why a lantern has more to get right
Four surfaces, a lot of edge and a kerb: every one is a chance for heat to slip out, and every one can be fixed.
A flat rooflight is one pane in one frame. A lantern is a small glazed roof with four or more sloping faces, a ridge, a corner at each junction and a long run of frame. More glass area means more heat loss through the panes. More frame means more chances for a cold bridge, where a conductive material carries cold straight through. And the whole thing stands on a kerb that has to join the roof insulation.
A lantern's height keeps the coldest glass further from your head, but warm air pooling in it is lost faster, so the spec still has to be warm.
A warm lantern
Low-e double or triple glazing, warm-edge spacers, a thermally broken frame, an insulated kerb tied into the roof, and a lined, sealed reveal inside.
A cold lantern
Older double glazing, an aluminium spacer bar, a frame with no thermal break, and a kerb with a gap in the insulation round it.
03Frames and glass
Choosing the glass and frame for warmth
The frame material and the glazing choice together set how the lantern behaves in the coldest week of the year.
Glass first. Double glazing with a low-e coating and argon is the standard choice, and it is enough for most kitchens and dining rooms. Triple glazing makes sense on a large lantern, a north facing roof or a room you want extra snug, though the unit is heavier and thicker. We weigh it up in is triple glazing worth it on a rooflight. We fit warm-edge glazing as standard, so the spacer between the panes is not plain aluminium.
Now the frame. Aluminium is slim and strong, and it is the usual choice for lanterns because it can span wide glass without bulk. Left alone it conducts cold, so the frame needs a thermal break, a strip of insulating material that splits the inside and outside halves. Our page on thermally broken frames explains it, and aluminium against uPVC lanterns compares both materials for warmth and looks.
Last, the kerb. If the roof insulation runs right up to and around it, with no gap, there is no cold ring. The whole recipe is set out on our warm specification page.
The glass gets the credit, but the kerb and the frame decide whether you feel it.
04Living with it
Making the most of a lantern from November to March
Four habits and one fitted extra keep the room comfortable when the weather turns.
Even a well-built lantern benefits from a little help on the coldest nights. Because it brings in so much daylight, a winter morning with low sun can warm the room faster than the heating does. In the evening, the story reverses, and glass is the coolest surface in the room. A blind closes that gap.
- Fit a thermal blind that closes at dusk and traps a layer of still air against the glass.
- Keep radiators or underfloor heating running, so the room does not start the evening cold.
- Put furniture where air can circulate, instead of blocking the warm flow from the heating.
- Wipe condensation from the frame and check that the room has ventilation.
Condensation on a lantern is a sign that warm, moist air is meeting a cold surface. With a warm-edge spacer and a thermally broken frame it appears far less, and ventilation takes care of the rest. If a lantern keeps dripping, read why a rooflight drips in winter.
Planning a lantern? Our roof lanterns page shows the range, and do skylights make a room cold covers the wider question for every kind of roof glazing.
Will a glass roof leave the room chilly from November on?
Will a lantern make my heating bill go up?
Replacing a plain flat roof with glass adds some heat loss, and the better the specification, the smaller that gain. Replacing an old cold lantern or dome with a modern one often improves matters. The extra daylight also means less artificial light.
Is a lantern colder than a conservatory?
A modern lantern on an insulated roof is far warmer than an old conservatory with a polycarbonate or single glazed roof, because the kerb and glass are both better, and the roof around it is solid.
Should I choose triple glazing?
Not by default. Good double glazing is enough for most rooms. Consider triple on a big lantern, on a north facing roof, or if the room is used all winter by people who feel the cold.
Does a lantern need Building Control approval?
A new lantern or a full replacement is notifiable under Part L, and we handle the notification. Only swapping the sealed unit in an existing frame is not.
