More than the roof they replace, less than most people fear, and the losses are not where people look. A modern aluminium lantern with argon-filled double glazing comes in around 1.2 to 1.5 W/m²K for the whole unit. The flat roof it sits in is around 0.15. So per square metre it is roughly eight to ten times leakier. But a lantern is a small share of the total envelope, and in a well-built extension it typically accounts for a modest slice of the heat leaving the room. The bigger question is whether the kerb under it was built properly, because that detail loses more heat than the glass on plenty of installations.
The figures, side by side
| Element | Typical U-value, W/m²K |
|---|---|
| New flat roof build-up | 0.15 to 0.18 |
| New cavity wall | 0.18 to 0.26 |
| Modern double glazed lantern, whole unit | 1.2 to 1.5 |
| Triple glazed lantern, whole unit | 0.9 to 1.2 |
| Old timber conservatory roof, single glazed | 5.0 and upwards |
Read that table twice. The gap between a lantern and a roof is real and large. The gap between a lantern and the polycarbonate or single glazed roof it usually replaces is larger still, and in the opposite direction. Anybody swapping an old conservatory roof for a glazed lantern is making the room dramatically warmer, not colder.
Whole-unit figures, and the number suppliers prefer to quote
Ask what the U-value refers to. There are two numbers and they are not close. The centre-pane figure describes the glass alone in the middle of the pane, away from any edge, and on a good sealed unit it might be 1.0 or better. The whole-unit figure includes the frame, the rafter bars, the ridge and the edge of every sealed unit, and it is the only figure that describes what your roof actually does.
On a lantern the difference between the two is unusually wide, because a lantern is mostly edges. A four-pane lantern has a ridge, four rafter bars, four eaves sections and the perimeter of four sealed units, all in a few square metres. Every one of those is a worse performer than the glass between them. A supplier quoting 1.0 for a lantern is quoting the glass.
The kerb, which is the loss nobody counts
A lantern sits on an upstand built off the deck, and that upstand has an inside face, an outside face and a top. It is a wall, standing in the middle of your roof, and it needs insulating like one.
Built badly it becomes a cold bridge running the entire perimeter of the lantern. Timber studwork with the insulation stopping at the deck, the covering dressed up the outside and plasterboard on the inside, gives you a continuous strip of poorly insulated construction round the whole opening. On a 2m by 1.5m lantern that is seven metres of it. Built properly, the roof insulation continues up the kerb without a break and laps the frame, so the insulation line is unbroken from deck to glass.
This is one of the four details we guarantee for ten years, and it is invisible the moment the plasterboard goes on. You cannot inspect it afterwards, which is precisely why it gets skimped.
Frames, thermal breaks and where cold arrives
Aluminium conducts heat about a thousand times better than the timber it replaced, which is a problem for a frame. The answer is a polyamide thermal break: a strip of low-conductivity plastic separating the outer aluminium section from the inner one, so there is no continuous metal path from cold side to warm side.
Break width is what separates a good lantern from a cheap one, and it is rarely advertised. Where it shows up is condensation. A frame with an inadequate break runs cold on its internal face in January, and moist kitchen air condenses on it and runs down the bars. That is a thermal fault presenting as a water problem, and the condensation page works through how to tell the two apart.
The Part L limit a lantern has to clear
Part L sets a limit for rooflights, and lanterns are assessed under it. The standard applied to a new rooflight in an existing dwelling has for some time sat around 2.2 W/m²K measured in the vertical plane, which converts to a more demanding figure once assessed in the plane of the roof. Any current lantern from a mainstream manufacturer clears it comfortably.
There is a second constraint that catches people out: the total area of glazing in an extension is limited relative to floor area, with an allowance and a trade-off calculation if you exceed it. A very large lantern combined with a wall of bi-fold doors can push you over. That is a design-stage conversation, and it is settled with Building Control before the order goes in, not after.
What you actually feel in the room in January
Downdraught, mostly. A cold glass surface cools the air touching it, that air becomes denser and falls, and you feel a draught even though nothing is leaking. Under a large lantern this is noticeable at the perimeter and is the honest downside of a lot of glass overhead.
The fix is warm surfaces, not more heating. Better glass, a proper thermal break and a well-insulated kerb keep the internal surface temperature up and the downdraught stops. It is also worth siting a heat source under the coldest part of the room rather than assuming the lantern is a problem to be heated around.
Solar gain, which runs the other way
A lantern is not purely a loss. It is pointed at the sky, so on a bright winter day it collects a useful amount of free heat, and a roof-mounted glazed area gathers considerably more low-angle winter sun than a north wall ever will. Over a heating season the gain offsets a meaningful part of the fabric loss.
The trade-off is summer, when the same geometry becomes a liability. Solar control coatings reduce the gain in both seasons, which is why the choice is not automatic. On a north-facing or shaded rear roof, plain low-emissivity glass with a high g-value is often the better annual answer.
Where the money is best spent
In order: get the kerb insulated continuously, because it costs almost nothing extra during the build and cannot be corrected later. Then specify a frame with a proper thermal break. Then argon fill and a soft-coat low-emissivity glass, which is standard on anything decent. Triple glazing comes last, because on a lantern the added weight, thickness and cost buy a smaller improvement than the same money spent on the perimeter.
We survey before quoting, and the kerb build-up is written into the specification with the insulation line described rather than assumed. Fifteen years installing rooflights across Oxford and 25 miles around, with a ten-year guarantee on the workmanship. Call 01865 704245.
Helpful reading on this
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