Do roof lanterns lose a lot of heat?
A modern lantern loses more heat per square metre than the insulated flat roof it replaces, and considerably less than the lanterns fitted twenty years ago. On a typical rear extension the lantern accounts for a modest share of the total heat leaving the room, and the two things that decide whether it feels like a problem are the frame specification and the way the kerb underneath is insulated.
The honest framing is not “does it lose heat”, because every piece of glazing does. It is “does it lose enough to change how the room behaves in January”, and that is answerable. This page works through where the heat actually goes and what to specify, which sits alongside the wider view on roof lantern installation.
Comparing a lantern with the roof it replaces
An insulated flat roof built to current standards performs several times better thermally than any glazed unit. Swapping part of that roof for glass therefore raises the heat loss through the roof plane, and pretending otherwise helps nobody.
The size of the effect depends on proportion. A lantern occupying a small share of a well insulated roof, in a room whose main losses are the rear elevation glazing and the floor, changes the picture less than people expect. A very large lantern over a small room is a different calculation, and it is one worth doing before the size is fixed rather than after.
Where the heat actually leaves a lantern
Four routes, and they are not equal. Through the glass, which on a modern low emissivity double glazed unit is the best performing part. Through the frame sections, which on a poorly specified lantern is the worst. Around the edge of each sealed unit, at the spacer bar. And through the kerb it sits on, which is not part of the lantern at all and is frequently the largest single loss.
People shop for glass and ignore the other three. A lantern with excellent glass, an unbroken aluminium frame and an uninsulated timber kerb will underperform a lantern with ordinary glass and both of the others done properly.
The thermal break, and what it is worth
Aluminium conducts heat extremely well, which is a virtue in a saucepan and a problem in a window frame. A thermally broken section separates the outer and inner halves of each extrusion with polyamide strips, so there is no continuous metal path from the cold side to the warm one.
Without it, the internal face of every bar sits close to outside temperature, and in a kitchen in December that means condensation running down the bars onto the plaster. It is the single most important line on a lantern specification and it is the one most often absent from a cheap quote. Ask for it by name.
The kerb, and the insulation that gets forgotten
A lantern sits on an upstand built off the roof structure, typically 150mm or more above the finished surface. That upstand is a vertical band of construction running right around the opening, and if it is built as bare timber with a covering dressed over it, it is a thermal weak point the full perimeter of the lantern.
Done properly it is insulated on its outer face, continuous with the roof insulation, so the line of insulation runs unbroken from the roof deck up the kerb to the underside of the lantern frame. On a two by three metre lantern that band is ten metres long. It is not a detail, it is a significant area, and it is invisible once the job is finished.
Cold downdraught, and why a room can feel colder than the numbers
Air in contact with a cold surface cools, becomes denser and falls. Under a lantern that means a slow current of cool air descending into the room, and because it arrives from above onto your head and shoulders, it registers far more strongly than the actual temperature drop warrants.
This is the mechanism behind most complaints about a cold room with a glass roof, and it responds to the frame specification and the glass surface temperature rather than to the heating. A warmer inner pane, achieved with a good low emissivity coating and a warm edge spacer, largely removes the sensation without changing the thermostat.
Winter gain, and the side nobody counts
The ledger has two columns. A lantern facing anywhere near south collects low winter sun at exactly the times a room needs it, and on a bright January afternoon a well glazed lantern over a kitchen is a net contributor rather than a net loss.
That is genuinely useful and it is also seasonal and unreliable, so it should not be used to excuse poor specification. The sensible way to read it is that a good lantern is roughly neutral across a heating season, while a bad one is a steady drain from October to March. The trade-off against summer heat is covered on roof lantern glazing and solar control.
Heating a room that has a lantern in it
Two practical points. First, warm air rises to the highest point in the room, which under a lantern is a pocket of glass several hundred millimetres above the ceiling line. That stratification is why an open plan kitchen with a lantern can feel warm at head height and cool at floor level.
Underfloor heating handles this better than radiators, because it warms from the bottom rather than driving a convection loop that ends at the glass. Second, a ceiling fan on slow reverse in winter pushes the warm layer back down and is the cheapest correction available. Neither is required. Both help in a tall extension.
What the Regulations ask of a new lantern
Fitting a lantern is notifiable work in England and it engages Part L, which sets a maximum U-value for new rooflights in an existing dwelling. Any reputable modern lantern with thermally broken frames and a coated double glazed unit meets it without difficulty, and the manufacturer’s declared figure is what gets submitted.
The figure to check is the whole unit U-value rather than the centre pane figure, because the two are not the same and the difference is the frame. Sunspire handles the Building Control notification as part of the job, and the declared values go in with it. Further detail sits on energy efficient rooflights.
Specifying for warmth without going dark
The combination that works on most Essex extensions is a thermally broken aluminium frame, a low emissivity coated double glazed unit with an argon filled cavity and a warm edge spacer, and an insulated kerb detailed continuously with the roof.
Triple glazing is available and it is rarely the right call in a lantern, because the extra pane adds weight, cuts light transmission and improves the U-value less than fixing the frame and kerb would. Spend on the break and the perimeter first. That order of priority is what separates a lantern that is comfortable in February from one that is merely well marketed.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements