More than through the insulated roof it replaced, inevitably, and more than a flat rooflight over the same opening because the sloping faces give a larger glazed area than the hole they cover. How much more depends on the glass, the frame sections and, more than either, on whether the kerb underneath was insulated properly.
How a rooflight loses heat generally
The mechanism and the figures are on does a rooflight lose more heat than a window.
What follows is what is particular to a structure with many bars and a larger surface than its opening.
The area problem
The one specific to this shape.
A lantern over a two metre square opening has considerably more than four square metres of glass, because every face is longer than its plan projection.
So the same opening loses more heat as a lantern than as a flat pane.
How much more
Depends on the pitch, and it is not trivial.
A steeper lantern has longer rafters and more glass on the same footprint, so the pitch chosen for appearance has a thermal consequence attached to it.
Where the balance sits is on what pitch should a roof lantern be.
The frame
A larger share of the loss than on any other rooflight.
A lantern has bars every few hundred millimetres, and aluminium conducts heat readily, so the frame is a meaningful part of the total rather than an edge effect.
Thermal breaks
What separates a good system from a cheap one.
A thermally broken section has an insulating barrier between the inside and outside faces of the aluminium, which stops the metal carrying heat straight through.
Without it the bars run cold and the room feels it.
Why cold bars show
They condense before the glass does. In a kitchen in January, water appearing along the bars rather than on the panes is telling you the frame is the coldest part of the assembly.
The kerb
Where most of the avoidable loss actually is.
An upstand insulated on one face, or not at all, is a continuous cold ring around the whole perimeter, and it loses more than the glass specification saves.
How it is built is on what is a builders kerb.
Why the kerb is missed
Because it is somebody else’s trade.
The lantern is bought with a performance figure and the kerb is built by a roofer or a builder to no particular standard, and the two are never assessed together.
It is the single most common weakness we see.
What good glass gives you
A meaningful step, and it is decided at order.
Low emissivity coatings, an argon filled cavity and a warm edge spacer together make a substantial difference to a lantern’s figure, and none of them can be added later.
What a spacer does is on what is a warm edge spacer.
Triple glazing
Better, heavier, and not always the right call.
A third pane improves the figure and adds weight the frame and the kerb have to carry, and on a lantern the frame is often the limiting element rather than the glass.
The argument is on is triple glazing worth it in a rooflight.
What the room actually feels
Radiant cold rather than a draught.
A large cold surface overhead takes heat from people beneath it by radiation, which is why a room can be at temperature and still feel cold under a poor lantern.
Where the heating is
Worth thinking about at design.
Underfloor heating under a lantern works well because it warms the surfaces the cold glass is drawing from, and a radiator on the far wall does considerably less.
What it gains in winter
Not nothing, and it is worth stating.
A lantern collects solar gain on bright winter days, and a south facing one puts real warmth into a room in the middle of a cold clear day.
The night time picture
Where the loss all happens.
A clear night sky is effectively very cold, and a glazed roof radiates to it, which is why the losses are concentrated in the hours nobody is looking at the lantern.
Blinds and the winter figure
A modest and real improvement.
Closed blinds trap a layer of still air against the glass, which reduces the loss a little and keeps the inner surface warmer, which matters more for comfort.
What is available is on do roof lanterns come with blinds.
Where it counts in a calculation
As part of the whole extension.
The regulations assess the envelope together, so a well insulated wall and floor can carry a large glazed area, and a poorly insulated one cannot.
The limits are on can a rooflight be too big for building regulations.
What we would specify
Three things, in order of value.
A fully insulated kerb, thermally broken frame sections, and a good sealed unit with solar control. The first is free of any product decision and the most often neglected.
Splaying the reveal and the cold line
Two reasons to do it and only one is about light.
Angling the internal faces of the kerb spreads daylight into the room, and it also puts more insulation depth into the corner where the kerb meets the ceiling, which is exactly where the cold line forms.
It is the same detail solving two problems.
Comparing two quotations on this
One question does it.
Ask what the kerb is insulated with and on which faces, because a quotation that answers precisely is from somebody who has thought about the thing that matters and one that does not is not.
What we insulate on a lantern
We insulate the kerb on every face and we say so in the quotation, because it is the part of a lantern installation that nobody sees and that decides how the room feels in January.
The whole picture is on the roof lantern installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.