01The idea
A frame with a gap in the middle on purpose
A thermally broken frame has a strip of insulating material built into it, so the cold outside edge and the warm inside edge never touch.
Metal carries heat very well. That is a good thing in a saucepan and a poor thing in a rooflight frame. If a single bar of aluminium runs from the outside of the frame to the inside, winter cold travels along it and lands on the inner face, where you can feel it and where damp air turns to water.
A thermal break fixes that by cutting the metal in two and joining the halves with a strip of something that barely carries heat, usually a tough polyamide plastic. The outer part of the frame stays cold. The inner part stays close to room temperature. The two are held together firmly, but heat has no straight metal path across.
So when you see "thermally broken rooflight frame" in a specification, it means a metal frame, almost always aluminium, made in two parts with an insulating barrier between them. Timber and PVC frames do not need one, because those materials are poor conductors already.
You will meet the idea on flat roof rooflights, roof lanterns and large fixed skylights, all of which are often built from aluminium. A small timber or PVC roof window on a pitched roof rarely needs the term at all. If someone quotes you for a big aluminium rooflight and does not mention the break, it is a fair question to ask what is in the frame.
- Outer aluminium section faces the weather and gets cold
- An insulating strip, the break, sits between the two halves
- Inner aluminium section faces the room and stays warm
- The glass sits against the warm half, held by a warm-edge spacer
02Why it matters
What a break does for the room
It makes the frame feel like part of the room instead of part of the roof.
Aluminium is popular for rooflights and lanterns because it is strong and can be made in slim, narrow profiles. That strength is the reason a thermal break is worth having. A big flat roof lantern has a lot of frame, and an unbroken metal frame on a large opening is a long run of cold surface above your head.
With a break, three things improve. The frame loses less heat overall, which helps the unit's U-value (how fast heat passes through, where a lower figure is warmer). The inside face stays warmer, so you feel less chill. And because the inner surface is warmer, the damp air in a kitchen or bathroom is much less likely to condense on it and run down the plaster.
That last point is a symptom of heat loss rather than a problem in itself. We cover how it shows up on our page about condensation on flat rooflights.
It also helps the wider build. A broken frame meets an insulated kerb without leaving a metal bridge between the warm lining and the cold roof deck. In a kitchen-diner under a lantern, that is the difference between a frame you forget about and a cold line you notice every winter morning.
The glass can be excellent and the room still feels cold if the frame is carrying winter in.
03Materials
Thermal breaks, timber and PVC compared
Each frame material gets to a warm result in a different way.
Timber and PVC are naturally slow to conduct heat, so a single section does the job. Aluminium is the one that needs the break, and it justifies itself on larger spans where a thin metal profile can hold a lot of glass. Our page on aluminium and uPVC roof lanterns goes into the trade-offs of each.
Not every aluminium frame is broken, so ask. A sales sheet that says "aluminium frame" and nothing else is not enough. You want the words "thermally broken" and ideally a stated U-value for the whole rooflight, not just for the glass in the middle, which always looks better than the unit as a whole.
For a rooflight that you want to stay comfortable under, our own starting point is a thermally broken aluminium frame on larger openings, or timber or PVC where the room and the roof suit them. The skylight you choose should be judged as a whole unit: glass, spacer, frame, kerb and lining together.
Frame materials at a glance
- Thermally broken aluminium
- Strong, slim and warm once the break is built in
- Plain aluminium
- Strong but a cold bridge, best avoided on a living room roof
- PVC
- Naturally warm, bulkier profiles
- Timber
- Naturally warm, needs a protective finish
04Your quote
What to ask when you are choosing
Five plain questions will tell you whether the frame is doing its share.
The frame is only one part of a warm opening. A broken frame on an uninsulated kerb still leaves a cold band in the roof, and a good break next to an aluminium spacer bar still has a cold edge. The parts have to work together, which is the whole point of our warm specification. The related page on warm-edge spacers and thermal breaks covers the glass edge.
If you want us to look at your roof, a free survey and written quote is the place to start, and we will tell you which frame suits the room.
- Is the aluminium thermally broken, and is that written in the quote?
- What U-value is quoted for the whole unit, not just the glass?
- Does the glass have a warm-edge spacer?
- Is the kerb or upstand insulated and tied to the roof insulation?
- Does the new unit meet the current Part L limit for a replacement rooflight?
Do you really need a break in the frame?
Is a thermally broken frame the same as a warm-edge spacer?
No. The thermal break sits inside the frame. The warm-edge spacer sits inside the sealed glass unit, around its rim. You want both, because a cold bridge in either one lets warmth escape.
Do all aluminium rooflights have a thermal break?
No. Cheaper or older aluminium frames may not, so check the specification. Ours are specified for warmth, and the details go in your written quote.
Does a thermal break stop condensation completely?
It cuts the risk by keeping the inner frame warmer, but room humidity and ventilation matter too. A warm frame, a warm-edge unit and a decent extractor in a kitchen or bathroom work best together.
Does it affect which U-value I should look for?
It helps you reach it. Our guide to what U-value a rooflight should have gives the targets, and the energy efficient rooflights hub sets out the wider picture.
