The short answer
No regulation says a skylight must have a thermal break. Part L sets a U-value instead, and a metal frame with no break between its inner and outer parts conducts cold straight through, which makes that figure hard to reach and invites condensation on the frame. Timber and uPVC frames insulate on their own, so the question mostly concerns metal and metal-clad units.
What a thermal break actually is
A thermal break is a strip of low-conducting material, often a plastic such as polyamide, set between the inner and outer halves of a metal frame. Metal carries heat well, so without the strip a cold outer face passes its chill straight through to the inner face, and the frame feels cold to the hand and sits cold against the room's warm air.
Aluminium is the usual metal in skylight and rooflight frames because it is light, strong and slim, and it is the material that needs a break most. Timber has natural insulating value, and uPVC is a plastic with chambers inside the profile, so neither depends on a separate strip in the same way. Roof windows from the main makers often use a timber core with a metal or polyurethane outer wrap, which gives the outside weather resistance while the inside stays warm.
So the right question to ask a supplier is not whether a break exists but how the frame as a whole performs. A data sheet should give a U-value for the complete unit, glass and frame together, not just the centre of the glass.
Where Part L comes in
For a roof window going into an existing house, Approved Document L Volume 1 (Table 4.2) sets 1.4, or a Window Energy Rating of Band B as the lowest acceptable. Rooflights have a row of their own. Neither row mentions thermal breaks, because the document sets a performance figure and leaves the design to the maker. A supplier who says a break is required is describing their product, not the law.
The frame matters to that figure all the same. A unit's U-value is worked out over the whole product, so a conductive frame drags the number up even when the glass is excellent. On a small rooflight the frame is a larger share of the area than on a big window, which is why a poorly detailed aluminium frame can fail a standard that the glass alone would pass.
The 2021 edition applies now, and the 2026 edition takes effect from 24 March 2027, with the tables renumbered 3.2 and 3.3. Our answer on the U-value a roof window needs covers the numbers. If a supplier cannot show a whole-unit figure that meets the row for the product, we move on to one that can.
Condensation on cold frames
Condensation forms when warm, damp indoor air meets a surface colder than the air's dew point. A frame with no break is the coldest part of a skylight, so it is where moisture appears first: beads along the edge, a wet sill, then dark mould in the corners or stained plaster in the reveal.
Approved Document C addresses condensation hidden inside the roof build-up, which it calls interstitial condensation, and expects a roof to be designed so that it does not harm the structure or its insulation. Moisture on a frame is on show rather than hidden, so it is a related problem rather than the same one. The cure overlaps all the same: keep the cold surface warmer, and keep the room's humidity under control.
A break does the first, and ventilation does the second. Kitchens, bathrooms and loft bedrooms with several people breathing at night all add moisture to the air. An extractor, trickle vents and an opening sash all help, and so does a deep, splayed reveal lined with insulation. Our answer on condensation on roof windows goes into the causes and the fixes.
Choosing a frame when you buy
Ask each supplier four things. What is the frame made of? What is the U-value of the whole unit, and under which standard was it tested? Does the frame include a thermal break, and where? And does the product's own data sheet describe the Window Energy Rating band?
On a sloping roof above a loft room, a timber or timber-and-wrap roof window from an established maker is a safe default, and it comes with a flashing set designed for your covering. For a flat roof, a rooflight with an insulated upstand and a thermally broken frame suits the situation, and cheaper units without those details tend to show the difference within a winter.
The cost difference is smaller than the cost of putting right a mouldy reveal. We list what the data sheet says in the survey notes, so that you can compare like with like. Our roof windows and skylights page covers the units we fit, and the survey is where the figures get checked.
