What a thermal break is, and why aluminium needs one
A thermal break is a strip of low-conductivity material that splits a metal frame into an inner half and an outer half, so heat in the room cannot run straight through the metal to the cold air above the roof. In a thermally broken rooflight, the inside of the frame stays close to room temperature instead of taking on the temperature of a February night.
Aluminium is the reason the idea exists. It is light, stiff, easy to extrude into slim profiles and it lasts for decades on a roof, which is why most modern flat rooflights, lanterns and many skylight kerb frames are made from it. It is also one of the best conductors of heat used in building. Its thermal conductivity sits somewhere around 160 W/mK, against roughly 0.13 for softwood and less than 0.2 for uPVC. Put a solid aluminium section between a room at 20°C and outside air at 0°C and it behaves like a cooling fin: the inner face drops towards the outside temperature, and the room's heat drains out along it.
The fix is to stop the metal being continuous. The profile is made in two parts, an outer section that faces the weather and an inner section that faces the room, and the two are joined by an insulating bridge. Heat still crosses, because nothing stops it entirely, but it now has to pass through a material that conducts it hundreds of times more slowly. The frame becomes part of the insulated envelope instead of a hole in it.
This guide is about that edge of the unit: the frame, the break inside it, and how they decide whether the underside of a thermally broken skylight stays dry and comfortable. The wider picture of glass, gas and coatings sits on the energy efficient rooflights hub.
The edge is where heat takes the short way out
Most of the area of a rooflight is glass, but a disproportionate share of its heat loss happens in the last few centimetres around the perimeter. That strip is where frame, glass edge, spacer bar and upstand all meet, and each can carry heat faster than the centre of the pane.
Think of the unit as a set of parallel routes for heat. The centre of a good double-glazed unit with a low-E coating and argon might pass around 1.1 W/m²K. The aluminium frame, if it were solid, might pass more than 5 W/m²K. Heat does not share itself out evenly across these routes; it flows fastest where resistance is lowest. So a slim band of solid metal around a highly insulating pane can drag the whole unit's figure up by a surprising amount.
There are three places along that edge where the break, or its absence, shows up:
- The frame itself. The profile that holds the glass and sits on the kerb. This is the part a thermal break is designed for.
- The glass edge. The spacer bar that holds the two panes apart. Older aluminium spacers conduct heat round the edge of the sealed unit; a warm edge spacer does the same job as a thermal break, but inside the glass.
- The junction with the roof. The upstand or kerb the frame sits on, and how its insulation lines up with the break in the frame.
A unit can get one of these right and the others wrong. A thermally broken frame sitting on an uninsulated timber kerb still leaves a cold ring around the opening. That is why the frame is worth looking at in the context of the whole edge, not as a line on a datasheet.
How the break is built into a rooflight frame
There are two main ways manufacturers break an aluminium profile, and a few frame materials that do not need a break at all because they are poor conductors to begin with.
The most common method uses polyamide strips. The inner and outer aluminium sections are extruded separately, with grooves along their edges, and one or two strips of glass-fibre-reinforced polyamide are slid into the grooves and rolled tight. Polyamide conducts heat at roughly 0.3 W/mK, and the glass fibre gives it enough stiffness and expansion behaviour to move with the aluminium through hot and cold seasons. Wider strips, or two strips with an insulated void between them, give a better frame figure. Some systems fill that void with foam for a further step.
The second method is pour and debridge. A single aluminium profile is extruded with a channel, the channel is filled with liquid polyurethane that sets hard, and a machine then cuts away the metal at the bottom of the channel so the two halves are joined only by the resin. It is a common method in curtain walling and some glazing systems.
Other frame materials avoid the problem in the first place. uPVC is a poor conductor, and multi-chamber profiles trap still air as well. Timber is a natural insulator, which is one reason many pitched roof windows use a timber core with an aluminium or polymer cladding on the weather side. Composite frames, such as glass-reinforced polymer, sit in the same group.
| Frame material | Conductivity (approx.) | Needs a break? |
|---|---|---|
| Aluminium | 160 W/mK | Yes |
| Steel | 50 W/mK | Yes, or a lined inner |
| Polyamide (break strip) | 0.3 W/mK | It is the break |
| uPVC | 0.17 W/mK | No |
| Softwood timber | 0.13 W/mK | No |
The figures are typical published values for the material, not for a finished frame. What matters in practice is the frame U-value, written Uf, which the manufacturer calculates for the actual profile shape. For how that feeds the whole-unit figure, see rooflight U-values explained.
Cold frames and the condensation line
The visible symptom of an unbroken frame is a line of water around the inside edge of the rooflight on cold mornings. The glass in the middle stays clear, the frame runs wet, and the drips land on the reveal, the worktop or the floor below.
Condensation forms when warm, moist room air touches a surface colder than its dew point. A room at 20°C with relative humidity around 50% has a dew point close to 9°C. A kitchen with the hob on, or a bathroom after a shower, can push humidity much higher and the dew point up towards 12°C or 14°C. So the question is simple: on a cold night, does the inner face of the frame drop below that temperature?
On a solid aluminium frame with outside air near freezing, it can. The metal inner face may sit only a few degrees above the outside air, well below any dew point in a lived-in room. On a thermally broken frame, the inner half is insulated from the outer half, so it stays much closer to room temperature and above the dew point in normal use.
Building physicists describe this with the temperature factor, written fRsi. It compares how warm an inside surface is with the gap between inside and outside. For junctions in homes, BRE guidance uses 0.75 as a threshold for keeping mould and surface condensation at bay under normal conditions. A well-designed thermally broken frame on an insulated kerb aims to clear that; a solid metal edge usually will not.
Even a good frame will show some moisture on the coldest mornings in a steamy room, because no break is perfect and humidity can climb very high. The difference is between a faint mist that clears as the room warms and a steady drip that stains the plaster.
What an unbroken frame does to the room below
A cold frame is felt as well as seen. The ring of cold metal chills the air that touches it, and that air sinks, so you get a gentle downdraught under the opening even when the glass is performing well.
Under a flat rooflight in a kitchen extension, that downdraught falls straight onto the island or the table beneath it. It is often why people say a skylight "feels cold underneath" even when the heating is on; the answer on that question covers the glass side of the same effect. With a thermally broken frame and a modern sealed unit, the inner surfaces stay warmer and the sinking cold air is much weaker.
The condensation line also damages the finishes around the light. Water running off the frame soaks the plasterboard reveal of the upstand, and over a few winters that shows as a brown tide mark or black mould in the corners. Those reveals are exactly where the daylight falls first as it enters the room. A clean, pale reveal bounces light further down the walls; a stained, damp one absorbs it. So the frame affects how bright the room looks, not only how warm it feels.
There is a quieter effect too. People close blinds under a cold rooflight on winter evenings to stop the draught, which means the light is shut out on the short days when it matters most. A warm frame lets the blind stay open until the light has gone.
The frame is only as good as the kerb it sits on
A thermally broken frame protects the edge only if the insulation line of the roof meets the break in the frame without a gap. If the break sits above an uninsulated timber upstand, the cold simply goes round it.
- Joists, with doubled trimmers either side of the opening to carry the loads of the cut joists around it.
- Deck (usually 18mm ply or OSB) with a vapour control layer laid on it.
- Insulation boards above the deck (a warm roof), often tapered to give the roof its falls.
- Waterproof membrane dressed up the upstand and over its top, so the joint is lapped, not sealed.
- Upstand, or kerb: insulated timber, finishing at least 150mm above the finished roof surface to keep standing water and splash off the joint.
- Rooflight unit fixed over the kerb, its frame skirt shedding water outside the membrane turn-up.
- Plastered reveal lining the opening from the ceiling up to the frame, finished white to bounce the light down.
- Roof falls to an outlet: designed at about 1:40 so the finished roof never falls less than 1:80.
On a flat roof, a rooflight sits on a kerb or upstand, usually at least 150mm above the finished roof surface so rain and standing water cannot reach the frame. That kerb is often built from treated timber. Timber is a better insulator than metal, but a 47mm or 50mm timber wall is still far weaker than the 120mm to 150mm of PIR insulation in a modern warm deck around it. The kerb therefore needs its own insulation, typically a layer of rigid board on the outside face, under the roof membrane, carried up to meet the underside of the frame.
The detail to check is alignment. Picture a line running through the insulation in the roof, up the kerb, into the thermal break of the frame and across the warm edge of the glass. If that line is continuous, the room is wrapped. If it steps sideways, for example insulation on the outside of the kerb but the break sitting towards the room, there is a short path of solid material that bypasses it. That path is where the condensation line tends to reappear.
Factory-made insulated upstands, in GRP or PVC with foam cores, solve much of this because they are designed to sit under a specific frame. On older Cambridge flat roofs, such as the 1960s and 1970s rear extensions behind semis in Cherry Hinton and Arbury, the original kerb is often bare timber with no insulation at all. Replacing only the rooflight on top of it gives a better unit on a cold base. Where the roof itself is being renewed, it is the moment to rebuild the kerb properly.
Pitched roof windows face the same problem in a different shape. The insulation between and under the rafters has to be carried tight to the window frame, usually with an insulating collar supplied for the purpose, before the lining goes on. Without it, the timber frame of the window is warm but the plasterboard reveal around it is not.
Reading the figures on a datasheet
Manufacturers publish several numbers for the edge of a unit, and it pays to know which one answers which question. The headline figure on a brochure is usually the glass, not the frame.
Ug is the centre-pane U-value of the glass. It says nothing about the frame at all. Uf is the frame U-value; for a thermally broken aluminium rooflight profile you might see figures somewhere around 2 to 3.5 W/m²K, depending on the width of the break and whether the void is filled, against well over 5 for an unbroken profile. Psi, written Ψ, is the linear heat loss along the joint between frame and glass, which is where the spacer bar matters. Ur or Uw is the whole-unit value that combines all of these for a stated size.
Three questions are worth asking about any rooflight you are comparing:
- Is the frame thermally broken, and what is its Uf? If only Ug is published, ask.
- What spacer is used in the sealed unit? A warm edge spacer and a broken frame work together; one without the other leaves a weak line.
- What is the whole-unit figure, at what size, and assessed in the horizontal position? Part L now looks at rooflights lying flat, which gives a higher number than the older vertical figure.
Size matters because the frame is a fixed width. On a small 600mm by 600mm skylight, the frame is a large share of the total area, so a poor frame hurts the whole-unit figure a lot. On a 2m by 1m flat rooflight, the frame is a smaller share and the glass dominates. The same frame can therefore look fine on a big unit and mediocre on a small one.
Frameless units and slim profiles
Frameless rooflights solve the edge differently: the glass is bonded to the kerb, often with a stepped outer pane that oversails the upstand, so there is no metal frame to break. The edge performance then depends on the kerb, the bond and the spacer.
This can work very well when the upstand is a purpose-made insulated one designed for that unit. It can work badly when frameless glass is bonded to a site-built timber kerb that was never insulated, because nothing is left to stop the cold at the perimeter. The comparison is covered in more detail in are frameless rooflights less energy efficient?
Very slim framed profiles raise a related point. A narrow sightline is attractive because the frame casts less shadow and the opening reads as more glass. But a narrow profile leaves less room for a deep thermal break. Some slim systems use high-performance polyamide or foam-filled breaks to make up for this; others trade some edge performance for the look. On a north-facing slope, where the unit will never gain winter sun to offset its losses, a slightly wider but better-broken frame is often the better choice. On a south slope the frame matters less for comfort, but condensation still depends on the coldest point, not the average.
Upgrading an older aluminium rooflight
Many homes around Cambridge have rooflights fitted in the 1980s and 1990s with unbroken aluminium frames, often with early double glazing or wired glass. They are the ones that run wet on winter mornings.
You will find them on flat-roofed kitchen extensions behind terraces in Romsey and Chesterton, over bathrooms in bungalows in the villages, and on the flat roofs of 1970s estates. The glass may still be sound, but the frame cannot be improved after the fact: a thermal break is part of the extrusion and cannot be added to a solid profile. Renewing the unit is the only way to change the edge. The guide to upgrading single-glazed rooflights covers the older wired-glass units in more depth.
A replacement is also the chance to look at the kerb. When the old unit comes off, the upstand is exposed, and it is quick to see whether it has insulation, whether the timber is sound, and whether the height still clears the finished roof by enough. A new thermally broken rooflight on a properly insulated kerb deals with the condensation line at its source. A replacement rooflight also has to meet current Part L standards for the unit, and we handle the Building Control notification where the work is notifiable.
Because the survey starts with the light, the same visit asks whether the old unit was in the right place. If the sun patch from the old skylight lands on a wall nobody looks at, it may be worth moving or resizing the opening while the roof is open, rather than repeating the old position with better parts. That is the point of the light plan that comes with every quote.
Questions people ask
Can a thermal break be added to my existing rooflight frame?
No. The break is built into the profile when it is made, by joining two aluminium sections with polyamide or resin. A solid frame cannot be split afterwards, so the edge only improves when the unit is replaced with a thermally broken one.
Will a thermally broken skylight stop all condensation?
It removes the cold frame as the main cause, so the steady drip around the edge on cold mornings should stop in normal use. Very high humidity, such as a kitchen with no extraction running, can still mist the coldest point briefly. Ventilation and a warm edge spacer finish the job.
Does the thermal break affect how much light comes in?
Not directly. The break sits inside the frame and is not visible. The frame width does affect the glazed area slightly, so a very deep, wide frame on a small unit trims a little daylight. Most modern broken profiles are slim enough that the difference is hard to see from the room.
Do timber or uPVC roof windows need a thermal break?
Not in the same way. Both materials are poor conductors, so the frame is already the warm part. Aluminium cladding on the outside of a timber roof window sits on the weather side of the timber, so it does not carry cold into the room. What still matters is insulating the lining around the window.
