Thermally broken rooflight frames, and where to look for the break

Aluminium is light, strong and thin, which is why it suits rooflight frames so well. It also moves heat faster than almost any material used in a building. Left as one solid section, a frame becomes a bridge from the cold outside to the warm inside.

Think of a metal spoon left in a mug of tea. The handle gets hot because the metal carries the heat along it. A frame in a winter roof does the same thing in reverse. The outer face sits in frost and wind. The inner face, a few centimetres away, is in a heated room. With no break between them, the inner face is pulled down towards the outdoor temperature and the room loses warmth through a strip of metal that may only be as wide as your thumb.

A rooflight makes this worse than a wall window does. Warm air rises and collects at the ceiling, so the frame sits exactly where the room is warmest, and the gap in temperature between the two faces is at its widest. The glass gets most of the attention, but around the edge of the unit the frame often does the damage. Our guide to how heat escapes through a rooflight puts the frame in context with the glass and the roof build-up.

Here we stay with the frame, because it is the part you can check on a sample and the part most often left unasked.

Ask a manufacturer for a cross-section drawing of the frame, or hold a corner sample in your hand. You are looking for a clear line of dark material running through the aluminium.

In a thermally broken frame the aluminium profile is extruded as two separate pieces. An inner piece and an outer piece run side by side. Between them sit strips, usually of glass-reinforced polyamide, that lock the two halves together mechanically. The strips are rolled or crimped into channels in the metal, so the frame keeps its strength while the path for heat is interrupted.

On a sample you can see the break in three places:

• At the cut end of the frame, as a dark band between two bright metal sections.
• At the corner, where the joint shows the two halves meeting without touching.
• In the drawing, as a shaded block of a different material from the metal around it.

If the section shows one continuous colour of metal from outside to inside, there is no break. If the brochure only shows the frame from the front, ask for the section. A front view tells you about looks and nothing about heat.

Rooflight frames have several components that can look alike from a distance. Only some of them affect heat flow, and sales talk can blur them together.

The thermal break is the insulating strip inside the profile that separates inner metal from outer metal. The pressure plate and cover cap on a roof lantern or a flat rooflight are the pieces that hold the glass in place from the outside. Fixings pass through them into the frame. On many systems the plate and cap are fitted on the outside face and sit clear of the inner half, so the screws do not bridge the break. Others have fixings that run straight through to the inner section, which gives heat a short route around the strip.

Then there is the glazing gasket, the soft seal that presses the glass against the frame. It cushions the glass and keeps water out. It is a seal first and an insulator second, so do not count it as the break.

Questions that separate these parts are more useful than one general question about whether the frame is insulated:

1. Does the break run the full length of the frame, including the corners?
2. Are the fixings on the outer half only, or do they bridge to the inner half?
3. Is the glass edge seated on the inner half, where it stays warmer, or on the outer?

A good supplier answers these from the section drawing, without hesitation. A vague reply tells you something as well.

A break only helps if the glass and the frame work together. The edge of the glass unit is its weakest thermal point, and the frame is what surrounds that edge.

Glass loses heat fastest at its perimeter. The spacer bar that holds the panes apart is a bridge of its own, which is why a warm edge spacer matters. We cover that in warm edge spacers and argon fill. If a warm-edge spacer sits in a frame with no break, the gain is partly wasted, because the frame takes back what the spacer saved. A good frame keeps the glass edge warm. A poor one lets it chill.

The depth of the rebate also matters. The rebate is the channel in the frame that holds the glass. A deeper rebate covers more of the glass edge and pushes the cold zone outwards, but it can also hide more of the glass from the room and reduce the daylight. A sensible compromise keeps the glass edge seated on the warm half of the frame and leaves the visible glass as large as the structure allows.

This is also why a frame cannot be judged on its own. A frame that performs well with double glazing may be wasted with a poor spacer, and a triple glazed unit needs a frame deep enough to take the thicker glass, with a break wide enough to go with it. If you are weighing a third pane, our triple glazing guide explains when it pays back and when the frame is the limit.

Three numbers describe a frame, and a supplier who understands the product will offer them without being pushed. They are worth asking for in writing.

The first is the frame's own heat-loss figure, a U-value for the frame section alone. It is lower for a broken section than for a plain one. The second is the overall U-value of the whole rooflight, which blends the glass, the spacer and the frame into one number. The third is the ratio of frame to glass, because a rooflight with a wide frame and a small pane is mostly frame, and the frame value counts for more.

The whole-unit figure is the one to compare between products. A centre-pane figure looks better on paper because it ignores the edge and the frame. Our answer page on whole-unit and centre-pane U-values explains the gap, and rooflight U-values explained shows how to read a certificate line by line.

The Building Regulations set the standard a new or replacement rooflight has to reach. Approved Document L is the place those limits live, and Part L and rooflights sets out what it asks for. We install to current Building Regulations, and a rooflight with a thermally broken frame makes it far easier to reach the whole-unit figure without adding a third pane.

Condensation shows where the break is missing. When warm, damp room air touches a cold surface, it leaves water behind, and the coldest surface in the room collects it first.

In a roof that surface is often the inner face of an unbroken aluminium frame. You see a thin line of droplets along the frame on a cold morning. Over a season the water runs onto the plasterboard reveal, marks the paint and can leave black mould where it sits. The glass may be perfectly clear while the frame is wet.

A thermally broken frame keeps the inner face warmer, closer to room temperature, so the water has less reason to condense there. It does not remove the need for ventilation. A bathroom, a kitchen and a room full of people all add moisture, and a warm frame simply pushes the problem further from the glass. If you have a frame that streams with water every winter, our page on why a skylight makes a room feel cold helps you work out whether the frame, the glass or the roof build-up is at fault.

The same thinking applies to the reveal around the opening. The frame sits on an upstand or a lining, and if those are uninsulated the frame is warm on one side and chilled through the kerb on the other. Insulating around a rooflight covers the upstand and reveal, and it is the other half of a good frame detail.

The principle is the same in every product, but the frame takes a different shape in each. It helps to know what to expect from the ones you are comparing.

A roof lantern has a lot of frame. The ridge, the hips, the bars and the eaves beam are all aluminium, and together they add up to a large length of metal standing in the weather. A break in every one of those members is the difference between a lantern that holds its warmth and one that becomes a cold cage above the kitchen table. The bars matter most, because they are the longest runs. If you are choosing one, roof lantern installation explains what we look at on the roof and the room.

A flat rooflight or a walk-on unit has a much smaller frame, often a single perimeter section on top of a kerb. The break here sits in that one section, and the kerb beneath it carries the rest of the insulation job. Many fixed flat rooflights use a frame and kerb set together, so the break needs to be part of the kerb design, not bolted on afterwards.

A roof window in a pitched roof commonly has a timber or polyurethane core, with aluminium or plastic covering on the outside. Those frames tend to be warm by nature, because the core is not metal. The metal you see on the outside is a weather skin, not a structural bridge. That is a different route to the same result, and it is why the question on these products is about the core material rather than a polyamide strip.

In every case the aim is to put something with poor conductivity between the exposed metal and the interior. The material that does the job changes from one product to the next.

A thermally broken section lets you give the frame a dark outside and a light inside, or the other way round. Because the two halves are separate pieces, each can carry its own colour.

That is a design gain as much as a thermal one. A white inner face keeps the ceiling line clean, and a dark grey outer face matches the window frames on the rest of the house. The break is the reason both are possible on one frame. It is also why dual-colour aluminium rooflights are offered at all.

The finish changes how the frame feels to touch. On a winter morning an unbroken frame feels cold to the hand, even in a heated room. A broken one is closer to room temperature. That small difference in comfort is easy to underrate, especially in a loft room where the frame is within reach of the bed or the desk.

A thermal break is a heat detail. It is not an acoustic one, and the two should not be confused. A frame can be warm and still let in the sound of a downpour.

Noise travels through glass, through the seals and through the upstand, and the frame matters chiefly as the place those three meet. A laminated inner pane and a well-compressed seal do the quieting. The break in the frame simply comes along with a well-made product. The two do share one thing: a product built with care at the frame tends to be built with care elsewhere, and that is worth noticing when you compare samples.

Warm and quiet often point the same way. A deeper, better-sealed frame helps both. Are energy efficient rooflights quieter too? explains where the overlap is real and where it ends. Whatever we recommend for a room is written into the quiet spec that comes with your quote, frame and glass together.

A quote that names the frame material and says nothing else is a quote you should question. The words on the page tell you what the supplier has thought about.

Look for the frame described by its construction, for example an aluminium frame with a polyamide thermal break and a stated finish on each face. Look for a whole-unit U-value, not a glass-only one. Look for the glass build, the spacer type and the gas fill written alongside, because the frame is one part of a set. Look for the kerb or upstand insulation stated as part of the job.

Be wary of a quote that says only "double glazed aluminium rooflight" or "energy efficient frame". Both phrases can cover a product with a break and one without. The extra cost of the break is usually small set against the whole job, and the difference in comfort lasts as long as the rooflight does.

We write the frame construction into the specification that goes with every quote, so you can compare it line for line with anyone else's. If you already hold a quote and want a second read, send it through the quote form and we will mark up what it leaves out.

These are the points people raise once they have a sample frame in their hands.

Can I tell if an existing frame has a break?

Look at the cut edge or the corner joint if you can see one. A dark line between two metal halves is a break. If the frame is a single colour of metal throughout and feels cold to the touch in winter, it probably has none. Older rooflights from before broken frames became common are the usual culprits.

Is a thermally broken frame worth it on a small rooflight?

On a small unit the frame is a larger share of the whole, so the frame matters more, not less. A small rooflight with an unbroken frame can lose a surprising amount of heat for its size. The break is a modest addition to the product and it is the detail that keeps the small rooflight comfortable.

Does the break make the frame weaker or wider?

The strips are engineered to carry the load, so a properly made broken frame is structurally sound. The profile can be slightly deeper than a plain one, because there are two halves and a strip to fit. The visible face on the inside and outside can still be slim.

Will a thermally broken frame fix a cold rooflight on its own?

Not by itself. The frame is one part of the unit, and the glass, the spacer and the insulation around the kerb all count. A broken frame with single glazing or a bare upstand will still feel cold. If an existing unit is the problem, upgrading an old rooflight looks at what can be improved and what needs replacing.

If you are planning a new rooflight or renewing a tired one, the wider set of choices sits in our energy efficient rooflights hub. When you are ready to talk about your own roof, get a quote and we will put the frame, the glass and the quiet spec in writing.