Thermal bridging at the kerb, not the glass

What a thermal bridge is

A path through the envelope with higher conductivity than what surrounds it. Heat takes the easy route, so a small area of poor performance carries a disproportionate share of the loss.

A rooflight kerb is a textbook case. In a warm roof the insulation runs continuously above the deck, and the kerb punches through it: timber, warm side to cold side, around the entire perimeter.

The arithmetic

A 1200 by 1800 rooflight has 6 metres of perimeter. An uninsulated 150mm kerb presents nearly a square metre of poorly performing envelope, at a U-value several times worse than the glass.

That is not a rounding error. It is comparable to leaving a patch of the roof uninsulated.

What it does besides waste heat

Creates a cold surface at the perimeter of the opening. In a kitchen or bathroom that is where condensation forms first: inside the frame, at the top of the reveal, in the corners.

Which then gets reported as a leaking rooflight, and occasionally results in a perfectly sound unit being replaced.

How it is done properly

Rigid insulation on the outer face of the kerb, carried up to meet the underside of the rooflight frame, so the insulation line is continuous from the roof plane up and over the upstand.

It is a sheet of insulation and an hour. It is invisible once the covering is dressed over it, which is exactly why it is skipped.

What to ask

Whether the kerb is insulated, and whether the vapour control layer is sealed to the frame. Those two questions tell you more about a quotation than any U-value on it.

The loss that appears on no specification

Every rooflight quotation quotes glass. Some quote it honestly, as a whole unit U-value, and some quote centre pane. Almost none quote the kerb, because the kerb is not part of the rooflight and belongs to whoever builds the roof.

Yet on a great many flat roof installations the kerb loses comparable heat to the entire glazed area sitting on top of it. It is the single largest avoidable loss in the whole assembly, and it costs a sheet of rigid board and an hour to prevent.

Putting numbers on it

Take a 1200 by 1800 rooflight. The glazed area is 2.16 square metres, and at a whole unit U-value of 1.3 that is a known and reasonable loss.

Now look at the perimeter. That unit has six metres of kerb around it. At 150mm high, the outer face of that kerb is 0.9 square metres of additional envelope.

An uninsulated timber kerb punches straight through the roof insulation for its full height, so that 0.9 square metres is performing at a U-value several times worse than the glass. Depending on the construction it may be worse than a solid brick wall.

In other words, you have added very nearly a square metre of poorly performing envelope to a roof, invisibly, to install two square metres of well performing glazing. That is not a rounding error. It is comparable to leaving a patch of the roof uninsulated.

Why it happens so often

Because it is invisible.

The insulation goes on the outer face of the kerb, and the roof covering is dressed up over it. Once the job is finished nobody can see whether it is there, and nobody will find out for certain without lifting the covering.

It is also nobody’s obvious responsibility. The rooflight supplier sells a unit. The roofer builds a kerb. Where a builder forms the kerb and a rooflight installer fits the unit onto it, the insulation sits precisely in the gap between the two trades.

And it is a separate material and a separate operation, which makes it exactly the kind of line that comes off a price under pressure.

What it feels like in the room

Two symptoms, and the second is what gets reported.

The room is colder than the glazing specification suggested. People notice this vaguely and blame the rooflight, and where they have paid extra for a good unit they are understandably unimpressed.

Condensation on the reveal. The inner face of an uninsulated kerb is cold. Warm moist room air touching it condenses, and water appears on the plaster around the opening, typically at the corners and along the lower edge.

To a homeowner that looks like a leak, and it is one of the commonest things reported to us as one. It appears in cold weather, unrelated to rain, and it is the room condensing on a cold band of plaster that should not be cold.

Left long enough it keeps the plaster and the timber behind it damp, and damp timber in a roof opening is how decay starts.

Getting it right

Rigid insulation on the outer face of the kerb, continuous around all four sides, taken up to meet the underside of the rooflight flange, and lapped properly with the main roof insulation so there is no gap at the base.

That last point matters. Insulation on the kerb that stops short of the roof insulation leaves a thermal bridge at the junction, which is the same problem in a narrower band.

The covering is then dressed up over the insulation and terminated properly, so the insulation is protected and invisible.

Some manufacturers supply insulated composite kerbs, which arrive as a unit with the insulation already integrated. They cost more, they are neat, and they remove the risk of the insulation being omitted by somebody who did not know it was meant to be there.

The vapour control layer, which goes with it

Insulation deals with conduction. The vapour control layer deals with air movement, and air leakage around an opening loses more heat than conduction through a small area of glass.

On a warm roof, where the insulation sits above the deck, the vapour control layer sits below it and has to be carried up the kerb and sealed to the rooflight frame. On a cold roof it sits below the insulation at ceiling level and is sealed around the opening.

Either way it has to be continuous. A cut edge left loose is an air path, and it carries moisture into the construction where it condenses out of sight and shows up eventually as wet insulation or decayed timber.

The two details are specified together and fitted together, and neither is visible afterwards.

The equivalent on a pitched roof

The insulation collar. Same principle, different geometry.

A roof window is set in an opening slightly larger than the frame, so it can be positioned and squared. That gap, typically 10 to 20mm all round, is an air path and a thermal bridge unless it is filled properly.

A purpose made collar is a moulded sleeve sized to the window rather than loose offcuts or expanding foam. Without it the reveal runs cold enough to condense, which produces exactly the same water on plaster that an uninsulated kerb does on a flat roof.

What to ask, and why it is a useful test

Ask directly whether the kerb is insulated on its outer face, and whether it is a named line on the quotation.

It is the fastest way to tell who has detailed the job and who is fitting a unit. An installer who has thought about the installation will answer immediately and will probably tell you more than you asked. One who has priced a window and a day’s labour will not.

On a pitched roof, ask the same about the insulation collar.

Neither adds much to a price and both change how the room performs for the next twenty five years.

Correcting an existing installation

On a flat roof it means lifting the covering around the kerb, which is disruptive and not usually worth doing on its own.

Where it is worth doing is when something else is already bringing you to that part of the roof: replacing the unit at the end of its life, recovering the roof, or dealing with a leak. At that point insulating the kerb is a small addition to work already in hand.

On a pitched roof, fitting an insulation collar to an existing window is a day’s work from inside: removing the lining, fitting the collar, reinstating and sealing the vapour control layer, then replastering. No roof access at all, which surprises people who assume a condensation problem means scaffolding.

Where people look, and where the problem is

Somebody notices water running down a flat rooflight in January and concludes the glazing is inadequate. Frequently the glazing is fine and the kerb is the problem.

The kerb is a box built up from the roof deck, and in most flat roof construction it passes from the warm side to the cold side. Unless it is insulated on its outer face, it is a conductive collar around the entire unit, standing into cold air on four sides plus the top.

Its internal face therefore runs cold. And because it sits at the very top of a warm humid room, where moist air collects, it is the surface that will collect condensation first, before the glass does.

How to tell which it is

Look at where the water is.

Condensation on the glass, glass cold, kerb reveal dry. A glazing question: specification and room humidity.

A band at the edge of the glass with the middle clear. The spacer. An aluminium spacer chills the inner pane at its perimeter.

Water on the kerb reveal with the glass clear. The kerb is bridged. No glazing upgrade will fix it.

That third pattern is extremely common and it is routinely misdiagnosed as a failed unit or a leak.

The detail that resolves it

Insulation on the outer face of the kerb, continuous with the roof insulation, running up to meet the underside of the rooflight frame with no break at the top.

The thermal line then passes outside the kerb rather than through it, and the internal face stays warm.

Insulated kerb systems exist as manufactured products, arriving square and with the insulation built in. On a job where a kerb is being built anyway, the cost difference against timber plus insulation is modest and the risk of it being forgotten disappears.

The vapour control layer at the same junction

It has to be carried up the inside of the kerb and taped to the frame continuously.

Where it is not, warm moist air enters the construction and condenses inside it, wetting timber where nobody can see. That is the damaging version of the same problem, and the symptom is damp appearing in the ceiling around the opening in cold weather rather than after rain.

Fixing it afterwards

Possible and disruptive. It means either taking the unit off to insulate the kerb externally, or lining the kerb internally with insulated board and losing some of the reveal.

Both are far more expensive than an hour on the day, which is why it is worth asking for photographs before the covering goes on.