Insulating a kerb so the frame does not run with condensation
An uninsulated kerb produces a cold frame, and a cold frame collects water every winter. The physics of that is covered on the thermal bridging page. This page is about the practical work: what the insulation is made of, how it is fitted, in what order, and how to tell whether it was done.
The detail in one paragraph
Insulation is carried up the outer face of the kerb, from the level of the main roof insulation to the top of the kerb, lapped so that there is no gap where the two meet, and the covering is then dressed over it and terminated under the trim. The kerb is now inside the thermal envelope instead of standing outside it.
That is the whole detail. Everything below is about the ways it gets done imperfectly.
Which materials suit the job
| Material | Performance per thickness | Handling at a kerb | Notes |
|---|---|---|---|
| PIR board | High | Cuts cleanly, holds an edge, takes a fixing | The usual choice. Foil faced, needs taping at joints |
| Phenolic board | Highest of the boards | Similar to PIR, slightly more brittle | Where the depth available is very tight |
| Mineral wool | Lower | Compresses, hard to hold on a vertical face | Better for fire performance, worse for this detail |
| Extruded polystyrene | Moderate | Rigid, closed cell, tolerates damp well | Useful on an inverted roof where it may get wet |
| Proprietary insulated kerb | High | Arrives already done | Removes the most missed step from site entirely |
For a normal warm deck kerb, rigid board is the answer, and the reason is that a vertical face needs a material that stays where it is put.
Thickness, and matching the roof
The insulation on the kerb face should be at least as effective as the roof insulation it is continuous with. Anything less makes the kerb the weak point again, just less weak than before.
Where depth is constrained, and at a kerb it frequently is because the covering and the trim have to fit over the top of it, a higher performance board in a thinner section is the way to keep the performance without the bulk. That is one of the few places where paying for phenolic over PIR earns its money.
What does not work is thin insulation applied because that is what fitted, without anybody checking what it achieves.
The lap, which is the part that matters
The most common defect in this detail is not missing insulation. It is insulation that is present and not connected.
Where the board on the kerb face stops short of the roof insulation, or butts against it with a gap, there is a continuous slot of air running round the whole perimeter. Air moves freely through a slot, so a gap of a few millimetres carries a surprising proportion of the heat that the whole detail was built to retain.
The lap has to be tight, and where a tight butt joint cannot be guaranteed the joint is taped or filled. On a foil faced board, taping the joint also maintains the vapour resistance across it.
Corners
Four faces meet at four corners, and a corner is where two boards have been cut to fit rather than laid in a run.
Cutting an accurate mitre in rigid board at the top of a kerb, in position, is more difficult than it sounds, and the temptation is to accept a gap and fill it. Expanding foam is used for this constantly and it is a reasonable filler for a small void, provided it is a small void being filled rather than a poor cut being disguised.
We would rather cut the boards accurately and use the foam for what remains, and the difference between those two approaches is visible for about ten minutes and invisible for the next thirty years.
Fixing it to the kerb
Rigid board on a vertical face has to be held there while the covering goes on, and there are two ways.
Adhesive
Insulation adhesive or foam applied to the kerb face, which holds the board without penetrating it. This is the cleaner option, because a mechanical fixing through insulation is a small thermal bridge of its own and a hole in any foil facing.
Mechanical fixings
Where the covering system requires them, or where the board needs holding immediately, plastic capped fixings rather than metal ones. A metal fixing through insulation into a timber kerb is a conductive path from the covering to the structure, which is the thing being avoided.
Which we use
Adhesive where the system allows it, plastic capped fixings where it does not, and never bare metal screws through the board.
The sequence on site
- Kerb built, or existing kerb corrected, and confirmed level and square.
- Vapour control layer carried up the inside face, ready to seal to the frame.
- Roof insulation laid up to the kerb.
- Kerb face insulation fitted, lapped tight to the roof insulation, joints taped.
- Covering dressed over the insulation, up the kerb face, above the water line.
- Trim fitted, covering terminated.
- Rooflight set, fixed down, and the vapour control layer sealed to the frame.
Step four is the one that gets skipped, because at the moment it happens the roof looks finished to anybody who is not looking for it.
Doing it on an existing kerb
Where a rooflight is being replaced onto a kerb that was never insulated, the question is how much has to come off.
The covering has to be lifted at the kerb face and for a short distance onto the roof, so that the new insulation can lap under it. That is a local operation rather than a recover, and it is entirely reasonable to do while a unit is off anyway.
The alternative, insulating the reveal internally instead, moves the cold surface rather than removing it and is a partial answer. It is worth doing where the roof genuinely cannot be opened, and it should be described as what it is.
Cold decks, where the line is lower
On a cold deck the insulation is between the joists, well below the top of the kerb, and the kerb face insulation has to reach down to meet it.
That is a longer run, it passes through the ventilated void, and it must not block the cross ventilation path. The way it is done is to insulate the kerb on its outer face down to the insulation line while leaving the air path across the void open past the kerb, which usually means the opening is positioned with the ventilation in mind rather than only the room below.
Where the void cannot be kept ventilated past the kerb, converting that part of the roof to a warm deck is frequently the honest answer, and it is why we raise the conversion question at survey rather than after.
The inside face, and the other half of the sandwich
The outer face of the kerb carries the insulation. The inner face carries the vapour control layer, and the two are doing opposite jobs at the same junction.
The insulation keeps the kerb warm. The vapour control layer keeps room air out of the construction. Neither substitutes for the other, and a kerb with excellent insulation and no vapour seal has traded surface condensation for interstitial condensation rather than solving anything.
Both are fitted while the roof is open and neither is reachable afterwards, which is why they belong in the same conversation and the same set of photographs. The vapour side is set out in full on the vapour control page.
Proprietary insulated kerbs
There is a way to remove this entire failure mode from site, which is to buy the kerb rather than build it.
An insulated composite kerb arrives with the insulation already bonded to the outer face, cut square, at a consistent thickness, with the corners already made. There is no board to cut, no lap to get tight and no corner to fill. The lap into the roof insulation is still made on site and is still the detail to watch, and everything above it has been done in a factory.
It costs more than timber and boards, it comes in standard sizes which may not suit an existing opening, and on a new roof where the size is being chosen anyway it is frequently the better buy. On a replacement onto an existing kerb it is rarely applicable, which is why we still build and insulate kerbs by hand more often than not.
How to inspect it
Once the covering is on, none of this is visible, so the inspection happens while the work is open.
What to look at, or to ask for a photograph of: insulation present on all four kerb faces, a tight lap into the roof insulation with no visible line of daylight, taped joints, accurate corners rather than large voids of foam, and no bare metal fixings passing through the board.
Five photographs taken before the covering goes on are worth more than any assurance afterwards, and we take them as a matter of course.
What it changes in the room
The measurable effect is that the frame and the reveal sit close to room temperature instead of close to outside temperature.
What that means day to day is no water running off the frame on cold mornings, no repeated staining at the head of the reveal, no mould growth on the plaster there, and a room that does not have a cold draught falling off the rooflight when the heating goes off.
It also means the insulation the roof was designed with actually performs, because a continuous perimeter bridge undermines a surprising proportion of a small roof.
Why it gets missed
It is worth being blunt about the reason, because it is not incompetence.
The kerb is frequently built by the builder and the rooflight fitted by somebody else, and the insulation on the kerb face falls between them. It happens at a moment when the roof looks finished. It is invisible immediately afterwards. And nothing goes wrong for a full year, by which time the connection between the omission and the symptom has been lost.
It is a sequencing problem rather than a skills problem, and the fix is somebody owning the whole detail.
What we insulate on a flat roof
We insulate the kerb on all four faces as standard, lap it tight into the roof insulation and tape the joints, use adhesive or plastic capped fixings rather than metal, and photograph the detail before the covering goes on. On a cold deck we keep the ventilation path open past the kerb and say so.
Where a kerb is already in place and uninsulated and the roof is not coming off, we say plainly what an internal remedy will and will not achieve, rather than presenting a partial answer as a fix. Most of the time the roof is open anyway at the moment a rooflight is being changed, which is exactly when this costs almost nothing to put right for good.
Four faces and a tight lap. It is twenty minutes of work and it decides whether the frame runs every January.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.