A roof either keeps the insulation below a ventilated void (cold roof) or wraps it around the structure with no ventilation needed (warm roof). Mixing the two, or blocking the eaves ventilation of a cold roof, traps moisture and rots the timbers. Spray foam does exactly that, which is why lenders now query it.
- Blocking the eaves when topping up insulation is the commonest cause of a damp loft.
- Cold roof needs ventilation. Warm roof needs an airtight vapour control layer. Pick one properly.
- Spray foam can make a house difficult to mortgage or sell, whatever its thermal performance.
- Insulating a conversion takes depth, and depth comes out of your headroom.
Cold roof and warm roof
Almost every loft insulation problem comes from not being clear which of these two the roof is.
A cold roof puts the insulation at ceiling level, between and over the ceiling joists. The roof space above it is outside the insulated envelope, so it is cold — and it must be ventilated, so that any moist air reaching it is carried away before it condenses on the cold underside of the roof. This is what an unconverted loft normally is.
A warm roof puts the insulation at rafter level, so the roof structure sits inside the insulated envelope and stays warm. There is no cold surface for moisture to condense on, so no ventilation void is needed above the insulation — but a continuous, well-sealed vapour control layer on the warm side becomes essential, because any moist air that does get into the construction has nowhere to go.
Both work. What does not work is a hybrid: insulation at rafter level with no vapour control and no ventilation, or insulation at ceiling level with the ventilation blocked. Those are the two failures we actually find.
A loft conversion almost always moves the roof from cold to warm, because the space below the rafters becomes the room. That change has to be done completely rather than halfway.
The giveaway for a failing roof build-up is damp or mould on the underside of the roofing felt, dark staining on the rafters, or frost on the inside of the roof on a cold morning. None of those is a leak.
The eaves: the mistake almost everyone makes
This is the single most common insulation fault in British lofts, and it is created by people trying to do the right thing.
A cold roof is ventilated at the eaves — air enters at the edge of the roof, passes over the insulation, and carries moisture away. The gap it needs is small but it has to be continuous and clear.
When loft insulation is topped up, the easy thing is to run the new layer right out to the edge of the joists, tight into the eaves. That blocks the ventilation path completely. It is a deeper layer of insulation and a worse roof, and the symptoms arrive the following winter:
Condensation on the underside of the felt, which drips onto the insulation and soaks it. Wet insulation stops insulating.
Mould on the rafters and the felt, usually worst at the eaves and on the north side.
Damp patches on the ceiling below, which get reported as a roof leak and are not.
Rot in the rafter feet and the wall plate, over longer periods, which turns an insulation problem into a structural one.
The fix is straightforward: pull the insulation back from the eaves and fit rafter trays — rigid plastic channels that hold the ventilation path open while letting the insulation come right up to them. They cost very little and they are the difference between a dry loft and a rotting one. Over-fascia or soffit vents provide the air supply at the eaves, which is why roofline replacement is the moment to get this right.
The spray foam problem
Spray polyurethane foam applied to the underside of the roof has been sold widely as a loft insulation upgrade. It has become a serious problem for homeowners, and it is worth being precise about why, because the reasons are not all the same.
It can trap moisture against the timbers. Closed-cell foam sticks to the underside of the roofing felt and the rafters, sealing the ventilation path. Where the detailing is not right, moisture has no route out and the battens and rafters stay damp. Open-cell foam is vapour-open and brings a different risk, in that moisture can pass into the foam and reach the timber.
It hides the roof. Once the underside is encapsulated, nobody can inspect the rafters, the battens, the felt or the nail fixings. A surveyor cannot see what condition the structure is in, and that uncertainty alone is enough to cause problems.
It makes re-roofing far more expensive. Tiles and battens normally lift off a roof cleanly. Foamed to the underside, the covering comes away in sections with the foam attached, and the roof typically has to be stripped and rebuilt rather than re-covered.
Lenders and surveyors now query it. A number of lenders will decline, retain, or require removal and a specialist report before advancing on a house with spray foam in the roof. That is a commercial reality regardless of whether a given installation is technically sound.
The practical position: if you are planning a loft conversion and there is spray foam in the roof, expect it to come out. Removal is messy, labour-intensive and sometimes damages the felt beneath, and the cost belongs in the budget from the start. If you are selling rather than converting, get a specialist report and documentation together early, because this is exactly the sort of thing that stops a sale at week six.
Insulating a conversion: depth versus headroom
| Approach | Where the insulation goes | Headroom cost |
|---|---|---|
| Between rafters only | In the rafter depth | None, but rarely enough on its own |
| Between and under rafters | Rafter depth plus a layer below | Reduces ceiling height |
| Between and over rafters | Rafter depth plus above the deck | None inside; raises the roof line |
| Rigid board, high performance | Thinner layer, same performance | Less height lost; higher material cost |
| Rafters deepened | Battened down to gain depth | Reduces ceiling height |
| Dormer cheeks and flat roof | Within the new construction | Designed in from the start |
In a marginal loft, high-performance rigid board is worth the extra cost because the money buys headroom as much as warmth — achieving the same standard in less depth can be the difference between a compliant room and an unusable one.
Ventilation in the finished room
- Background ventilation in the new room, usually trickle vents in the rooflights or windows.
- Mechanical extract in any ensuite, ducted to the outside air — never into the roof void, which simply moves the moisture into the structure.
- A continuous vapour control layer on the warm side of the insulation, with joints sealed and penetrations taped. Downlights, cables and loft hatches all puncture it, and each puncture is a route for moist air.
- Ventilation to any remaining cold roof void, because a conversion usually leaves triangles of unheated space behind the dwarf walls and above the flat ceiling. Those voids are still cold roofs and still need ventilating.
- An insulated, draught-sealed hatch to any remaining void.
- Insulation to the dwarf walls and the flat ceiling, continuous with the rafter insulation so there is no gap at the junction.
Point four is the one most often missed. A conversion rarely insulates the whole roof plane: there are usually small voids left behind the knee walls and above the ceiling, and they remain cold roofs with the same ventilation needs as before. Sealing the room up beautifully and forgetting those voids produces condensation in exactly the places nobody looks.
What it costs
These are the loft conversion figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Velux / rooflight conversion | £25,000–£35,000 | Simplest type |
| Dormer conversion | £35,000–£50,000 | Adds head height & space |
| Hip-to-gable | £45,000–£65,000 | End-of-terrace / semi |
| Average loft conversion | ~£35,371 | Type & extras dependent |
Spray foam removal is not included in any of those and is a substantial separate cost, as is any repair to the felt or timbers it reveals. Where a loft already has spray foam, that is the first thing to get priced rather than the last.
Check the eaves before the roof. Blocked ventilation causes most of what gets reported as a leak, and rafter trays are an inexpensive fix.
Frequently asked questions
What is the difference between a warm roof and a cold roof?
A cold roof has the insulation at ceiling level with a ventilated roof space above it. A warm roof has it at rafter level so the structure stays inside the insulated envelope, needing no ventilation void but an airtight vapour control layer instead. Problems come from mixing the two.
Why is my loft damp after adding insulation?
Almost always because the new insulation was run tight into the eaves and blocked the ventilation path. Moist air can no longer escape, so it condenses on the cold underside of the felt. Pull the insulation back and fit rafter trays, which hold the gap open inexpensively.
Is spray foam insulation a problem when selling?
It can be. A number of lenders will decline, retain funds, or require removal and a specialist report before advancing on a house with spray foam in the roof. It also hides the roof structure from inspection and makes future re-roofing far more expensive.
Does spray foam have to be removed for a loft conversion?
Usually yes. The roof build-up has to be designed and inspected, and encapsulated rafters cannot be assessed. Removal is messy, labour-intensive and can damage the felt beneath, so where a loft already has spray foam it is the first item to get priced rather than the last.
How much insulation does a loft conversion need?
Enough to meet the performance standard in Part L, which in practice means insulation between and under the rafters, or between and above them. The depth comes out of your headroom, which is why high-performance rigid board is worth the extra cost in a marginal loft.
Do I still need loft ventilation after a conversion?
In the remaining voids, yes. A conversion rarely insulates the whole roof plane — there are usually cold voids left behind the knee walls and above the flat ceiling, and they still need ventilating. Forgetting them produces condensation exactly where nobody looks.
Get the build-up right once
Cold roof or warm roof, ventilated or sealed — we specify one properly rather than leaving a hybrid that rots the timbers.