Spray foam is the one insulation measure where the installed price is not the important number. Foam applied to the underside of a roof can be difficult for a surveyor to inspect and can interfere with the ventilation a cold roof depends on, and that combination is why it has become a recurring problem at the point of sale or remortgage rather than at the point of installation.
Open cell and closed cell are different products
Open cell foam is lighter, softer and vapour open. Closed cell is denser, rigid and acts as a vapour barrier. Sprayed against the underside of tiles or slates in a traditional cold roof, either can change how that roof breathes, and closed cell in particular can trap moisture against the timbers and the underlay. The right answer depends on the roof construction, and a survey of that construction should come before any quote.
Ventilation is the real question
A traditional pitched roof is ventilated: air moves through the loft, carries moisture away and keeps the timbers dry. Filling the rafter zone without providing for that airflow, or without a properly designed warm roof build up, risks condensation where nobody can see it. The ventilation requirements are set out in the approved document, and a scheme that does not explain how it meets them is not a scheme.
Why lenders and surveyors ask
A surveyor who cannot see the roof timbers cannot report on them, and a valuer faced with an unknown is cautious. That caution has been expressed as retentions, as conditions requiring removal, and sometimes as a refusal, and it applies to sale as much as to remortgage. Removal is labour intensive and can damage the underlay and the timbers, so the cost of taking it out can exceed the cost of putting it in.
What to establish before buying it
Which product, on which roof construction, with what ventilation strategy, and what written documentation you will hold afterwards: a survey of the roof, the product data, the installer's certification and a guarantee that is actually backed. Where the aim is a warmer house at lower cost, compare it honestly with loft insulation at ceiling level and with a properly designed warm roof, and check whether a government backed scheme covers the alternative.
Questions people ask about spray foam insulation cost
Will spray foam stop me selling the house?
Not automatically, but it frequently triggers questions from surveyors and lenders, and the outcome has ranged from a request for documentation to a condition that it is removed.
Is open cell foam safer than closed cell?
It behaves differently. The question is whether the roof's ventilation and moisture strategy works with the product, which depends on the construction.
How much does removal cost?
It is labour intensive and can damage the underlay and timbers, so it is often more expensive than the original installation.
Is loft insulation a better buy?
For most houses with a cold roof, insulating at ceiling level is cheaper, reversible and does not raise the questions foam raises.