The short answer
A spray foam guarantee is worth something only if the installer is still trading and the wording covers the faults you worry about. It is a promise from the installer, not a decision by your lender. Lenders set their own policy on foam, so RICS advises asking your mortgage provider directly, whatever paperwork you hold.
What a guarantee actually is
An installer guarantee is a written promise to put right defects in that firm's own work for a stated period. It is a promise from a company, so it is only as strong as the company. If the firm has closed, been dissolved or changed hands, the paper may be worth very little, however long the term it states.
It is also narrow. Most guarantees cover workmanship or the product, and many exclude the consequences that worry buyers and lenders: condensation in the roof void, rot in timber behind the foam, damage that follows a roof leak. Read the exclusions before the headline term. They usually describe the very problems you are asking about.
How lenders and surveyors treat the paperwork
A lender is lending against the building, and what interests it is the condition of the property and how easily it could be sold. An installer guarantee answers a different question. It tells the lender who might pay for a defect, not whether the roof timbers are sound or whether the roof can dry.
RICS published its consumer guide in March 2023. It names condensation that goes unchecked inside a roof as the chief hazard, and it tells owners to put the question to their lender before any foam is sprayed. A BBC investigation reported on 20 November 2024 found some lenders refusing such homes outright and others weighing each property individually. A guarantee does not shift your lender from one camp to the other. Our guide to the RICS position on spray foam goes through the guide in more detail.
When a guarantee still helps
A guarantee is useful evidence. It shows who put the foam in, when, and which product was used, and a buyer's surveyor will ask for exactly that. Keep it with the invoice, any product data sheets and photographs taken during the work.
If the installer is still trading, a claim is best made early, because delay can be used against you. Where the foam went in years ago and the firm cannot be traced, the document still names the product, which helps whoever assesses the roof later. A guarantee also sits alongside the bigger question many owners are really asking, which is what happens to the covering if the foam has to come out. We answer that in does removing spray foam mean a new roof.
Dates, terms and the problem of proof
Two practical points decide whether a guarantee can be used at all. The first is the start date. A term of, say, a stated number of years runs from the day of installation, so a guarantee that looks generous on paper may have little time left by the time you come to sell. Check the date against the invoice, because the two should agree.
The second is proof of the defect. A guarantee that excludes anything caused by moisture puts the burden on you to show that the moisture did not come from elsewhere, such as a leaking gutter or a cracked chimney flashing. That is hard to do with the foam still in place and the timbers hidden behind it. A roof survey that records the condition of the timber, the flashings and the covering gives you something to hold up against such an exclusion.
Photographs matter here more than most people expect. A dated set of pictures taken before and after removal shows what the wood looked like, and it is the kind of evidence a surveyor, a lender and an installer will all treat seriously. We take them as a matter of course and pass them to you with the written findings.
None of this makes a guarantee worthless. It makes it one piece of evidence among several, and it is rarely the piece that settles a lender's decision. If you are selling, hand the buyer's solicitor the whole bundle together: guarantee, invoice, product sheet, photographs and any survey report, so that nothing has to be chased later.
Listed buildings and older roofs
On an older or listed house the foam question reaches well beyond the guarantee. Historic roofs are built to shed water and to move air, and timber that is sealed behind foam can stay damp for long periods. Where the house is listed, B&NES Council issues the consent for roof work, through an application that is separate from planning.
A guarantee for foam installed without that conversation will not cure a consent problem. It will not shield an owner from the cost of undoing the work either. We cover that side in our answer on spray foam in listed buildings.
Questions to put before you rely on one
Before you treat a guarantee as protection, ask five things. Is the company still trading? Does the wording name condensation, rot and roof timbers, or exclude them? Who inspects when you make a claim? Can it pass to a buyer? And does your lender accept the property with the foam in place?
If the answers point towards removal, the sensible move is a survey of the loft space, and ours is free, with photographs of your own roof. Any work that follows is priced on paper, and ours carries a 10-year workmanship guarantee. Before any decision on foam that may be hiding moisture, read how we judge damp behind foam. The spray foam insulation hub gathers the rest of the questions in one place.
