Spray Foam Insulation: What Every Homeowner Needs to Know

Spray foam insulation has been marketed as a game-changer for energy efficiency, promising lower heating bills, improved airtightness, and even structural reinforcement. But is it really the magic bullet for keeping homes warm and energy-efficient? As professional surveyors, we take a cautious but fair approach, weighing up the pros and cons based on guidance from the Residential Property Surveyors Association (RPSA), Royal Institution of Chartered Surveyors (RICS), and the Property Care Association (PCA), alongside research from the Health and Safety Executive (HSE). If you’re considering buying or selling a house with spray foam insulation, the following information may be of interest to you.

One of the biggest concerns raised by the RPSA and PCA is that spray foam can make it difficult to inspect a roof properly. Once applied, the foam adheres tightly to the roof structure, covering up rafters and making it hard to spot problems such as rot, leaks, or timber decay. If issues go unnoticed, they can worsen over time, potentially leading to costly repairs (RPSA/PCA, 2023).

This is why over 75% of UK mortgage lenders are wary of properties with spray foam insulation (PCA, 2023). If a surveyor can’t see the condition of the roof, they’re more likely to raise a red flag, which could impact your ability to sell or remortgage your home.

Both the RICS and the PCA warn that incorrect spray foam installation can lead to moisture being trapped against the timbers, accelerating decay. This is especially true with closed-cell foam, which forms a rigid, impermeable layer. If ventilation isn’t carefully managed, condensation can build up, pushing timber moisture levels above 20%—the point at which fungal decay can set in (RICS, 2023).

However, the HSE offers a vital counterpoint: when applied correctly, closed-cell foam can actually reduce the rate of timber deterioration. Because it has such a low permeability, it can limit external moisture ingress and help stabilise timber moisture levels (HSE, 2023). The key is ensuring that the existing structure is already dry and that ventilation pathways are adequately maintained.

Another issue flagged by RICS is the risk of excessive expansion pressure on roof timbers. Some spray foams expand up to 1.5 times their initial thickness, which can put stress on older timber structures, leading to distortion or cracking (RICS, 2023).

Additionally, the RICS make it clear that spray foam should never be applied directly to roof tiles or slates (RICS, 2023). Doing so can create problems with water runoff, trap moisture underneath, and even make tile replacement or maintenance impossible without damaging the insulation layer.

In the South East of England, where property values are among the highest in the UK, poorly installed spray foam has been linked to devaluations of 5-10% in some cases (UK Finance Lenders Panel, 2023). Survey data from the UK Finance Lenders Panel shows that 85% of lenders in the region reject mortgage applications where unverified spray foam insulation is present.

Meanwhile, removing spray foam insulation—often necessary to secure lending—can cost anywhere from £3,000 to £8,000, depending on roof size and foam type (PCA, 2023).

Despite the risks, there are situations where closed-cell spray foam can be beneficial, provided it is applied correctly. HSE research highlights that:

• It can improve timber longevity by preventing moisture absorption (HSE, 2023).
• It can enhance structural stability, thanks to its compressive strength of 200-300 kPa (HSE, 2023).
• It’s effective in certain properties where external moisture ingress is an issue and ventilation is properly managed.

However, success depends entirely on the quality of the installation. Before proceeding, homeowners should:

– Ensure ventilation pathways are maintained to prevent moisture build-up.

– Use an accredited installer with experience in moisture control.

– Keep all installation records to provide reassurance to future buyers or lenders.

When our surveyors inspect a property with spray foam they will follow best practices to help establish whether it’s been installed appropriately and will:

• Check for documented evidence of installation, including product details, installer credentials, and ventilation plans (RICS, 2023).
• Look for ventilation pathways—if the foam covers all airflow routes, there may be hidden moisture problems.
• Use non-invasive testing methods such as thermal imaging to check for condensation build-up behind the foam.
• Confirm that the foam is not applied directly to tiles or slates, as this can cause water retention issues (RICS, 2023).
• Advise homeowners on lender concerns—if no evidence of a professional installation exists, they may face valuation issues.

Spray foam insulation is not a one-size-fits-all solution. While it offers potential benefits, particularly in moisture control and energy efficiency, the risks of poor installation, trapped moisture, and mortgageability issues mean homeowners and purchasers alike must proceed with caution.

If you’re considering spray foam, do your homework. Choose the right installer, maintain proper ventilation, and keep detailed records. And if you’re buying a property with spray foam, always get an expert surveyor’s opinion before committing.

At the end of the day, insulation should improve your home’s comfort and value, not cause you headaches down the line.