Spray foam insulation – blessing or curse?

A professional property survey can flag up a wide range of problems with a building, but were you aware that issues with spray foam insulation have become one of the most common concerns we encounter across London and the South East?

Spray foam insulation has been available in the UK for over 30 years. It’s a liquid foam that is sprayed onto the underside of roof pitches to provide an additional layer of insulation. Spray polyurethane foam (SPF) is sometimes installed as part of an energy-efficiency upgrade, or as a temporary measure on a roof in poor condition. In recent years, it has become more popular with homeowners as an alternative to traditional loft insulation materials such as fibreglass, wool or insulation boards.

Mortgage lenders have become increasingly cautious about properties with spray foam insulation. BBC research published in late 2024 found that a quarter of the UK’s biggest mortgage providers would not lend on homes with spray foam in the roof, and surveyors and valuers are often required to flag its presence or seek a specialist report.

Why use spray foam insulation?

SPF is applied as a lightweight liquid foam that sets and hardens in situ after application. It is typically sprayed as a 100mm thick layer onto the underside of the roof. Installation can release fumes during application and curing, so homeowners should follow the installer’s safety and re-entry guidance. The material cannot be decorated over and it is also more expensive than traditional types of insulation.

On the upside, SPF is a better insulator than mineral wool loft insulation, meaning a thinner layer of SPF is required to achieve the same insulating effect. And as it is applied to the roof slope, it doesn’t get in the way of boarding the floor of the loft space.

SPF contractors are keen to stress the benefits of an installation as a long-term energy-saving investment into the property, claiming it can reduce energy bills, protect water tanks and pipes from freezing and keep the loft space dry. In addition, the rigid foam is said to stop roof tiles from slipping and give extra strength to the roof structure.

Two types of SPF insulation are available: open-cell and closed-cell. Closed-cell SPF is firm to the touch and dries rigid, stopping moisture from passing through it. By comparison, open-cell SPF is spongy when pressed and is less efficient as an insulator. However, it does have good sound-absorbing properties and will allow water vapour to pass through it.

What is the problem with spray foam insulation?

One of the main concerns with SPF insulation is that, if the roof is not designed and installed correctly, it can disrupt moisture movement and increase the risk of condensation in the roof space. The effect can be more pronounced with closed-cell SPF, but even the open-cell ‘breathable’ type can cause problems in places where foam and roof covering materials meet.

The moisture build-up over time can cause wood rot in the roof timbers. What’s more, defects in the roof covering itself may be concealed from view resulting in water penetration which also leads to timber decay. Damp issues in a roof can be caused by water penetration or condensation, so a building survey should be carried out to establish the exact cause.

It is also worth noting that spray foam insulation should not be installed in a listed or other protected building without prior consent, and it is often unsuitable for traditional roofs in older properties because it can trap moisture, hinder inspection and complicate future repair work.

What do mortgage lenders think?

Lender guidance for surveyors regarding spray foam insulation varies, ranging from requests for a structural engineer or roofing contractor report to lowering the valuation on account of the potential cost of roof renewal, to declining the loan outright. For property surveyors and valuers who encounter SPF when carrying out a survey or mortgage valuation, the latest guidance notes for the specific lenders must be front of mind.

Not all mainstream lenders have stated a clear policy on their lending criteria for properties with SPF insulation. If you currently have a mortgage and are considering having spray foam insulation installed, or if you are thinking of purchasing a property with SPF insulation in situ, you should check with your mortgage provider as soon as possible.

At Squarepoint Surveyors in London, we specialise in providing professional building advice to a wide range of clients and properties. For expert assistance and advice from our specialist building surveyors and party wall surveyors, please get in touch with us for a no-obligation consultation.