Spray Foam Insulation Open or Closed Cell?

Every property needs insulation. It’s what keeps our homes so warm and toasty even through the harsh bite of English winters, and it’s an important part of the building process. But while having insulation itself is a legal requirement, the type of insulation is up to the builder to choose. In this article we’re going to cover what spray foam insulation is, some of the key differences, whether retrofitting it is a good idea, and what documentation you need to request if you choose to.

What is Spray Foam Insulation?

Spray foam insulation is a type of polyurethane foam insulation. It’s an alternative to traditional building insulation, and it’s a common choice for the underside of roof coverings between timber rafters or roof trusses. It’s easy to apply – just spray and it sets to form an insulating layer, which is why builders have been using it for over 30 years. Like every type of insulation, it comes with its own pros and cons, and it typically comes in two types  –closed-cell and open-cell.

What’s the Difference Between Open Cell and Closed Cell Insulation?

Open Cell Insulation: A lot of modern installations with pitched roofs will use an open cell type of spray foam insulation. These sprays have a lighter cellular structure that isn’t totally encapsulated, making them lighter and low-density. This allows it to expand more easily to fill gaps, and is a better choice to install into complex roofs, or where access is restricted. It adheres strongly to the roof structure, which means it can be cut away more easily, allowing it to be trimmed back and lined with plasterboard and vapour control layers in loft extensions.

There are also some products on the market that are sold as ‘vapour permeable’, which means they allow moisture to pass through them. These do have their benefits, but it means you need to carefully plan how to manage or mitigate the condensation issues that can occur.

Closed Cell Insulation: This type of spray insulation is usually found in older installations, and is a much denser product with a closed cellular structure. That means it doesn’t expand to the same volume as the open cell insulation, and is much more rigid once it’s dried in place. It has great adhesion to roof structures, which can make it very difficult to remove. Closed cell insulation is less vapour permeable than its open cell alternative, and is marketed as having better thermal performance at lower volumes. So, if you’ve got limited space, it can be a good option.

However, it’s also marketed as a stabilising substance for roof coverings to secure loose tiles or slates. This is something we would never recommend they’re used for, since any damage or deterioration to the foam could let water in, which could become trapped and lead to all sorts of problems. Instead, you should always opt for repairing the roof.

Which Type of Insulation is Right for my Roof?

It’s very difficult for us to say without looking at your roof, because every case is different. What we will say is that changing the position of the insulating layer and the level of ventilation of the roof is making fundamental change to how the roof was initially designed to perform. So, if you’re planning to upgrade your insulation, it’s important to consider the issues it could cause before it’s installed. At the very least, this should always include a condensation risk or hydrothermal assessment. This helps installers assess whether there will be any risk or damage because of the change. They should also assess the moisture content of the timbers to confirm it’s satisfactory.

We recommend you bring in a suitably qualified person to fully assess the construction and its condition, including:

The presence of underlay
What type it is
The condition of the underlay
Signs of damp or water ingress 
The condition of the exterior coverings
Flashing details

And more besides.

You should also check with your building insurance provider whether installing spray foam insulation will affect your policy before you install it. Most insulations of this type are combustible, which means they can contribute to the spread of fire and increase fire damage if one should break out. It’s a big drawback, and one you need to think about carefully.

Do I Need any Documentation?

Before you go ahead with spray foam installation, you should make sure you have a full pre-installation survey done and that the documents are fully recorded and provided for you to review. You should also check that your installer has a third-party accreditation (look for the Product BBA or KIWA certification). With this your installer should be able to demonstrate that they’ve handled this product before and installed it in full, in line with the requirements and safety guidelines. Once it’s completed, you should make sure you have a copy of the warranty or guarantee, and check if it’s backed by any insurance if the installer stops trading for any reason.

However, even with a comprehensive documentation pack, you may need to undertake further investigations to satisfy future purchasers or lenders. If you’re considering having spray foam insulation installed, or it’s already in your home and you’re worried about the risk of damage and potential future problems, we recommend speaking to a qualified chartered building surveyor to get some impartial advice.

At Harrison Clarke, we are a specialist firm of chartered building surveyors. That means we know an awful lot about how houses are built, and what materials are the better options for them in different circumstances. If you’d like to know more about what we do and how we can help you, just get in touch by phone on 023 8155 0051, or email us at info@harrisonclarke.co.



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Paul Badham, associate director and building surveyor at Harrison Clarke chartered surveyors.

About the author

Paul Badham,
BSc (Hons) PGDipSurv MRICS

Associate Director

Paul joined the team at Harrison Clarke in 2021, having previously worked with Daniells Harrison Chartered Surveyors, Vivid Housing and First Wessex Property Services. Paul first started surveying in 2008, becoming a Chartered Building Surveyor in 2015. 

Paul brings experience from his previous career in shipping, and has a degree in Shipping Operations, as well as a Post Graduate Diploma in Surveying. Paul is an RICS Registered Expert Witness and is experienced in technical due diligence, dilapidations, party wall, and contract administration.