Skip to main content
Blog Post 28 June 2024

Spray foam insulation explained

Insulating your home is a good idea. It can make your home cosier and reduce your energy bills.

You might have heard news stories about insulation projects going wrong. Often these stories are about spray foam insulation.

To help you get the facts, and understand the issues, our energy experts have written this blog to explore some of those spray foam headlines and dig into what you need to know about this material.

But first, let’s be clear about what spray foam insulation is and how it’s used.

What is spray foam insulation?

Spray foam insulation is a liquid polyurethane foam that installers apply using a spray gun in the area they want to insulate. The foam then sets, forming an insulating layer. As it’s a liquid, installers can apply it quickly. They can also apply it in tricky areas that may be more complicated and time consuming than if you were to use solid insulation materials.

 Spray foam can be used to insulate the:

  • roof
  • walls
  • floors

There are two main types of spray foam insulation, closed-cell and open-cell.

Closed-cell foam

Closed-cell polyurethane foam is one of the most effective insulation materials commonly used in homes. It sets solidly and contains lots of gas pockets that can slow the escaping heat from a space. It can also help support the structure of the insulation area.

Its main drawback is that it’s not vapour permeable. That means that any moisture in the surrounding air or other building materials can’t travel through the closed-cell foam insulation and escape.

Open-cell spray foam insulation

This type of spray foam is the more vapour permeable, which is generally a good thing. It’s also lighter and more flexible, but it is a less effective insulator. This means you’ll need to use a thicker layer than closed-cell foam to get the same insulating effect.

How is spray foam used?

Foam can be used in various parts of the home:

  • Roofs: if a home has a pitched roof with an uninsulated loft, foam is sometimes sprayed onto the underside of the sloping roof. This is one alternative to laying mineral wall insulation at joist level (or what you might call the horizontal ‘floor’ of the loft).
  • Walls: foam is sometimes sprayed into empty cavity walls, usually in situations where standard cavity wall insulation isn’t considered suitable.
  • Floors: foam can be sprayed to the underside of a suspended timber floor, often using a robot to access the underfloor space. This is a less disruptive alternative to lifting the floorboards and fitting insulation material from above.
A technician spraying spray foam insulation on to a wall.

What issues have there been with spray foam insulation?

Most of the issues we hear from householders and other professionals around spray foam are to do with roof insulation. Some of these are related to the product or system itself.

Here are some common things we hear said about spray foam:

There are also issues caused by false claims or poor-quality installation:

And perhaps the biggest headline grabbing story:

A spray foam gun with some foam on the nozzle.

Are the risks with spray foam insulation real?

There’s always some risk when you make changes to a property, but some insulation options are much higher risk than others.

Spraying vapour impermeable foam directly to the underside of an old and poorly maintained roof is one of the highest risk insulation options there is. However, insulating at joist level with mineral wool is one of the lowest risk options for insulating the loft (if you have that option).

Whatever options you’re considering, you can help to minimise risk by:

  • Getting multiple quotes from accredited installers (such as TrustMark-registered companies).
  • Looking for genuine customer reviews and ratings.
  • Making sure your property is well maintained – especially the roof, gutters, downpipes and drains.

Do a bit of research so you understand your options, rather than relying on guidance from a single company, especially if they approached you first. Read our top tips for avoiding rogue traders and installers.

What are my options if I want to insulate my home?

Insulating a loft or roof

If you want to insulate your loft or roof, consider all the alternatives, including:

  • standard loft insulation with mineral wool (which is the cheapest, easiest and most efficient option for most people).
  • dry lining the sloping roof if you want to turn your loft into a heated space.

If you’re considering spray foam for your roof, read this guide from The Royal Institution of Chartered Surveyors (RICS). This offers advice about spray foam and what you should consider, helping you make an informed decision.

Insulating a cavity wall

If you’re looking to have cavity wall insulation installed, then for most homes polystyrene bead insulation is the most suitable type of insulation specified by installers.

Occasionally spray foam might be more suited to your property and your installer should advise.

Insulating a suspended floor

If it’s a suspended floor you’re looking to insulate, and you haven’t got a cellar underneath to give you easy access, then an installer can either:

  • lift the carpet and floorboards and then apply insulation and refit the floorboards.
  • pay for a company to use a robot to spray foam from the void below the floor.

Find out what type of floor you have and how to insulate it.

Last updated: 28 June 2024