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Closed Cell vs Open Cell Spray Foam: The Differences

If a survey has mentioned spray foam in your loft, the next question is often which type it is. Here's how closed cell and open cell foam differ, and why it matters.

Two kinds of spray foam

Spray foam insulation is sprayed as a liquid that expands and sets. The names describe what the bubbles inside are like once it cures. In open cell foam, the tiny cells are broken and connected, so air can move through the material. In closed cell foam, each cell is sealed, making the foam much denser and more rigid. Both have been sprayed onto the underside of roofs in UK homes, often marketed as a quick insulation upgrade.

Open cell spray foam

Open cell foam is soft and spongy, usually pale cream or white. It expands a lot when sprayed, so it fills gaps well, and it's relatively light. Because its cells are open, it lets water vapour pass through, but it can also absorb and hold moisture. If condensation or a small roof leak gets into it, that moisture can sit against the timbers.

Closed cell spray foam

Closed cell foam is firm to the touch, often yellowish, and bonds tightly to whatever it's sprayed on. It resists water getting through and adds some stiffness to what it's stuck to. The downside is that it largely stops the roof structure from breathing. Moisture that does get in, from a slipped tile or condensation, has nowhere to go, and the timber behind can't be seen or easily inspected.

Side by side

  • Feel: open cell is soft and squashy; closed cell is hard and rigid.
  • Density: open cell is light; closed cell is much denser.
  • Moisture: open cell can soak up water; closed cell resists it but traps what's behind.
  • Adhesion: both stick to timber and felt; closed cell bonds harder.
  • Removal: open cell is usually quicker to take out; closed cell takes more effort.

Why lenders and surveyors raise concerns

The worry isn't really which type you have. It's that foam sprayed directly onto rafters and the roof covering hides the timber, can trap moisture, and makes it hard for a surveyor to confirm the roof is sound. That's why some lenders are reluctant to lend on affected homes, whichever foam was used. Our spray foam insulation problems guide explains these issues in more depth.

What the type means for removal

Both kinds can be removed. Open cell foam tends to come away in softer chunks, while closed cell needs more cutting and scraping to free it from the timbers and underside of the roof. The thickness, how much of the loft is covered and the access all matter too. Once it's off, the rafters, battens and membrane can finally be inspected, and any damage dealt with. The cost side is covered in our spray foam removal cost guide.

Can foam be left in place?

Some owners ask whether foam can simply stay. That's a decision for you, your surveyor and your lender. In many cases the issue only surfaces when the house is sold or remortgaged, so it's worth understanding the position early rather than under time pressure.

What to do next

If you're not sure which foam you have, a photo of it and the loft is a good start. If a sale or remortgage is held up, getting a quote for removal early helps keep things moving.

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Frequently asked questions

How can I tell if I have open or closed cell spray foam?

Open cell foam is usually soft, spongy and squashes easily under a finger. Closed cell foam is firm and rigid, almost like a hard plastic. A surveyor or specialist can confirm which you have.

Is closed cell spray foam better than open cell?

Neither is simply better. Closed cell foam is denser and resists water, but it's also harder to remove. Open cell foam is lighter but can hold moisture. Concerns from lenders and surveyors apply to both when foam is sprayed onto roof timbers.

Is closed cell spray foam harder to remove?

Generally yes. Because it's rigid and bonds strongly, it takes longer to cut and scrape away from rafters and the underside of the roof covering than softer open cell foam.