Open Cell vs Closed Cell Foam: What's the Difference?

The Two Types of Spray Foam Used in UK Homes

When spray foam insulation was applied in UK properties — often through government energy efficiency schemes — two distinct types were commonly used. Understanding which type you have is the first step in assessing your situation.

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Open-Cell Spray Foam

What it is: A softer, more flexible foam that expands significantly on application. It has a sponge-like structure with interconnected air pockets.

Appearance: Light, yellowish-white, soft to the touch, slightly flexible.

Properties:

• Lower density (~0.5–1 lb/ft³)
• Breathable — allows some moisture vapour to pass through
• Expands to fill irregular gaps
• Less thermally efficient than closed-cell

Mortgage Risk: Moderate to High — lenders are still concerned because it can trap moisture against timbers and obscure structural inspection, even if it is breathable.

Removal: Generally easier and less expensive than closed-cell. It can often be peeled or cut away from surfaces without causing significant timber damage.

Typical removal cost: £40–£65 per m²

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Closed-Cell Spray Foam

What it is: A rigid, dense foam with a tight cellular structure. When cured, it becomes almost plastic-hard.

Appearance: Hard, yellowish-brown, completely rigid, cannot be compressed.

Properties:

• Higher density (~2 lb/ft³)
• Vapour barrier — does NOT allow moisture to pass through
• Very strong structural adhesion
• Highly thermally efficient

Mortgage Risk: High — closed-cell foam is non-breathable, so moisture trapped in timber has no escape route, significantly accelerating rot. It also bonds extremely tightly to roof timbers.

Removal: Significantly harder and more expensive than open-cell. Chiselling it away risks splitting or damaging the timber it's bonded to.

Typical removal cost: £65–£100 per m²

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How Do I Know Which Type I Have?

| Feature | Open-Cell | Closed-Cell | |---|---|---| | Texture | Soft, spongy | Hard, rigid | | Colour | Light cream/white | Yellow-brown | | Can you press a finger into it? | Yes | No | | Does it crumble easily? | Yes | No |

If in doubt, a specialist can identify the foam type during a free survey visit.

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Does the Type Affect My Mortgage Situation?

Yes — but both types cause mortgage problems. The difference is:

• Open-cell may occasionally satisfy lenders with a specialist report (rare)
• Closed-cell almost always requires full removal before lenders will proceed

Neither type has a guaranteed outcome without professional assessment.

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Key Takeaway

Knowing your foam type helps you understand the likely cost and complexity of removal. Both types are taken seriously by lenders — but closed-cell is the more serious concern and typically costs more to remove.