Can Spray Foam Insulation Cause Damp or Rot?

If your loft feels warm and sealed-up thanks to spray foam, you might assume all is well — no drafts, no cold air, no issues. But what’s happening behind that foam could be another story.

One of the most serious concerns around spray foam insulation in the UK is its potential to trap moisture and accelerate timber decay. And because it’s sealed tight to the underside of your roof, you often can’t see the warning signs until it’s too late.

Here’s how it happens — and what to do if you suspect spray foam might be causing more harm than good.

Why Spray Foam Traps Moisture

Closed-cell spray foam — the most commonly used type in UK lofts — is designed to be impermeable. That means no air, and crucially, no water vapour passes through it.

While that may sound energy-efficient, it comes with a downside:

• No airflow means condensation gets trapped
• That moisture sits against timber rafters and boards
• Over time, this creates the perfect conditions for mould, rot, and decay

Older roofs in particular — ones with clay tiles or no membrane — were never designed to be sealed up so tightly. They rely on passive airflow to stay dry.

The Risks to Your Roof

When spray foam covers the entire underside of a pitched roof, it hides everything — including:

• Wet patches
• Mould growth
• Softening or warping timbers
• Insect activity

Because the foam is stuck directly to the wood, any trapped moisture has nowhere to go. And if there are roof leaks, they're often absorbed silently into the structure.

Left unchecked, this can lead to:

• Structural timber damage
• Failed HomeBuyer Reports
• Expensive roof repairs
• And in some cases, full roof replacement

Will You Always See the Signs?

Unfortunately, no.

That’s the real issue — most homeowners with spray foam don’t know there’s a problem until a surveyor spots something, or an issue forces the foam to be removed.

By then, it can be years of slow damage behind a barrier you can’t see through.

What’s the Solution?

If you suspect damp, or just want peace of mind, the best option is removal and inspection.

At Snug & Secure, we safely remove spray foam by hand, check the condition of the roof timbers underneath, and replace it with breathable loft insulation that allows your home to ventilate naturally.

If we spot signs of rot or trapped moisture, we’ll show you photos — and explain your options clearly.

Final Thoughts

Spray foam doesn’t cause leaks — but it can trap the consequences of them.

If you’ve noticed condensation in your loft, a musty smell, or just want to make sure your roof’s in good shape, get in touch for a no-pressure inspection. Or check out our full blog for more guides like this one.