Can Spray Foam Insulation Rot Your Roof?

It’s one of the most common worries we hear from UK homeowners:
"I’ve heard spray foam can rot your roof — is that true?"

The short answer is: not directly — spray foam itself isn’t corrosive or harmful to timber. But the way it’s applied, and how it affects ventilation, can absolutely create the conditions for rot to take hold.

Here’s what you need to know before deciding whether to keep it, remove it, or investigate further.

How Spray Foam Works

Spray foam insulation (especially closed-cell foam) is applied directly to the underside of your roof — sealing over:

• Rafters
• Tiles or membranes
• Junctions and nails

This forms an airtight barrier — great for stopping draughts, but also blocks natural airflow through the loft.

Why Roof Timbers Need to Breathe

In a typical UK loft, small amounts of moisture rise from below due to:

• Shower steam
• Cooking vapour
• Natural humidity in the home

Normally, that moisture escapes through vents or breathable felt. But when spray foam seals everything up, it can:

• Condense behind the foam
• Soak into rafters and joists
• Create a damp microclimate just beneath the surface

Over time, this encourages fungal growth, mould, and eventually — timber rot.

Early Signs of Trouble

If you can access your loft, check for:

• Musty smells
• Peeling foam or discolouration
• Damp patches on the rafters (if visible)
• Signs of softening or surface mould on timber

The issue? Foam hides problems really well — so many homeowners don’t realise there’s damage until it’s already progressed.

Can It Be Prevented?

Only with proper ventilation — and even then, most older lofts weren’t built to accommodate sealed insulation systems like spray foam.

If you already have it installed, the safest route is to:

• Have it professionally assessed
• Remove it if there are signs of moisture buildup
• Replace it with breathable insulation that meets modern standards

Snug & Secure specialises in this process — and we’ll always show you what’s going on beneath the surface before making any recommendations.

Final Thoughts

Spray foam might not “rot” your roof by itself — but it can trap the moisture that does. That’s why surveyors, mortgage lenders, and insurers are cautious when they see it.

If you’re unsure what’s behind your insulation, or want peace of mind before things get worse, get in touch or browse the rest of our blog for real, no-pressure advice.