Is Spray Foam Insulation Waterproof?

If you’ve had spray foam installed — or are considering removing it — you might be wondering: “Does it block water?” or “Is it waterproof?”

The answer is… sort of.
Spray foam, particularly closed-cell foam, is water-resistant. But in a loft or roof space, that’s not always a good thing. In fact, this so-called waterproofing is one of the reasons spray foam causes problems in UK homes.

Let’s look at how spray foam handles moisture — and why it’s often better to let your roof breathe.

What “Waterproof” Really Means

Closed-cell spray foam:

• Forms a solid, dense barrier
• Blocks both airflow and water vapour
• Repels liquid water on the surface

That makes it great for things like commercial roofs or basements — but not for the timbers in your loft, which are designed to breathe and release moisture gradually over time.

Open-cell foam, on the other hand:

• Is more porous
• Allows some vapour to pass through
• Still absorbs moisture if exposed directly

Why “Waterproofing” Can Be a Problem in Lofts

Your roof isn’t just a lid on the house — it’s part of a delicate system that includes:

• Air circulation
• Heat flow
• Moisture release

When you seal it tightly with closed-cell foam, you can create a perfect storm for trapped condensation. That leads to:

• Timber rot
• Mould
• Hidden damp patches behind the insulation

We’ve removed spray foam from dozens of lofts where water damage was slowly brewing underneath a “waterproof” layer.

But Doesn’t It Help Prevent Leaks?

Not really. Spray foam isn’t a substitute for a watertight roof. If there’s a leak in your tiles or flashing:

• The water may still get in
• It can become trapped between the roof deck and the foam
• It can go unnoticed for months or years

A breathable, well-ventilated loft actually makes leaks easier to spot and fix before they do real damage.

What If My Home Has Poor Ventilation?

Then spray foam is even riskier. Lofts without proper airflow are more likely to develop:

• Condensation
• Poor air quality
• Insulation failure over time

In these cases, the best solution is often to remove spray foam and replace it with breathable insulation designed to regulate moisture.

You can learn more about our removal process here.

Final Thoughts

Spray foam might be “waterproof” — but that doesn’t mean it’s right for your loft.

In most UK homes, especially older properties, a breathable insulation system is far safer, more sustainable, and less likely to cause problems with surveys or lenders.

If you’re not sure whether your insulation is helping or hurting, get in touch for straightforward advice — or check out our full blog for more guidance.