Does Spray Foam Insulation Cause Humidity or Condensation in the Loft? (2025 Guide)

Spray foam insulation promises energy efficiency—but can it backfire by causing humidity or condensation problems in the loft? In this guide, we explore whether spray foam can trap moisture, how to spot the early signs, and what to do if your loft is suffering from damp-related issues in 2025.

1. How Spray Foam Works in the Loft

Spray foam expands to seal gaps and crevices in the roofline. While this reduces heat loss, it also significantly alters how your loft ‘breathes.’

Unlike traditional loft insulation, spray foam—especially closed-cell types—can block natural airflow. When this happens, any internal moisture from living spaces below can get trapped.

This increases the risk of:

• Humidity buildup
  
  
• Interstitial condensation (inside the roof structure)
  
  
• Timber rot and mould growth
  
  

2. Why Loft Condensation Happens

Condensation forms when warm, moist air hits a colder surface. In a typical loft, this happens when:

• Bathrooms or kitchens vent into the loft
  
  
• Living spaces below aren’t properly ventilated
  
  
• The roof is sealed too tightly
  
  

Spray foam insulation can contribute to this if it:

• Closes off soffit vents
  
  
• Covers timbers directly (blocking moisture escape)
  
  
• Fails to allow adequate airflow through the eaves
  
  

3. Warning Signs of Humidity or Condensation in a Spray Foam Loft

If you already have spray foam in the loft, watch for these red flags:

• Damp smell or mustiness
  
  
• Visible condensation on rafters
  
  
• Mould growth along ridge beams or insulation
  
  
• Discolouration or dark patches in the foam
  
  
• Swollen timber or soft wood
  
  

If these symptoms are present, your loft’s ventilation may be compromised—and structural damage may be developing.

4. How to Reduce Condensation in a Spray Foam Loft

To minimise humidity and protect your property:

• Check that soffit and ridge vents are clear
  
  
• Add roof vents or a mechanical ventilation system
  
  
• Ensure bathroom and kitchen fans are vented externally, not into the loft
  
  
• Use a dehumidifier in colder months
  
  
• Speak with a surveyor if mould or damp is already present
  
  

5. When to Consider Spray Foam Removal

In many UK cases, removing the spray foam is the only long-term solution. Consider removal if:

• Mould or rot is advanced
  
  
• Your roof timbers feel soft or smell musty
  
  
• A surveyor has flagged the issue
  
  
• You’re facing a mortgage delay due to survey problems
  
  

Learn more: Spray Foam Removal Without Damage

6. Can Spray Foam Ever Work Without Causing Damp?

Yes—but only when it’s applied professionally with ventilation considerations in mind. Open-cell spray foam, for example, is more breathable and less likely to trap water vapour.

Still, even good installations can fail over time due to roof ageing, blocked vents, or changing moisture patterns.

A routine loft inspection is recommended every 1–2 years to ensure no early signs of humidity are developing.

Frequently Asked Questions

Can condensation form under spray foam?
Yes—especially if the foam is applied directly to the roof deck and airflow is blocked.

Will adding more ventilation help?
Yes, but only if it’s strategically placed and doesn’t compromise roof insulation.

Can condensation damage the spray foam?
Over time, yes. It can cause delamination, dark staining, and internal decay of timber.

Should I remove spray foam if I have condensation?
In serious cases—yes. Especially if timber rot or surveyor concerns are involved.

How do I check for moisture issues in a spray foam loft?
Use a moisture meter, inspect visually, or hire a RICS-accredited building surveyor.

Explore trusted Spray Foam Removal Locations or speak to Snug & Secure for expert advice on insulation issues in your loft.