Spray-foam insulation makes homes unable to be mortgaged
Spray-foam insulation applied to UK roofs is rendering many homes unmortgageable, because lenders cannot inspect the covered timbers and fear hidden moisture damage and rot. Commenters explain how improper use of foam can trap condensation or roof leaks against wood, turning minor defects into major structural risks that are costly to diagnose and fix. The debate contrasts UK experiences with stricter building-science approaches elsewhere, while highlighting the role of unqualified installers, weak regulation, and parallels with other “green” upgrades like leased solar panels that complicate property sales.
Mechanism of Damage & Risk
- Main claimed mechanism: spray foam (especially closed-cell) can trap moisture against wood, preventing it from drying and leading to rot, mold, and structural decay.
- Condensation risk: warm, humid interior air can leak into insulation, cool against cold roof/wall surfaces, and deposit water in timber or foam.
- Multiple commenters stress that insulation must be paired with a well-designed vapor/moisture barrier strategy; two vapor barriers with moisture between them are especially dangerous.
- Foam also hides roof structure, making it hard to visually inspect timber for damage or leaks; lenders and surveyors treat this as a major unknown.
- Some argue the real problem is undetected leaks: water can infiltrate via a failing roof and be trapped by foam for years without visible drips, accelerating hidden decay.
Mortgage, Resale, and Lender Behavior
- Lenders dislike unquantified risks. If roof timbers cannot be inspected without removing foam, they may refuse mortgages entirely.
- This makes properties much harder to sell to typical buyers who need financing, even if the foam was installed correctly.
- Homeowners may need to pay thousands to remove foam simply to enable inspection and sale, effectively turning a “green upgrade” into a liability.
Regional Practices & Disagreement
- In the UK, spray-foaming under roof tiles is seen as risky and often associated with “cowboy traders” doing poor surveys and installs.
- Some note that spray foam is widely used and recommended in North America when properly designed (e.g., closed-cell under roof decks, correct venting and moisture control).
- Others counter that long-term problems (hidden moisture, corrosion in metal structures, off‑gassing smells) are documented and underappreciated.
Alternative Approaches & Best Practice
- Several participants favor mineral wool, PIR boards, traditional vented attics, explicit vapor barriers on the warm side, and careful dew point design.
- Tight building envelopes should be paired with deliberate mechanical ventilation (HRV/ERV) rather than relying on “leaky” old-house self‑ventilation.
Systemic & Regulatory Issues
- Thread highlights “cowboy” contractors, weak oversight, and negative externalities: careful homeowners are penalized because lenders cannot distinguish good from bad installs.
- Some argue stronger regulation and standards would prevent both botched work and blanket lender refusals.