Spray-foam insulation makes homes unable to be mortgaged

Original Article ↗ Hacker News Discussion ↗

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.