Few renovation discoveries make your stomach drop like finding a dark patch on a joist. If spray foam is nearby, it is natural to blame the foam. The honest answer is more useful: properly selected and installed spray foam does not create mould. Moisture does.
What mould actually needs
Mould needs moisture and something organic to grow on. Wood, drywall paper and settled dust can provide that food. Spray polyurethane foam is not usually the meal, but it can sit beside materials that are. If a roof leak, plumbing leak or damp foundation is ignored, mould can still grow on the surrounding structure.
When spray foam helps
Spray foam can reduce the movement of warm, moist air through cracks. That matters in Vancouver Island attics and crawl spaces, where humid indoor air can reach a cold surface and condense. Closed-cell spray foam can also resist vapour movement at suitable thicknesses, depending on the product and assembly.
Where installations go wrong
Problems begin when foam is sprayed over wet or mouldy wood, when an exterior leak is left unresolved, or when the new assembly cannot dry in the direction the designer expected. Foam should never be treated as paint that makes a moisture problem disappear. The source must be found first, the substrate must be clean and dry, and ventilation or drainage details still need to work.
A useful pre-installation question
Ask the contractor: Where will this wall, roof or crawl space dry if moisture gets in? A confident answer should include the air barrier, vapour control, drainage and ventilation strategy, not simply the words spray foam.
If your home in Victoria, Sooke, Langford, Mill Bay or the Cowichan Valley already has a musty smell or visible staining, investigate that before adding insulation. HD Horne Sprayfoam & Insulation Ltd. can assess the insulation and air-sealing side of the assembly and help determine when another building professional should be involved.
