Does spray foam trap moisture in a Queensland roof or subfloor?

It can, and in our humid climate that's the thing I'd worry about first. Closed-cell polyurethane foam sets like a hard skin against your roof timbers or floorboards. The Australian Building Codes Board's Condensation in Buildings Handbook spells out the mechanism: closed-cell insulation typically has a low vapour permeance, while open materials like mineral wool let moisture through. So foam seals moisture in instead of letting it escape. In a damp SE-QLD roof cavity or subfloor, that sealed skin can hold moisture against the timber so it never dries out, and that's the documented road to hidden rot and mould the ABCB warns breeds in concealed spaces. Cellulose in your ceiling and polyester under your floor are both vapour-open, so the structure keeps breathing and drying the way it's meant to.

Because we live in a warm, humid climate, and warm air carries a lot of moisture. When that moist air meets a cool surface inside a roof or subfloor, it condenses back to water. The Australian Government's yourhome.gov.au guide ties insulation directly to this: it says insulation must be installed correctly to manage condensation risk, and that thermal bridges lead to condensation problems. A vapour-open insulation lets that moisture pass through and dry out. A vapour-closed product like closed-cell foam can stop it escaping, so the damp sits against your timber. In a dry southern climate foam gets away with more; here in SE-QLD the humidity is exactly what makes a sealed, low-permeance skin risky.

For our climate, yes, and that's why we use them. Pumped cellulose in a ceiling is a paper fibre; it's hygroscopic, so it takes up a bit of incidental moisture and then dries back out rather than sealing it in. Polyester batts under a floor are vapour-open and hydrophobic; they don't waterlog and they let the subfloor keep breathing. Neither one creates the sealed, low-permeance skin that closed-cell foam does. I want to be straight with you about the limit: no insulation waterproofs your roof, and any insulation performs poorly if it gets badly wet, so a real leak still needs the roof fixed. But for handling the everyday humidity of an SE-QLD home, a breathable product is the safer bet.

Not easily, and that's the second problem with foam. Once closed-cell foam cures it bonds to whatever it was sprayed onto, the roof timbers, the floor joists, the pipes and the wiring. Australian installers describe removal as difficult and largely irreversible; taking it off generally means grinding it back off the structure and repairing what's damaged underneath, and it complicates future termite inspection and roof repairs. So if moisture does get trapped and the timber starts to suffer, you can't just lift the insulation out to deal with it. Cellulose and polyester both come back out, cellulose vacuums out of a ceiling, and polyester batts lift straight back down from between the joists in an afternoon.