Will spray foam cause my timbers to rot?

No. Sprayed foam insulation has the same ‘breathability’ as hard wood so does not cause timbers to rot, in fact as foam insulation reduces the chance of surface and interstitial condensation your timber should have a prolonged life. If timbers are already damp, or have some form of rot or woodworm, they should be treated and allowed to dry before foam is applied. This allows the foam insulation to adhere to the surface correctly, thus ensuring a fully maintained layer of insulation.