Is the spray foam waterproof

Polyurethane foams can be made ‘open cell’ or ‘closed cell’ Open cells foams are not water proof and allow water to permeate via the open cells. Twin component foam is designed to be closed cell. However, scientifically, no polyurethane foam is ever 100% closed cell as the cells sacs making up the foam can and do rupture and this will leaves a percentage of open cells. The percentage of open cells is, however, quite low. A ‘closed cell’ foam is best described as water resistant rather than water proof in much the same way as timber is water resistant but not technically water proof. In practice, closed cell foams afford excellent water resistance and can be applied under roof slates and tiles to prevent the ingress of rain and snow. A depth of foam of 90 mm will give a vapour permeability of less than 1 perm (55ng/m2.s.Pa) which affords vapour barrier protection.