> You nearly never want to install insulation without a vapor barrier. And especially not with spray foam
Closed cell foam insulation is a vapor barrier, which is where the problem comes from. You must never have two vapor barriers as any moisture that is between them cannot get out and this will cause mold and rot.
Punching a small hole in a wall and putting foam is fine so long as the foam and the existing vapor barrier are touching each other and thus form one barrier. However if there is something between the vapor barrier and the foam you have two barriers and problems.
Closed cell foam insulation is a vapor barrier, which is where the problem comes from. You must never have two vapor barriers as any moisture that is between them cannot get out and this will cause mold and rot.
Punching a small hole in a wall and putting foam is fine so long as the foam and the existing vapor barrier are touching each other and thus form one barrier. However if there is something between the vapor barrier and the foam you have two barriers and problems.