Do I need a vapor barrier with spray foam insulation?

Whether you need vapor barrier with spray foam insulation depends on type of spray foam, climate, and building code, as closed-cell spray foam itself acts as vapor barrier, while open-cell does not, similar to different storage protections. Closed-cell spray foam at 2 inches or more typically has low vapor permeance under 1 perm, acting as Class II vapor retarder, and at 1.5 inches may meet vapor barrier requirement, so additional vapor barrier like plastic sheeting often not needed and can even trap moisture if added incorrectly, as closed-cell itself resists moisture. Open-cell spray foam is vapor permeable, around 8 to 16 perms at 3.5 inches, so it allows moisture to pass, and in cold climates, building code may require additional vapor retarder like paint or membrane on warm side to prevent condensation. Building codes in International Residential Code require vapor retarder in certain climate zones, but spray foam exception allows closed-cell at certain thickness to count as vapor retarder. So you generally do not need additional vapor barrier with closed-cell spray foam at recommended thickness, but may need with open-cell in cold climates, consult local code and ensure no double vapor barrier trapping moisture, as improper vapor barrier can cause mold behind foam, so check climate zone and foam type.