There is some debate on whether or not you need a vapor barrier with spray foam insulation. Some say that the closed-cell structure of the foam prevents moisture from entering, and therefore no vapor barrier is necessary. Others recommend using a vapor barrier any time there is insulation installed in order to avoid moisture buildup and potential problems with mold or rot. The best way to decide if a vapor barrier is necessary in your situation is to speak with an expert at your local hardware store or foam insulation installer. They will be able to advise you on the best course of action based on the climate where you live and other specific factors about your home.