Many people wonder if a vapor barrier is necessary when they insulate with spray foam. The answer is a bit complex as there are two kinds of spray foam insulation commonly used in homes: closed cell and open cell. Each has a different ability to control vapor diffusion. Generally speaking, closed cell has greater vapor diffusion control than open cell. (Closed cell also has a higher R-value per inch than open cell). However, other factors come into play, including local climate, the formulation of spray foam used and the thickness of the spray foam application. With most spray foam projects involving experienced spray foam contractors, it is advisable to consult your contractor when it comes to the need for adding a vapor barrier. Your contractor will be familiar with the formulation used, the proposed thickness, the local climate and local building codes. All will impact the potential need for a vapor barrier.
A bit more information may be helpful:
Generally speaking, a vapor barrier is more likely to be required when open cell spray foam is used because open cell lacks the vapor diffusion characteristics of closed cell.
The U.S. Department of Energy has studied the issue and has published guidelines for vapor barriers in spray foam applications based on different climates in the U.S. There are many areas where open cell foam should be accompanied by vapor barriers!
It is very important to check your local building codes as they will address the need for vapor barriers in spray foam projects.
Remember that as with most things, the quality of your spray foam application is only as good as the materials used and the quality of the installation. Consult an expert!