What is the difference between a vapor retarder and a vapor barrier?

• A. A vapor barrier has very low permeability to moisture diffusion (typically a barrier); a vapor retarder slows diffusion but allows some moisture passage; placement depends on climate.
• B. A vapor retarder completely blocks moisture; a vapor barrier allows all diffusion.
• C. They are the same thing.
• D. Vapor barrier is for wind; vapor retarder is for water.

Answer: (A)

Moisture moves through building assemblies mainly by diffusion, and the difference between a vapor barrier and a vapor retarder is how easily water vapor can pass through them. A vapor barrier is designed to be very low in permeability, blocking most of the diffusion of moisture. The idea is to prevent interior moisture from migrating into cooler wall cavities where it could condense and cause problems.

A vapor retarder, on the other hand, slows down diffusion but doesn’t stop it completely. It reduces the rate of moisture movement enough to manage condensation risk, but still allows some vapor to pass through. This is useful in assemblies where completely blocking diffusion could trap moisture in spots where it could cause mold or damage, or where seasonal changes in humidity make a totally impermeable barrier undesirable.

Placement of these materials depends on climate because the direction and amount of vapor drive change with temperature and humidity. In practice you choose barrier or retarder placement to keep vapor from condensing in the most vulnerable parts of the envelope, balancing interior and exterior moisture drives. That’s why the statement describing a vapor barrier as having very low permeability and a vapor retarder as slowing diffusion (with placement depending on climate) is the best fit.