A vapor barrier separates the two environments vapor barrier vapor retarder vapor retarder efficient water vapor barrier vapor retarder U.S. Department of Energy Vapor Barriers or Vapor Diffusion Retarders energy.gov vapor retarder U.S. Department of Energy vapor diffusion retarders vapor retarder vapor retarder energy.gov Vapor Barriers or Vapor Diffusion Retarders vapor retarder vapor barrier U.S. Department of Energy vapor retarder U.S. Department of Energy A vapor barrier separates the two environments vapor barrier