PHS Sd Variable Plus is a robust airtight membrane with a variable Sd value. It’s reinforced mesh material is designed to create an airtight barrier within the home. With variable diffusion resistance, this membrane can adapt diffusion rates depending on the environment. The membrane creates an airtight layer that dynamically adjusts based on the humidity of the environment, preventing condensation build up and moisture damage. PHS Sd Variable Plus is highly recommended for flat roof builds and retrofits.