Perm-A-Barrier VPL Low Temp (LT) is a fluid-applied, vapor-permeable air barrier membrane for low temperature applications. Perm-A-Barrier VPL Low Temp (LT) is a two-component, acrylic membrane that cures to form a resilient, monolithic, fully-bonded elastomeric sheet when applied to construction surfaces at temperatures between 20 °F (-7°C) and rising to 60 °F (16°C).