What does "perms" refer to in the context of vapor barriers?

In the context of vapor barriers, "perms" refers to a measurement of permeability, specifically the permeability rating of a material. This rating determines how easily water vapor can pass through the material. A vapor barrier is designed to control moisture movement and prevent condensation within building assemblies, which can lead to mold and structural damage.

The term "permeability" in this context is crucial because it helps assess the effectiveness of a vapor barrier in various environmental conditions. Materials with a low perm rating are considered effective at preventing moisture from migrating, while those with a high perm rating allow more vapor to pass through. Understanding the perm rating is essential for contractors when selecting appropriate materials for construction projects in areas where moisture control is vital. This knowledge ensures that installations meet necessary building performance and durability standards.