Vapor Barrier
A vapor barrier is a layer of material designed to slow or stop the diffusion of water vapor through a building assembly. The classification is based on perm rating: a Class I vapor barrier (≤0.1 perm) like polyethylene sheet stops vapor almost completely; Class II (≤1 perm) is a vapor retarder; Class III (≤10 perm) is a vapor-permeable layer. Whether you need a vapor barrier — and which side of the assembly it goes on — depends entirely on climate zone and assembly type.