August 13, 2008

Rainwater is absorbed into the brick veneer after a rainstorm. Water vapor is then transported from the rain-wetted brick veneer through the exterior sheathing into the exterior wall cavities. by a process called vapor diffusion. This water vapor condenses on the cavity side of the interior polyethylene vapor barrier. When sufficient moisture condenses on the surface of the polyethylene vapor barrier, it runs down the surface of the polyethylene where it accumulates on the bottom plate of the wall. If sufficient water accumulates at the bottom plate, mold growth will occur, ultimately followed by wood decay.