Air sealing is critical to prevent mold growth in a structure. When there’s a lack of exterior or interior sealing, cold outside air collides with warm inside air within the walls, there is a risk of condensation — ideal conditions for mold growth. At the same time, air movement is also important in our buildings. We introduce a ton of moisture in our buildings during the construction process and that moisture needs to escape! We also introduce moisture in our buildings in a variety of other means: cooking, showers, breathing, and more. Here in the northwest, the amount of moisture transmission through a 1 inch2 hole in your average 4’ x 8’ sheet of drywall during a normal winter (heating) season—assuming 70˚ interior and 40% RH—would be nearly 30 quarts of water. That’s a lot of moisture migration. As a result, avoiding the risks and liabilities of improper air sealing, requires controlling air and vapor movement in the building. While this can be addressed from the building exterior, the building interior, or both, we will consider approaches for the building interior. In order to understand vapor control and interior air, we need to take a historical view of both what has been used, and is currently being used for interior air sealing. VAPOR BARRIER One of the original solutions was Poly Sheeting. This vapor barrier is not permeable, meaning that it keeps interior moisture inside and exterior moisture outside. As it traps mold-friendly moisture in the wall assembly, builders and contractors who know best practices no longer use it. VAPOR RETARDER An improvement to Poly Sheeting, at least in theory, is the Kraft Faced Batt. This vapor retarder is a little more permeable than Poly Sheeting, but in the end, not much better. The perm rating is up to 1.0. Despite the higher number, by definition, Kraft Faced Batts are still not permeable, therefore you risk liability by trapping moisture in the wall assembly. As demonstrated in the illustration below, you do get some moisture movement to both sides of the wall, but the perm rating limits the drying potential in the wall assembly. A perm rating of 10 or higher is required for a barrier to be considered permeable. “SMART” MEMBRANE Next in the advancement of interior membranes came the CertainTeed Smart Vapor Control Membrane. CT was the first to market with this product. Shortly after, another company ventured into this segment with a similar, slightly improved product. In the end, both these smart membranes have moisture movement going in both directions of the wall assembly. With these products you have the risk of moisture getting into the wall assembly as the membrane is open in both directions. AIR AND VAPOR CONTROL LAYER SYSTEM The latest and best solution for internal air and vapor control is the Majrex Air & Vapor Control Membrane. Unlike other solutions, Majrex is the only unidirectional membrane. This enables moisture to only travel in one direction, enabling interior moisture to escape without letting exterior moisture into the wall assembly. Majrex air and vapor control membrane can also be used for blown-in insulation, creating both an air and vapor control product in one. Applied air tight, this solution can meet passive house standards in air sealing. Plus, installation is aided by an incredible tape solution. Simple application allows for an airtight seal around interior penetrations and windows. A 5’ roll with protruded core keeps the material clean. And featuring cutting and bonding guides on the membrane helps keep your install straight and clean.