Vapor barriers are a complex subject, and as new materials have entered the industry, the topic has only gotten more complicated, writes JLC's Ted Cushman.
One new entry in the market is Majrex by Swiss firm Siga. It's an advanced vapor control membrane that not only adjusts to variable humidity, but actually has a greater moisture permeability in one direction than in the other
The material is intended for use in cold climates, where its purpose is to protect insulated wall and roof assemblies from indoor-generated moisture during winter, but still allow the insulated cavities to dry out towards the occupied space during summer.
Based on “dry-cup” and “wet-cup” ASTM testing, the less-permeable face of Majrex is rated at just 0.16 (“dry”) to 1.3 perms (“wet”); that’s the side you install facing the conditioned space. The more permeable face opens up from 0.17 (“dry”) to as much as 3.8 perms (“wet”); that side is placed facing into the fiber-insulated wall or roof assembly. So all else being equal, moisture can move out of the wall or roof into the conditioned space three times faster than it can move in the opposite direction, into the walls or roof. Siga rep Keith Bidwell showed JLC a simple demonstration (above): If you seal wet wood into two envelopes made of Majrex, one with the vapor-open side in, and one with the vapor-open side out, only one piece of wood will dry out.
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