Another unique situation for vapor diffusion is when moisture storing claddings such as masonry or precast concrete are used. When these types of claddings are exposed to wetting, they can absorb and store significant amounts of moisture. Then, if the sun comes out and heats up these wet claddings, this can create a high humidity condition on the exterior of the building which will cause significant inward vapor drive even in a cold or mixed climate. In other words, when moisture storing claddings are used in cold climates, sometimes the direction of vapor diffusion can reverse.

As discussed for the other wall types with respect to the differences between warm and cold climates, when the vapor drive reverses, the design with respect to vapor diffusion control changes. The figure below illustrates this reversal of vapor drive for a wall with a moisture storing cladding.

This reversal of the vapor drive creates a problem, because typically walls are designed to deal with vapor diffusion in only one direction. There are a number of different ways to design a wall to accommodate this effect.

• Provide a well-ventilated cavity between the moisture storing cladding and the back-up wall
• Use an exterior insulated wall assembly
• Use a split insulated wall assembly with suitable quantity of exterior insulation
• Use a moderately permeable sheathing membrane (i.e. approx. 8 to 18 US perms) to provide some inward vapor control
• Use a smart vapor retarder on the interior of the wall instead to provide adaptive vapor diffusion control to allow for inward drying

Overall, this reversal of the vapor drive for walls with moisture storing cladding is typically less of a concern than is the more consistent outward vapor drive.