Tags: #architecture #buildingdesign #buildingscience #corrosion #hygrothermal #materialsscience #mgo #waterresistance

Water vapor permeance of WRBs: For some assemblies, it really doesn’t matter.  Here, I examine the semi-permeable range for two climate extremes. 

This wall utilizes structural insulated sheathing comprised of MgO, polyurethane foam, and an outboard 10-mil WRB modeled as either 1-perm or 10-perm. Results are plotted as 30-day moving averages for Months 3 thru 16 of a 40-month 1-D WUFI analysis. Climates reflect ASHRAE ‘Year 1’ data for Minneapolis and Houston. 

The assembly fares extremely well for both climates as indicated by RHs at, or below, 80%. Predicted outcomes show very low risks for corrosion and microbial growth – especially when considering coated fasteners and the low bio-nutritive values of MgO and polyurethane foam. 

How does it work? The WRB is outboard of the MgO and foam, so appreciable diffusion from the exterior does not occur. This makes hot, humid climates a cinch.   During cold periods, vapor drive may occur from the interior.  And as it does, RH increases as air cools at the outer reaches of the foam.  This continues until a reverse moisture gradient is established – opposite of the winter time thermal gradient.  A simple and effective assembly is achieved for all climates.

For too many years, the tail has wagged the dog – we have allowed vapor permeance of the WRB to dictate our assembly design.  True hygrothermal resilience implies quality performance for a wide range of climates, including rare, ‘unexpected’ climate conditions. Simplicity is key. And for assemblies like these, water resistance and insulation thickness/continuity are more important than WRB permeability.

If you look closely at the chart, you will see that a happy medium is reached. The slightly more permeable membrane does better in cold climates and the slightly less permeable membrane does better in hot, humid climates. But both ‘semi-permeable’ membranes do extremely well in both climates.