Tags: #architecture #buildingdesign #buildingdurability #buildingmaterials #buildingscience #materialsscience #resilience #waterresistance

Water resistance of glass-faced gypsum sheathing. Comparison of an integral WRB panel to conventional sheathing products.

This graph compares moisture content of gypsum sheathing resulting from five-hour ponding tests. As I’ve described in prior posts, I’m using a ponding height of just 0.17 inch. These data represent means from four replicates per product – each sampled from separate 4’ x 8’ panels. 

The integral WRB panel performed no better than conventional glass-faced sheathing. In fact, three products performed significantly better. Clearly, shared mechanisms of wetting and moisture transport are in play. 

So how does the integral panel claim WRB status? That’s easy – employ test protocols and criteria such as ASTM E331 and AATCC 127/AC38. Neither of these methods assess water accumulation within the substrate itself (i.e. the gypsum core). Instead, they assess water penetration through the entire substrate under very short durations. This interpretation defies the fundamental intent of building codes while also vastly underestimating the ramifications to real-world durability. 

 Because glass-facers absorb water, it’s critical that their protective layers do not. In the eye of the code, the purpose of the WRB is to A) resist liquid water and B) prevent water accumulation within the assembly. With respect to moisture accumulation, the sheathing is arguably the most important element of the assembly.