Sheathing & WRB

Sheathing
Sheathing is typically oriented strand board (OSB) or preferably plywood. Plywood is preferred due to its moisture durability, nail holding power, air tightness, and smart vapor retarder properties. As plywood gets wet, its permeability increases which promotes drying. Although, the benefits of air tightness and moisture durability are reduced when a self-adhered weather resistive barrier is applied. 

Weather Resistive Barrier (WRB)
The WRB protects the wall assembly from moisture that finds its way past the cladding and with our recommendation of self-adhered, it serves as the primary air barrier. To be effective, flashing from any fenestration or penetration from services must be well integrated with the WRB and properly lapped to allow drainage by gravity— don’t tuck your pants into your boots during a rainstorm. 

There are drainable and non-drainable varieties, and their specification is dependent on the wall assembly. The drainability is achieved through intentional crinkles or dots or zigzags a few mm tall on the surface of the WRB. 

In our recommended wall with a highly permeable rigid insulation, a non-drainable, self-adhered WRB will suffice. Additionally, our assembly includes a rainscreen that provides many of the benefits of drainable WRB. 

If an impermeable rigid insulation, for example any with foil facing, is used, then a drainable WRB is recommended. It is not necessary, but it will increase drying potential to the exterior. If stucco is used, then some sort of drainage gap is required between the rigid insulation and sheathing and a drainable WRB could work.

Some products such as Zip, Force Field, and OX IS, provide sheathing with an integrated weather resistive barrier, and sometimes rigid insulation in a single product. These products speed up installation and reduce the number of trips around the building. The labor savings can offset some of the incremental material costs. Also, on foam integrated products, shear capacity is reduced and must be reviewed against structural requirements. 

• Detailing methods (basic: tape ply seams; best: use adhered WRB)
• Adhered WRB: self-healing (reference "bathtub test"), airtight, holds integrity vs. floppy things

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