CCBST 2017: Design Limits for Framed Wall Assemblies Dependent on Material Choices for Sheathing Membranes and Exterior Insulation

There are many opinions in industry with regard to appropriate material choices for sheathing membranes and exterior insulation of framed wall assemblies. Opinions vary on what is sensible for the vapour permance of materials outboard of the framing so that not only will wetting and drying will be in harmony, but other interests such as costs, wall thickness, and energy efficiency targets can be met.

sheathing.jpgSome common positions held by industry stakeholders in heating dominated climates include:

  1. Drying to the exterior is paramount, therefore the higher the permeance the better,
  2. Vapour flows in both directions, so there is a “Goldilocks” permeance that should be determined on projects, especially for walls with absorptive cladding,
  3. The vapour permeance of the materials outside of the sheathing is irrelevant as long as enough exterior insulation is provided outboard of the sheathing.

Many of these opinions rely on assumptions for rational test conditions such as indoor humidity, construction moisture, air leakage, and rain penetration, which may or may not represent realistic conditions.

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