Design Limits for Framed Wall Assemblies Dependent on Material Choices for Sheating Membranes and Exterior Insultation

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

  • Drying to the exterior is paramount, therefore the higher the permeance the better.
  • Vapour flows in both directions, so there is a “Goldilocks” permeance that should be determined on projects, especially for walls with absorptive cladding.
  • 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. In addition the majority of past work is focused on field area of the wall rather than at framing and junctions. This is an oversight since walls often get wet by rain at framing junctions, during and after construction, and the membranes at junctions are often not the same as in the field area.

Many industry stakeholders struggle to make sense of these seeming contradictory viewpoints and how to assess the benefit of specific products beyond the context of code minimums. An approach to overcome this struggle is to develop design guidelines that outline the design limits of materials applied to specific configurations, where hygrothermal performance criteria will be satisfied. An example of a design limit is the maximum amount of moisture that can penetrate past the moisture barrier due to a rain leak without the framing being at risk of deterioration.

Author: Ivan Lee, Patrick Roppel, Mark Lawton

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