Summary

• Durability issues are related mainly to solar driven moisture, where the brick veneer is absorbing and storing rain water, which is then driven into the assembly by solar-induced vapour pressure, resulting in extended periods of elevated moisture content in the wood sheathing and in the stud cavities or the 2 lb. spray foam for certain climates. Due to the slightly higher water vapour permeance of Plywood in higher relative humidity environments, it appears to be more susceptible to the solar driven inward moisture drive issue than OSB.
  
  In order to be used as an effective air barrier, the 2lb spray foam product must be CCMC approved as part of an air barrier system. Please refer to product technical specifications and manufacturer instructions.

Energy & Thermal Performance

• This wall is subject to some thermal bridging due to exposure of framing elements to outside temperature conditions.
• The framing factor for this wall at 16" o.c. is 23% (i.e. 23% of the wall is wood only and 77% is insulated)
• Advanced framing as defined by NBC 9.36.2.4. (1) (e.g. insulated headers, 2 stud corners, ladder blocking, and in-line framing) can potentially reduce the framing factor by 10% to 20%)
• Maximum nominal R-value of cavity insulation is typically limited to R19,20,22,24 (fiberglass batt) to R30 with medium density spray-applied insulation

Exterior Moisture/Wetting

• The brick veneer with the vented or ventilated air space behind it acts as a rain screen
• However, the relatively low permeability of the spray foam will increase the amount of time that it takes for the spray foam/stud assembly to dry out should it get wet from a rain leak
• The asphalt impregnated paper over the wood sheathing and lapped over a flashing at the bottom of the cavity to direct any water out through the weep holes provides a secondary drainage plane
• Care must be taken at penetrations (windows etc.) by use of proper detailing and use of flashings/ sealants etc. so that water does not leak into the assembly

Air Leakage Transported Moisture from Inside

• The spray foam filling the stud cavity in this assembly effectively manages the leakage of warm, moist interior air, thus essentially eliminating the risk of damage from this mechanism
• In order to be used as an effective air barrier, the 2lb spray foam product must be CCMC approved as part of an air barrier system. Please refer to the product's technical specifications and manufacturer’s instructions
• Additional air leakage control from the inside will be provided by the polyethylene vapour retarder layer recommended for vapour diffusion control, as well as the gypsum board

Water Vapour Diffusion from Outside

• Brick is a moisture absorbing cladding which requires special consideration. Once wetted, the sun can drive the moisture into the assembly if it occurs immediately after a heavy rainfall. Solar driven moisture can raise the relative humidity (rh) for extended periods of time in the vented cavity behind the brick veneer, and raise the rh in the wood sheathing and stud cavity/batt to dangerous levels adequate for the growth of mould and wood deteriorating fungi (rot)
• A well vented cavity behind the brick veneer (vented at both the top and bottom) is recommended to reduce the potential for moisture to be driven into the wall

Water Vapour Diffusion from Inside

• Vapour diffusion into this assembly from indoors will be controlled by the 2 lb. closed cell spray foam, as a thickness of 2" (50 mm) or more provides more than the required vapour diffusion resistance (1 perm or 57 ng/pa.s.m²) required by the building code, and thus the installation of a vapour retarding membrane (such as polyethylene or a vapour retarder paint) on the inside behind the gypsum board or painted onto the gypsum board, should not be required

How to Improve Durability

• In climates and elevations (east and west) where solar-driven moisture is a problem, the use of a non-absorptive cladding such as vinyl siding or coated fibreboard / cement board instead of a brick veneer would be preferred to minimize the solar-driven moisture. Where a brick veneer is to be used, increasing the rate of outside air flow through the cavity behind the brick veneer may help dissipate moisture to the outside. This can be achieved by means of a ventilated cavity, using weep holes at the bottom and vent holes at the top of the brick veneer, and by minimizing the amount of mortar droppings and other obstructions which can potentially block air flow through the cavity. Another strategy is to reduce potential solar gains and rain deposition onto the walls by means of shading with vegetation, fences etc., and increasing the size of roof overhangs where possible will also help.

Ease of Construction

• This wall is easily constructed through traditional stick frame methods on-site
• Exterior wood sheathing provides both structural resistance to "racking" and a nailing substrate for cladding materials
• Insulation, weather barrier and air barrier details and materials are readily available and understood within the Canadian industry
• Materials such as studs, wood sheathing panels and/or insulation sheet goods are readily available in pre-cut lengths for 8' and 9' wall heights
• Some spray-applied insulation products meet CSA standards for air barrier properties
• Spray-applied insulation products must be applied within given ideal temperature ranges as per manufacturer instructions
• Foam plastics (board stock) or other spray applied insulation products will likely require fire protection prior to occupancy as per the NBC/Provincial Building Codes

Affordability: Cost Implications

• Reduction in wood use framing stud members possible (19.2" OR 24" o.c.) with no additional engineering required.
• Alternative bracing methods can be substituted for the wood sheathing panel (e.g. T-slot inlet bracing) - however, details for wall bracing, tall walls, and more than 3-storey construction may require additional engineering
• Advanced framing as defined by NBC 9.36.2.4. (1) (e.g. insulated headers, 2 stud corners, ladder blocking, and in-line framing) can potentially reduce overall lumber costs by upwards of 10 to 20% (i.e. for softwood and panel products)
• Some spray-applied insulation products may require changes to electrical wire gauge requirements within enclosures

Esthetics: Architectural Design

• This wall assembly design can be used up to 3 storey construction under most prevailing building codes
• Exterior wood sheathing provides a nailing substrate for cladding materials including various siding applications (vertical or horizontal)

Additional Sources of Information

• Designing Energy Efficient Building Enclosures, FPInnovations
  
  
  High R Value Enclosure Assemblies, Building Science Corporation
  
  
  Guide to Attaching Exterior Wall Coverings through Foam Sheathing to Wood or Steel Wall Framing, FSC