Summary

• Durability issues are related mainly to solar driven moisture, where the brick veneer is absorbing and storing wind-driven rain, which is then pushed into the assembly by solar-induced vapour pressure, resulting in extended periods of elevated moisture content in the sheathing and in the stud cavities for certain climates.
  
  The use of a non-absorptive cladding such as vinyl siding or coated fibreboard/ cement board instead of a brick veneer would be the best solution, avoiding the solar-driven moisture problem all together in climates where it is a problem.
  
  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.

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 24" o.c. is 20% (i.e. 20% of the wall is wood only and 80% 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%)

Exterior Moisture/Wetting

• The brick veneer with the vented or ventilated air space behind it acts as a rain screen
• 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

• Air leakage into the wall must be managed by means of a continuous interior or exterior air barrier (preferably both), combined with proper detailing at any connection or penetrations (window openings, electrical boxes, plumbing penetrations etc.), which will also help reduce heating and air conditioning costs
• If the leakage of warm moisture-laden interior air into the wall assembly is not managed, there is a risk of interstitial condensation on the wood sheathing, however, it has the opportunity to dry to the exterior because there are no impermeable materials outboard of it.

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 is controlled by 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
• Wall assembly has very good drying potential towards the outside:
  o Fibreglass batt is very vapour permeable
  o Wood sheathing is relatively vapour permeable when on the cold side of a wall, where rh is typically higher
  o Asphalt impregnated 30 minute paper is vapour permeable, and
  o The air space behind the brick will be vented to the outside through the weep holes at the bottom of the brick veneer (and the assembly will perform even better if ventilated with both weep holes at the bottom and at the top of each storey)
  

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

Affordability: Cost Implications

• 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)

Esthetics: Architectural Design

• 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