Summary

• This is a very durable wall overall in all the climate zones listed, due mainly to the rain screen properties of the vinyl siding. The vinyl siding is non-absorptive to moisture, thus avoiding solar-driven moisture issues, while maintaining excellent drying potential towards the outside due to the "air-leakiness" of the siding (should any moisture find its way into the wall). The rest of the wall assembly is also relatively vapour permeable outboard of the polyethylene vapour retarder, maintaining excellent drying characteristics towards the outside.
• This is a very durable wall overall in all the climate zones listed, due to both the rain screen properties of the vinyl siding combined with the benefits of the 2 lb. closed cell spray foam in the 2x6 stud cavity.
  
  The vinyl siding is non-absorptive to moisture, thus avoiding solar-driven moisture issues, while maintaining good drying potential towards the outside due to the "air-leakiness" of the siding should any moisture find its way behind it, and the asphalt impregnated 30 minute paper acts as a secondary drainage plane behind the vinyl siding, with lapped joints/seams and lapped over a flashing at the bottom of the assembly. The low vapour permeability of the closed cell spray foam essentially eliminates the possibility of warm, moist indoor air leaking into the assembly by providing a continuous interior air barrier, and also provides water vapour diffusion resistance eliminating the need for an additional vapour control layer such as polyethylene or vapour retarder paint inboard of it. Care should still be taken around penetrations to avoid rain water leaks into the assembly by proper detailing and the use of flashings / sealants etc.
  
  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 technical specifications and manufacturer instruction.

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

• Any liquid water leaking past the vinyl siding will be intercepted by the asphalt impregnated 30 minute paper with lapped joints, lapped over a flashing at the bottom of the wall assembly which acts as a secondary drainage plane and will be drained to the outside
• The vinyl siding is non-absorptive and rain shedding
• 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

• Solar driven moisture is not an issue with vinyl siding and other similar non-absorptive and rain-shedding sidings

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

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