Summary

• 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 1/2 lb. open cell spray foam in the 2x6 stud cavity. The vinyl siding is non-absorptive to moisture, thus minimizing 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 1/2 lb. open cell spray foam essentially eliminates the possibility of warm, moist indoor air leaking into the assembly by providing a continuous interior air barrier. This assembly does however need a vapour control layer on the warm side, such as polyethylene or vapour retarder paint on the gypsum board inboard of it due to the high vapour permeability of the 1/2 lb. open cell spray foam, but the benefit of the high vapour permeability is that the assembly has good drying potential toward the outside. Care should still be taken around penetrations to minimize rain water leaks into the assembly by proper detailing and the use of flashings / sealants etc.

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
• 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 must be 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, as open cell spray foam has too high a permeability to control vapour diffusion into the assembly on its own and without the additional vapour diffusion control may result in elevated sheathing moisture content levels in the winter months, especially in north and south facing walls
• Any moisture which does get into the assembly can dry towards the outside because the relatively good drying potential towards the outside:
  o 1/2 lb. open cell spray foam is 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)
  
• Should moisture get into the assembly, it 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 relative humidity is typically higher
  o Asphalt impregnated 30 minute paper is vapour permeable
  o vinyl siding is "air-leaky" or "breathable", allowing convective drying to the outside
  

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