Summary

• This is a durable wall overall in all the climate zones listed, due to both the rain screen properties of the fibre cement board lap siding combined with the benefits of the 1/2 pcf open cell spray foam in the 2x6 stud cavity. The fibre cement board lap siding is water shedding on all painted surfaces, thus minimizing solar-driven moisture issues, while maintaining good drying potential towards the outside due to the ventilation behind the siding for convective drying should any moisture find its way behind it, and the asphalt impregnated 30 minute paper acts as a secondary drainage plane behind the fibre cement board lap siding, with lapped joints/seams and lapped over a flashing at the bottom of the assembly. The 1/2 pcf open cell spray foam helps manage the leakage of warm, moist indoor air into the assembly by providing a continuous interior air barrier in each stud cavity, but additional air leakage control is needed as it cannot seal wood to wood framing connections and many other details. This assembly needs a vapour control layer on the warm side, such as polyethylene, or where codes allow a vapour retarder paint on the gypsum board or a “smart” variable permeance vapour retarder, due to the high vapour permeability of the ½ pcf open cell spray foam. The benefit of the high vapour permeability is that the assembly has good drying potential toward the outside. Care must still be taken around penetrations and transitions to avoid rain water leaks into the assembly by proper detailing and the use of flashings / sealants etc.

  Notes

  • Fibre cement board is susceptible to freeze-thaw deterioration, so it is important to maintain minimum clearances as per manufacturer guidelines from the ground or wall/roof intersections to avoid excessive water absorption, and contact with standing water should always be avoided
  • Due to the caustic nature of the product, corrosion resistant fasteners must be used as per manufacturer guidelines
  • Butt joints where two adjacent fibre cement board lap siding boards meet must be installed with a piece of flashing behind the joint as per manufacturer guidelines, with the bottom lapped over the top of the siding board beneath to direct any water back out onto the surface
  • Any field cut edges must be caulked, painted or primed as per manufacturer guidelines to avoid moisture absorption
  • Best practice is to install fibre cement board lap siding with the fastener heads driven snug against the siding (not flush), as the fastener head can then act as a spacer providing a small air gap where the two boards overlap shingle style, as this provides ventilation for convective drying and allows the assembly to better act as a rain screen

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
• The effective R value calculations and the durability analysis were completed for cement board assuming a minimum 10mm cavity space between the cladding and the substrate.It should be noted that some fiber cement board products can be installed over solid-foam insulation board up to 1-in. thick without the 10mm cavity or furring strips for additional fastening. Local prevailing building codes and manufacturers installation instructions should be deferred to in every case.

Exterior Moisture/Wetting

• 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
• Fibre cement board lap siding is rain shedding and acts as a rain screen when installed properly in accordance to manufacturer instructions, with all field cut edges caulked, painted or primed and by installing flashing behind each siding butt joint, and at penetrations and transitions with proper detailing
• Any liquid water leaking past the fibre cement board lap 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
• Any water leaking past the asphalt impregnated 30 minute paper and into the framing and stud cavity will have a difficult time drying to the outside due to the low vapour permeance of the 2 pcf closed cell spray foam and the relative air tightness preventing convective drying, so extra care must be taken at all penetrations and transitions (windows etc.) so water does not leak in behind the asphalt impregnated paper, and also that built-in construction moisture is managed to reasonable levels

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
• Even though the open cell spray foam filling the stud cavity helps reduce air leakage, 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.) will still be required, which will also help reduce heating and air conditioning costs.

Water Vapour Diffusion from Outside

• Solar driven moisture is not a significant issue with fibre cement board lap siding and other similar rain-shedding sidings provided the siding is caulked, painted or primed on all exposed surfaces and field cut edges and installed in accordance to manufacturer guidelines

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
• 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
• Length of fastener may need minor adjustment to ensure proper penetration depth into framing member. When applying fiber cement board siding to furring strips, which is applied on foam sheathing or other non-nailable substrates (e.g. gypsum sheathing) witha combined thickness up to 4 inches thick, the foam sheathing and other non-nailable substrates do not have adequate nail holding capacity. In this scenario the nail holding substrate for the cladding is the furring strips.Fastener depth and type must be specifed by the designer in accrodance with the manufacturers instructions.

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
• Some fiber cement board products will require a 3/8 in. air gap (Rainscreens, Furring, Etc.), when installing cladding on a Multi-Family/ Commercial project. Local prevailing building codes should be deferred to for rainscreen guidelines and applications.

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)
• Fiber cement as do all building materials, expands and contracts with changes in relative humidity and temperature. Most manufacturers recommend designing for this type of movement on buildings with long runs of continuous siding.
• Fiber cement board is available in both plank(single long traditional siding lenghts) and panel (up to 4x8 sheet/panel) configurations. This allows for a wide range of exterior finishes and architectural features.

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