Summary

• This is a durable wall overall in all the climate zones listed, due mainly to the rain screen properties of the fibre cement board lap siding, whether installed directly over a water resistive barrier such as asphalt impregnated paper 30 minute paper, or installed over vertical furring as required in some jurisdictions (best practice). 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 into the wall. The rest of the wall assembly is also relatively vapour permeable outboard of the polyethylene vapour retarder, maintaining good drying characteristics towards the outside.

  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 19.2" o.c. is 21.5% (i.e. 21.5% of the wall is wood only and 78.5% 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 reasonable amount of moisture which may leak into the wall through penetrations can dry to the outside due to the ventilation behind the fibre cement board lap siding, and the good drying characteristics of the whole assembly.
• 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

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
• This wall assembly has good drying potential towards the outside due to the ventilation behind the fibre cement board lap siding allowing for convective drying

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 from the inside must be controlled by the installation of a vapour retarding membrane (polyethylene, or with a vapour retarder paint or variable permeance “smart” vapour retarder if the code allows) on the inside behind the gypsum board or painted onto the gypsum board according to code. Should moisture get into the assembly, it has good drying potential towards the outside:
  
  • Fibreglass batt is very vapour permeable
  • Wood sheathing is relatively vapour permeable when on the cold side of a wall, where the relative humidity is typically higher
  • Asphalt impregnated 30 minute paper is vapour permeable
  • Fibre cement board lap siding is well ventilated behind it, 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
• 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

• 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 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

• 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