Summary

• This is a relatively durable wall overall in all the climate zones listed, due to both the rain screen properties of the fibre cement board lap siding, and the benefits of the insulative sheathing, whether installed directly over a water resistive barrier such as asphalt impregnated paper, directly over the insulative sheathing, or installed over vertical furring outboard of the insulative sheathing 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 behind it. The asphalt impregnated 30 minute paper and the insulative sheathing act as secondary drainage planes behind the fibre cement board lap siding, and also reduce the risk of interstitial condensation by raising the temperature of the wall assembly inboard of it. However, the low vapour permeability of the insulative sheathing reduces the outward drying potential of the assembly inboard of it, so extra care must be taken to minimize the leakage of warm, moist indoor air into the assembly by means of a continuous interior air barrier, and rain water leaks into the assembly must also be managed by proper detailing and use of flashings/ sealants etc. Construction or built-in moisture must also be managed to reasonable levels. This is especially critical when the assembly is combined with a polyethylene vapour barrier behind the gypsum board, which eliminates the drying potential towards the inside. If the code allows, the polyethylene vapour barrier should be replaced with either vapour retarder paint, or a variable permeance “smart” vapour retarder.

  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

• 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
• Continuous exterior Insulation significantly minimizes thermal bridging and enhances overall effective R-value of the entire assembly
• Due to the limited permeance of exterior insulation materials, keeping the wall dry through detailed flashing and rigorous air barrier applications is important
• 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

• The outward drying potential of this wall is greatly reduced by the layer of insulative sheathing, and if a polyethylene vapour retarder is used, it cannot dry to the inside either, 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 the insulative sheathing, and also that built-in construction moisture is managed to reasonable levels.
• 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, or by the layer of insulative sheathing flashed to the outside, which both act as secondary drainage planes and will drain liquid water back to the outside

Air Leakage Transported Moisture from Inside

• The effect of the insulative sheathing is a reduced risk of interstitial condensation on the wood sheathing or in the stud cavity from warm moisture-laden interior air leaking into the wall assembly, as the insulative sheathing raises the temperature inboard of it, causing the dew point in the heating season mostly to fall outboard of the inner surface of the insulative sheathing
• For colder climates, the thickness of the insulative sheathing must be increased to maintain the location of the dew point falling outside of the inner surface of the insulative sheathing (see code for inboard to outboard thermal resistance ratios)
• 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

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 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

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
• 19.2" stud spacing will typically require exterior sheathing and/or insulation panels to be installed horizontally, whereas 16" and 24" stud spacing can accommodate vertically installed panels (most panel goods have fastener spacing guidelines printed on the material based on vertical installation)
• Insulation, weather barrier and air barrier details and materials are readily available and understood within the Canadian industry
• Constructing walls with exterior insulation is rapidly becoming common practice in some Canadian Zones
• Handling, application and process integration of rigid insulation by Suppliers and Framers can initially affect cycle time - however, production cycle times quickly return to normal after the first few built units
• 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
• Exterior insulating sheathing can be an effective exterior air barrier when joints and seams are properly sealed with compatible air barrier qualified tapes, mastic, or caulking
• 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

• Cost of exterior rigid insulation material and labour vary widely from region to region
• 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
• Wall thickness adjustment could require minor increase of foundation wall thickness (e.g. 8" to 10" foundation width) and increase costs
• Wall thickness adjustment could require minor jamb extensions or additional trim details on openings in the enclosure (i.e. for windows and doors)
• 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

• 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)
• Wall thickness adjustment could require minor increase of foundation wall thickness (e.g. 8" to 10" foundation width) and increase costs
• Wall thickness adjustment could require minor jamb extensions or additional trim details on openings in the enclosure (i.e. for windows and doors)
• Exterior wall dimension width may have minor effect on interior dimensions (i.e. stair widths on exterior walls and overall interior useable square footage)
• 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