Wall ID: 3959

Summary

• This is a durable wall overall in all the climate zones listed, due mainly to the rain screen properties of the painted bevelled wood siding installed over vertical furring as required in some jurisdictions (best practice). However, the thicker 2x8 assembly has a slightly reduced drying ability compared to a 2x6 assembly and a colder cladding temperature resulting in poorer performance in an Edmonton climate. The bevelled wood lap siding is water shedding on all painted surfaces, thus minimizing solar-driven moisture issues, while maintaining excellent 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 variable permeance vapour retarder also allows drying towards the inside.
  
  Notes:
  • Any field cut edges must be painted and primed as per manufacturer guidelines to avoid moisture absorption
  • Best practice is to install bevelled wood 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%)
• If wood siding is installed over a standard 16" on center stud built wall and furring strips can be lined up with the studs, then a minimum strapping thickness of 10mm (3/8 inch) is permissible.If the strapping is NOT applied directly over studs, then 25mm (full 1 inch thick) strapping MUST be used. See manufacturers installation guidelines in all cases.

Exterior Moisture/Wetting

• Care must be taken at all penetrations and transitions (windows, etc.) by use of proper detailing as well as sealants and/or flashing to ensure water does not leak into the assembly.
• The lapped board/bevelled wood siding is rain shedding, and therefore acts as a rain screen when installed properly in accordance to manufacturer instructions. All field cut edges must be caulked, painted or primed. Flashing must be installed behind each siding butt joint.
• Any liquid water leaking past the siding will be intercepted by the asphalt impregnated 30 minute paper. It must have lapped joints and be lapped over flashing (located at the bottom of the wall assembly) to act as a secondary drainage plane and drain any liquid water to the outside.
• Unlike polyethylene, a variable permeance "smart" vapour retarder will help the assembly dry to the inside and help reduce risk of moisture accumulation. 
• The lapped board/beveled wood siding must be installed over 3/4" or thicker vertical furring to provide ventilation behind the cladding.
• Built-in construction moisture must be managed to reasonable levels.
• The lapped board/bevelled wood siding is non-absorptive and rain shedding.

Air Leakage Transported Moisture from Inside

• Air leakage into the assembly must be managed by means of a continuous air barrier (preferably both interior and exterior). Proper detailing at any connection or penetrations (window openings, electrical boxes, plumbing penetrations etc.), 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 siding in the furring space allowing for convective drying.

Water Vapour Diffusion from Outside

• Solar driven moisture is not a significant issue with vinyl, fibre cement, metal, Maibec type coated wood siding (with all surfaces coated) or other similar non-absorptive and rain-shedding sidings, provided they are detailed properly, they have a ventilated (openings at the top and bottom of the assembly) air gap minimum 3/8" behind them, and that they are installed in accordance to manufacturer guidelines.
• The non-absorptive lapped board siding does not have significant solar driven moisture issues when installed properly in accordance to manufacturer instructions. All field cut edges must be caulked, painted or primed. Flashing must be installed behind each siding butt joint.

Water Vapour Diffusion from Inside

• Vapour diffusion from the inside must be controlled by the installation of a vapour retarding membrane (such as polyethylene, 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 very 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 relative humidity is typically higher
  • Weather barrier is vapour permeable
  • Fibre cement siding is ventilated, allowing convective drying to the outside
  

Drying Potential

• Should the moisture get into the assembly through penetrations, it has good drying potential to the outside. Air movement behind the lapped board siding over vertical furring is important.
• Built-in moisture must be managed to reasonable levels.

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
• Prior to installation, furring strips shall be installed. It is very important to ensure adequate air circulation and drainage between the siding and sheathing so that walls can dry in all seasons and conditions. It is critical to follow manufacturer installation requirements and local prevailing building codes regarding drainage details and/or rain screen requirements.
• Wood siding cannot be applied DIRECTLY over/in contact with concrete forms (ICF) or continuous exterior insulated foam sheathings . See wood siding manufacturers installation requirements for fastening instructions when applying to ICF or insulated foam sheathing assemblies.

Affordability: Cost Implications

• Reduction in wood use framing stud members is 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)

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)