Summary

• The addition of the semi-rigid rock wool wall insulation reduces the risk of interstitial condensation due to interior air leakage or vapour diffusion from the inside by raising the temperature of the wall assembly inboard of it.
  
  The high permeability of the semi-rigid rock wool insulation may allow for solar-driven moisture to penetrate into the wall assembly at times, but this risk can be managed by means of a well ventilated cavity behind the brick veneer, allowing the wall to dry out again quickly.
  
  Due to the slightly higher water vapour permeance of Plywood in higher relative humidity environments, it appears to be more susceptible to the solar driven inward moisture drive issue than OSB.

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

Exterior Moisture/Wetting

• The brick veneer with the vented or ventilated air space behind it acts as a rain screen
• The semi-rigid rock wool wall insulation with its hydrophobic treated fibres, and the spunbonded polyolefin with lapped and taped joints, lapped over a flashing at the bottom of the cavity provide secondary drainage planes to direct any water out through the weep holes at the bottom of the brick veneer
• 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.

Air Leakage Transported Moisture from Inside

• The effect of the semi-rigid rock wool wall insulation 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 dew point in the heating season will mostly fall outboard of the wood sheathing and the spunbonded polyolefin
• For colder climates, the thickness of the insulative sheathing should be increased to maintain the location of the dew point falling outside of the wood sheathing
• 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

• A well vented cavity behind the brick veneer (vented at both the top and bottom) is recommended to reduce the potential for moisture to be driven into the wall
• There is a possible risk of solar driven moisture problems due to the high vapour permeability of the insulative sheathing, resulting in periods of time where the wood sheathing layer behind it is may be at elevated levels of moisture content
• Another solution would be to use a non-absorptive cladding such as vinyl siding or coated fibreboard, avoiding the solar-driven moisture problem all together

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

How to Improve Durability

• In climates and elevations (east and west) where solar-driven moisture is a problem, the use of a non-absorptive cladding such as vinyl siding or coated fibreboard / cement board instead of a brick veneer would be preferred to minimize the solar-driven moisture. Where a brick veneer is to be used, increasing the rate of outside air flow through the cavity behind the brick veneer may help dissipate moisture to the outside. This can be achieved by means of a ventilated cavity, using weep holes at the bottom and vent holes at the top of the brick veneer, and by minimizing the amount of mortar droppings and other obstructions which can potentially block air flow through the cavity. Another strategy is to reduce potential solar gains and rain deposition onto the walls by means of shading with vegetation, fences etc., and increasing the size of roof overhangs where possible will also help.

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
• Constructing walls with exterior insulation is rapidly becoming common practice in some Canadian Zones
• 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
• Due to the semi-rigid nature of the exterior rock wool wall insulation, strapping may be required in order to attach exterior cladding materials (e.g. vinyl or shiplap type siding)

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
• 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)
• Thickness of exterior rigid insulation that is 2" or more may require strapping for adequate fastening of exterior cladding and increase costs
• 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)
• 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)

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