Summary

• The primary durability concern for this assembly in some climate zones is solar driven moisture.
  
  The addition of the exterior insulative sheathing reduces 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 drying potential of the assembly inboard of it, so care must be taken to minimize the leakage of warm, moist indoor air into the assembly by means of a continuous air barrier. Rain water leaks into the assembly must also be managed by proper detailing and use of flashings and/or sealants. For colder climates the thickness of the exterior insulative sheathing will need to be increased to keep the dew point outboard of its inner surface.
  
  Also, increasing cavity insulation R-values (R-28 vs. R-19) decreases the temperature of the exterior sheathing, making it more susceptible to condensation accumulation.
• The addition of the exterior insulative sheathing reduces 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 drying potential of the assembly inboard of it, so care must be taken to minimize the leakage of warm, moist indoor air into the assembly by means of a continuous air barrier. Rain water leaks into the assembly must also be managed by proper detailing and use of flashings and/or sealants.
  
  For colder climates the thickness of the exterior insulative sheathing will need to be increased to keep the dew point outboard of its inner surface. The primary durability concern for this assembly in some climate zones is solar driven moisture.
  
  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. Also, increasing cavity insulation R-values (R-28 vs. R-19) decreases the temperature of the exterior sheathing, making it more susceptible to condensation accumulation.

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%)
• 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

Exterior Moisture/Wetting

• The brick veneer with the vented or ventilated air space behind it acts as a rain screen
• The asphalt impregnated 30 minute paper over the insulative sheathing and lapped over a flashing at the bottom of the cavity, and the insulative sheathing with taped joints over the wood sheathing provide secondary drainage planes to direct any water out through the weep holes
• 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 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

• The risk of solar driven moisture problems is somewhat reduced by the addition of the layer of relatively low permeability insulative sheathing, but in certain climates enough water vapour still gets driven into the wall at times to elevate the relative humidity in the stud cavity
• 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

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

• To reduce the potential risk of trapping air leakage transported moisture between the interior vapour barrier and the XPS, extra care and attention must be paid to air barrier detailing especially around any penetrations, intersections, and transitions. The other area to pay attention to are all the exterior details to reduce the potential for rain water to leak into the assembly from the outside, such as around windows, doors, and any other exterior penetrations or transitions. Clearly, proper detailing of air barriers and flashing are crucial to achieve a durable and better built wall assembly.

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
• Length of fastener may need minor adjustment to ensure proper penetration depth into framing member
• 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

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)

Esthetics: Architectural Design

• 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