Summary

• While sheathings with extremely low permeance might raise questions of trapped vapour within the assembly, thousands of such walls exist and seem to perform relatively well. Several factors contribute to this performance such as drying towards the outside through gaps in the polyiso sheathing, and better than expected tightness of the polyethylene vapour barrier/retarder to both air leakage and vapour diffusion.
  
  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.
  
  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%)
• 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

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

• For colder climates, the thickness of the insulative sheathing must be increased to maintain the location of the dew point falling outside of the stud cavity and the wood sheathing (see code for inboard to outboard thermal resistance ratios)
• Due to the water vapour impermeable foil facing on the exterior surface of the insulative sheathing, combined with the polyethylene vapour retarder/barrier on inner surface of the stud cavity, this assembly has a "double vapour barrier" resulting in reduced drying potential and the possibility of trapping moisture within the assembly. Thus, air leakage into the wall must be carefully 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
• An exterior air barrier approach by taping the joints on the foil face of the insulative sheathing may not be desirable for this assembly as it may reduce any air leakage related drying potential towards the outside, but here the asphalt impregnated 30 minute paper now becomes important as a drainage plane
• The positive 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 dew point in the heating season will mostly fall outboard of the inner surface of the polyiso

Water Vapour Diffusion from Outside

• The risk of solar driven moisture problems is essentially eliminated by the foil facing on the layer of insulative sheathing
• 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

• Due to the vapour impermeable foil facing on the insulative sheathing, there is a risk of interstitial condensation, thus vapour diffusion from the inside must be controlled by the installation of a vapour retarding membrane (polyethylene or vapour retarder paint) 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 moisture between the interior polyethylene vapour barrier/retarder and the foil faced polyisocyanurate insulative sheathing, extra care and attention must be paid to interior air barrier detailing, especially around any penetrations or intersections/transitions to reduce the chance of air transported moisture from leaking into the wall. The other area to pay extra attention 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. Also, by not taping the joints between polyiso sheets, a small amount of air leakage may help dry any moisture which does find its way into the assembly back to the outside, but this must be combined with a drainage plane outside of it which is the asphalt impregnated 30 minute paper.

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

• 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