Walls Above Grade

Cladding such as wood siding, stucco or brick veneer provides a first plane of protection against precipitation ingress in above-grade wall assemblies. In geographic locations where the probability of wetting due to precipitation is high and the drying potential of the walls is low, a capillary break (an air space) must be provided between the first and second plane of protection in exterior walls. This configuration is commonly called a rainscreen. The need for a rainscreen is determined by the Moisture Index (MI) for a particular geographic location. MI values are provided for many locations in Canada as part of the climatic data tabulated in building codes.

The capillary break, or air space, allows precipitation that penetrates past the cladding to readily drain on the backside of the cladding, or alternately on the sheathing membrane, which provides a second plane of protection against precipitation ingress. The air space also allows materials to dry by means of evaporation and exchange of air.

Capillary breaks can be provided by:

  • a 10 mm (7⁄16 in.) gap between the sheathing membrane and the cladding (usually furring strips are used) so that water droplets do not infiltrate between the cladding and the sheathing and can drain back to the exterior;
  • a mesh material not less than 10 mm (7⁄16 in.) thick and at least 80 per cent open;
  • common horizontal metal and vinyl siding, without contoured insulated backing, that is loosely fastened to the sheathing and is designed with a 10 mm (7⁄16 in.) space at the bottom of each horizontal strip; or
  • the air space in masonry cavity walls allowing moisture to drain to the exterior.

In order to be effective, a capillary break must be provided over the full height and width of a wall, except where openings (such as windows and doors), service penetrations (such as pipes, ducts and electrical outlets) or flashings are present. Where furring is used to provide the gap, it must not cover more than 20 per cent of the wall area.

A screen should be installed at the bottom of the capillary break or air space at foundations, windows and doors to prevent insects from getting in. The top of the capillary break should not vent into the attic space as this can lead to condensation on surfaces in the attic. Although blocking at the top of the capillary break restricts ventilation, it does not impede drainage, which is the more important aspect of water penetration control for rainscreen construction.

Figure 18 shows three different approaches for insulating within and beyond the stud space and foundation wall. In warmer climate zones, the required level of thermal resistance can be achieved using batt insulation in the stud space (Figure 18a). In climate zones where required effective insulating values cannot be provided within the framing space alone or where higher insulation levels are desired, additional insulation must be provided on either the inside or the outside of the framing (Figure 18: b and c).

Figure 18

Location of thermal insulation

Source : Canada Mortgage and Housing Corporation (CMHC)