The performance of the building envelope was a major consideration in light of the leaky condo crisis which still afflicts all man- ner of buildings on the south coast of British Columbia and stems, in part, from a lack of detailing of building envelope elements, poor implementation of details on site and a lack of ongoing maintenance of the building’s envelope. Face-sealed technology, especially using stucco cladding, represents the majori- ty of building envelope failures, and is largely inappropriate for a rain forest environment. A face-sealed building is literally sealed at the outside face, using caulking to seal all exte- rior penetrations and other potential avenues for water ingress. Problems occur if these caulked areas fail, allowing water through the building envelope. If there are no paths for moisture to leave the envelope, it accumulates leading to mould and rot. Increasingly, rainscreen systems are being used where an airspace is put behind the cladding. This has been used in brick veneer construction for years, allowing moisture that penetrates the outside skin (through capillary action, wind pressure, or other means) to evaporate or otherwise escape. Because of the airspace, wind pressure on the cladding is equalized, preventing moisture from being blown onto the substrate behind the cladding. A double line of defence against moisture is made: the cladding and airspace forms the first line of defence, the building paper and sheathing behind the airspace form the second. Excess moisture escapes through weep-holes at the bottom of the airspace cavity at each floor level. When these holes in the rainscreen system that let water to escape are zealously caulked by maintenance crews used to face-sealed systems, moisture is trapped and probable decay ensues. Everyone involved with the on- going care of the building should be taught how rainscreen systems work to understand how to properly maintain them.
The Nanaimo Youth Housing project is clad in fibre-cement horizontal siding and acrylic stucco. Both surfaces were set onto a similar rainscreen base of exterior plywood sheathing, two layers of ‘30 minute’ asphalt- impregnated building paper (lapped like shingles to shed moisture) and 1x4 pressure treated vertical wood strapping. The wood strapping creates a 3/4” air cavity behind the cladding and allows for the fastening of the cladding system. For stucco, a material that is only semi-solid when applied, a further step is needed before application. Spanning the strapping, an as- phalt impregnated fiberglass board provides a firm substrate upon which to apply the wire mesh and stucco. Although a rainscreen system is designed to handle moisture penetration of the building envelope, it is still preferable to keep water outside the envelope. Junctions between materials are potential spots for infiltration — properly lapped materials allow water to drain away from the building envelope. All horizontal surfaces should have some degree of slope to shed water, and roof overhangs prevent rain from reaching trouble-prone areas of the building envelope where there are many junctions or envelope penetrations. These all help the building envelope perform properly during the full extent of its designed service life. c
a section of the building envelope: flash - ing is inserted in the stucco cladding at each floor level to drain the wall cavity behind.
The NYH project was designed by Bas Smith, MAIBC and David Poiron, MAIBC, while David was an intern architect at Bas Smith Architect Inc. in Victoria. David now runs his own architectural firm, Da - vid Poiron Architect Inc. in Nanaimo.
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