Implications for the Housing Industry by Alberta building moisture problem

The report contains a full discussion of properties, problems and recommendations for sheathing membranes, stucco and vinyl cladding, flashings, windows, doors, balcony connections, small penetrations, foundations, air leakage and drainage.

The most important point is that walls should be designed and constructed to shed water and drain Implications for the Housing Industry penetrating moisture to the exterior. Each wall and its exposure must be assessed in relation to the expected conditions. Homes exposed to high wind conditions require upgraded construction practices and materials to prevent costly repairs for moisture-related damage.

Additional implications for:

Sheathing membranesˇŞThe sheathing membrane provides a majority of the rain water resistance, particularly with stucco claddings. More water-resistant membranes will not solve the problems and may increase the potential for condensation to be trapped within the wall. Correctly lapped and installed building paper or house wrap will perform adequately for most applications.

Vinyl or stucco claddingˇŞInstall cladding and trim to shed and drain moisture to the exterior. Consider using rainscreen systems. Stucco mix proportions, sand impurities and additives to improve the workability, may impair the strength, imperviousness, water repellency or frost resistance of the cured material.

FlashingsˇŞUse flashings with significant slope to the exterior, adequate extension beyond the face of the wall to create a proper drip edge, and adequate vertical extension behind the sheathing membrane on the wall (at least 50 mm, for normal exposure and 100 mm for severe exposure.) Install flashings above all penetrations through the wall and at all window sills. Minimize joints in flashings whenever possible.

Windows and doorsˇŞSelect windows that are designed based on rainscreen principles, including the connections between frames and perimeter seals. Have windows installed by experienced glazing tradespeople rather than framers. Install windows and doors immediately upon delivery to minimize theft and damage. Use installation practices that are appropriate for the specific exposure and window type, focusing on providing an interior air seal, drained rough opening cavity and adequate flashings.

Balcony connectionsˇŞPrevent water leakage into the wall below by installing flashings and sheathing membranes. Use manufacturer's recommended details that will allow for the future installation and replacement of decking materials.

Small penetrationsˇŞDesign flashings and sheathing membrane details for small penetrations similar to those for large openings.

FoundationsˇŞExtend sheathing membrane and cladding at least 50 mm below the joint between the floor framing and concrete foundation. Provide a drip flashing to prevent water from draining down the face of the foundation wall.

Air leakageˇŞEnsure that the most air and vapour tight components of the wall are directly to the interior of the insulation. Incorporate material choices and sealing techniques that will eliminate any trapping of moisture within the wall. Continue the air seal to the window and door assembly or other penetration.

The Alberta study demonstrated that although leakage from exterior moisture sources is causing some problems in Alberta dwellings, there are specific improvements that can be made by the housing industry to decrease moisture leakage and the resulting problems.



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CMHC- Canada Mortgage and Housing Corporation

"the federal government's housing agency, is responsible for administering the National Housing", formerly the Central Mortgage and Housing Corporation