RDH Building Science
    RDH Building Science
    RDH Building Science

In conventional compact low-slope roofs, insulation is sandwiched between two vapour-impermeable layers: on top, the roof membrane, and underneath, the concrete deck or metal deck (with an additional air- and vapour-retarding membrane). This approach can be problematic should water get into the roofing assembly—either during construction, due to a roofing membrane leak, or from air leakage from the interior.

This paper describes Phase 1 of an experimental program involving three compact low-slope roof assemblies on metal decks that were constructed side by side in a field exposure facility in Waterloo, Ontario (Climate Zone 5-6). One assembly was constructed as a reference or base case, with two vapour barriers. The two other assemblies were designed and constructed to allow drying by vapour diffusion to either the top or bottom side. All three assemblies were subjected to periodic wetting by the injection of controlled amounts of water, and moisture movement was tracked using embedded moisture, temperature, and relative humidity sensors. It was found that the roof assembly with a high-vapour-permeance membrane on the metal deck (i.e., the inward drying assembly) was most effective in drying water following each intentional wetting. Implications for design and construction are discussed.

This paper is included in the RCI Proceedings for the 2018 RCI Canadian Building Envelope Technology Symposium, and is published with permission.

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