One of the most overlooked layers in an Edmonton flat roof installation is the vapour retarder. Warm, moist indoor air rises into the roof. In Edmonton's cold winters it condenses and soaks the insulation from below, causing hidden damage, lost energy efficiency and early roof failure. This guide explains what a vapour retarder does and where it goes in the roof assembly. It also explains why it's especially important for restaurants, gyms, pools and manufacturing buildings with high indoor humidity.