Unvented Cold Deck Flat Roof

Where the insulation is placed above the structural deck warm deck and either above or below the waterproofing.

Unvented cold deck flat roof.

There is no need for additional ventilation. Exterior foam can reduce air leaks limit condensation at the sheathing layer interrupt thermal bridging through the rafters and make ice dams unlikely. It is universally recommended that cold deck flat roof construction should be avoided. However in many situations the insulation may need to be primarily or entirely below the flat roof membrane and between the roof joists if so what are the best options.

Unvented flat roofs a technical discussion flat roofs have often been built with the insulation all on top of the vapor closed drainage plane as this avoids dew point condensation issues inboard of the roofing. The higher the interior moisture level the thicker the rigid insulation required. One of the best ways to insulate a roof whether sloped or flat is to install rigid foam on top of the roof sheathing. Drying of unvented roofing assemblies is by definition only provided to the interior.

Unvented flat roof assembly. In unvented flat or low slope roofs with parapets perform a full flood test to ensure leak free installation of roofing membrane.