Unvented Warm Flat Roof

The portion of the 2nd story roof not covered with the deck is a flat roof w a parapet.

Unvented warm flat roof.

Insulation and air barrier are applied directly to the underside of the roof deck are common in applications where designers are seeking cost effective and energy efficient roof assemblies. An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot. However architects of these compact roof assemblies must consider vapor permeance and arrangement of the materials to produce acceptable hygrothermal performance and low risk of condensation in the resulting conditioned attic space. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck.

The contractor is wondering whether or not the flat roof should be vented. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. One of the best ways to insulate a roof whether sloped or flat is to install rigid foam on top of the roof sheathing. The enclosed void of a warm pitched roof does not require to be ventilated.

Low slope and steep slope unvented roof systems i e. Drying of unvented roofing assemblies is by definition only provided to the interior. With the introduction of vapour permeable plastics underlays the need to create ventilated voids above inclined insulation and the requirement to ventilate cold roof lofts may be reduced. Air permeable insulation such as fiberglass batts dense packed cellulose or blown in fiberglass can t be used alone for an unvented assembly since these types of insulation can allow moist indoor air to reach the cold roof sheathing leading to condensation or moisture accumulation.

This document provides a brief description of different types of unvented roof assemblies and the benefits of unvented roof construction. In unvented flat or low slope roofs with parapets perform a full flood test to ensure leak free installation of roofing membrane. Exterior foam can reduce air leaks limit condensation at the sheathing layer interrupt thermal bridging through the rafters and make ice dams unlikely.