Vented Attic Roof Assembly

Section r806 4 unvented attic assemblies.

Vented attic roof assembly.

Because the drying capacity of unvented roof assemblies is inherently lower than vented ones it is especially critical to control built in moisture during construction. 2012 irc section r806 5 unvented attic assemblies unvented attic assemblies spaces between the ceiling joists of the top story of the roof rafters shall be permitted if all of the following conditions are met. Unvented roof systems where the 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. 2009 irc section r806 4.

When hvac equipment is in the attic insulation and an air barrier at the roof line brings that equipment within the conditioned enclosure envelope thereby allowing it to operate in more favorable temperature and humidity conditions without air. A secondary goal is to vent moisture that infiltrates the attic from the conditioned home. A vent space clearance of at least 1 must be maintained at the location of the vent and between insulation and roof sheathing. Sc historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof.

The unvented attic space is completely contained within the building thermal envelope. Unvented roof assemblies are becoming an increasingly common construction alternative to traditional vented roofs. They are designed without ventilation openings and the attic is conditioned like the rest of the living space. This section outlines the conditions for unvented attic roof assemblies.

Un ven ted roof ˌənˈven t əd ro of an unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with insulation. With vented attic assemblies moisture that diffuses into the attic space. In colder climates the main purpose of a vented roof is to keep the roof cold and reduce the formation of ice dams. Control construction moisture content with tenting or temporary roofing when necessary.

Note that this would not apply where an unvented roof assembly is used as indicated in section r806 4. Unvented attic assemblies offer numerous performance advantages versus vented attic assemblies. However designers of compact roof assemblies must consider vapor permeance and arrangement of the roofing materials to produce acceptable hygrothermal performance and low.