Unvented Roof Assembly

Dallas tx is located in a mixed humid climate region.

Unvented roof assembly.

This is a mis interpretation of the code. In some climates it requires a vapor barrier. Figure 4 shows the potential for condensation of an unvented roof assembly in dallas tx. Sc historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof.

It s not and so code doesn t require r value ratios. Unvented roof assemblies are becoming an increasingly common construction alternative to traditional vented roofs. 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. They are designed without ventilation openings and the attic is conditioned like the rest of the living space.

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. However designers of compact roof assemblies must consider vapor permeance and arrangement of the roofing materials to produce acceptable hygrothermal performance and low. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. Figure 6 describes open cell spray foam as air permeable.

2009 irc section r806 4. Windstorms for example can cause vented soffits to collapse creating excessive pressure which may lead to window blowout and roof loss. An unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with insulation. Ventilation becomes more difficult the more complicated a roof s construction is.

Read all of r806 5 carefully. Unvented roofs have a number of advantages. An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck.

Recent building codes only allow unvented roof assemblies for pitches 3 12 and greater irc 2015 section r806 5 2 7. Note that this roof assembly has rigid insulation installed above the roof deck in order to control the condensation potential.