Unvented Roof Assembly

Dallas tx is located in a mixed humid climate region.

Unvented roof assembly.

Figure 4 shows the potential for condensation of an unvented roof assembly in dallas tx. 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. The unvented attic space is completely contained within the building thermal envelope. It s not and so code doesn t require r value ratios.

An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot. 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. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck. They are designed without ventilation openings and the attic is conditioned like the rest of the living space.

Figure 6 describes open cell spray foam as air permeable. This roof assembly has no insulating sheathing installed above the roof deck. Read all of r806 5 carefully. 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.

Note that this roof assembly has rigid insulation installed above the roof deck in order to control the condensation potential. Recent building codes only allow unvented roof assemblies for pitches 3 12 and greater irc 2015 section r806 5 2 7. However designers of compact roof assemblies must consider vapor permeance and arrangement of the roofing materials to produce acceptable hygrothermal performance and low. Windstorms for example can cause vented soffits to collapse creating excessive pressure which may lead to window blowout and roof loss.

Unvented roofs have a number of advantages. In some climates it requires a vapor barrier. An unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with insulation. Sc historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof.

2009 irc section r806 4. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction.