Unvented Roof Assembly

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 assembly.

They are designed without ventilation openings and the attic is conditioned like the rest of the living space. Unvented roof assemblies are becoming an increasingly common construction alternative to traditional vented roofs. 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. Recent building codes only allow unvented roof assemblies for pitches 3 12 and greater irc 2015 section r806 5 2 7.

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. In some climates it requires a vapor barrier. 2009 irc section r806 4.

Figure 4 shows the potential for condensation of an unvented roof assembly in dallas tx. This is a mis interpretation of the code. An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot. Read all of r806 5 carefully.

Note that this roof assembly has rigid insulation installed above the roof deck in order to control the condensation potential. Dallas tx is located in a mixed humid climate region. It s not and so code doesn t require r value ratios. Sc historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof.

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 roof assembly has no insulating sheathing installed above the roof deck. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck. Ventilation becomes more difficult the more complicated a roof s construction is.

Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. An unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with insulation.