Vented 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.

Vented roof assembly.

Sc historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof. Some manufacturers don t warranty or offer only a limited warranty when their products are used over an unvented roof assembly. The total insulation r value for the assembly will be an r 39 r 20 r 19. Mapping out the roof assembly for a new house in climate zone 6b steve mackay has settled on long i joists insulated with a mix of closed cell spray polyurethane foam and blown in fiberglass.

The roof assembly has enough insulation and it is properly installed. An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot. Control construction moisture content with tenting or temporary roofing when necessary. This can reduce their service life by roughly 10.

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. 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. To be placed on the underside and in direct contact with the roof sheathing. 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.

Moisture content in roof structure must be less than 18 prior to covering. The building envelope is tight enough to prevent warm moist air from escaping from the building interior into the roof assembly. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck. He doesn t plan on venting the roof and he wants to be certain his design will be problem free.

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. They are designed without ventilation openings and the attic is conditioned like the rest of the living space. Based on table r806 4 r 20 rigid board insulation will be required to be placed on top of the roof deck to keep the condensing surface temperature at or above 45 f. Unvented roof assemblies are becoming an increasingly common construction alternative to traditional vented roofs.