Vented Atic Roof Assembly

In a roof attic assembly the builder has two choices.

Vented atic roof assembly.

If less analysis is required. 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. Roof sheathing to be air sealed.

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. Closed cell spray polyurethane foam ccspf at bottom of roof sheathing. The total insulation r value for the assembly will be an r 39 r 20 r 19. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck.

A vented roof system can only work properly and remove the moisture from a roof assembly if. 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 quest for greater energy efficiency in buildings has led to increasing interest and need for insulation improvements in hvac equipment more airtight construction new materials and assemblies. R value of ccspf to be 40 of total assembly.

Shingles that are installed on unvented roof assemblies operate at slightly higher temperatures roughly 2 f to 3 f warmer than shingles on vented assemblies. The building envelope is tight enough to prevent warm moist air from escaping from the building interior into the roof assembly. Acceptable if above is not feasible. 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.

The thermal boundary can be at the ceiling plane leaving the attic space above uninsulated and vented or the thermal boundary can be located at the roof line creating a sealed insulated conditioned or semi conditioned attic space. Senior engineer manager at icynene john broniek makes the case for the unvented attic assembly and why insulating the underside of the roof makes sense. 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. Ci to be installed in multiple layers to limit insulation by pass from gapping of insulation.

This can reduce their service life by roughly 10.