Vented Atic Roof Assembly

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.

Vented atic roof assembly.

An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot. 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. Ci to be installed in multiple layers to limit insulation by pass from gapping of insulation. 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.

The total insulation r value for the assembly will be an r 39 r 20 r 19. To be placed on the underside and in direct contact with the roof sheathing. Roof sheathing to be air sealed. Some manufacturers don t warranty or offer only a limited warranty when their products are used over an unvented roof assembly.

A vented roof system can only work properly and remove the moisture from a roof assembly if. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. 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. 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.

If less analysis is required. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck. 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. Acceptable if above is not feasible.

In a roof attic assembly the builder has two choices. 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. 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 building envelope is tight enough to prevent warm moist air from escaping from the building interior into the roof assembly.

Closed cell spray polyurethane foam ccspf at bottom of roof sheathing.