Vented Flat Roof Assembly

Let s take the example of a cathedral ceiling renovation project on older home in bc.

Vented flat roof assembly.

6 2 vented and unvented roof assemblies vented roofs for pitches of 3 12 or above when detailed correctly vented roofs have a long successful track record in the pacific northwest. It has the shape of a mushroom once it is completely installed. That setup makes it easy to insulate the ceiling with thick fluffy insulation like cellulose or fiberglass. However in many situations the insulation may need to be primarily or entirely below the flat roof membrane and between the roof joists if so what are the best options.

Before constructing a flat roof assembly please refer to the unvented roof assembly comments under 6 2 vented and unvented roof assemblies and if applicable 6 3 parapet walls. Unvented flat roofs a technical discussion flat roofs have often been built with the insulation all on top of the vapor closed drainage plane as this avoids dew point condensation issues inboard of the roofing. Unvented roof assemblies should be insulated either with air impermeable insulation rigid foam or spray polyurethane foam or with a combination of foam and air permeable insulation. Vented roofs provide the capacity for moisture to be removed from the underside of roof sheathing via convection currents in the vent cavity.

Historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof. However they help to not only stabilize the temperature within the roofing system and attic but also allow moisture to escape. 25 mm of ccspf against the underside of the roof deck to prevent air leakage through the roof assembly and to prevent moist indoor air from reaching the inside of the roof sheathing where it can condense. This is a long slender aluminum vent that has a larger cover that is a domed shape.

If you don t want to use sips or nailbase read this article tolearn the four basic approaches to insulating unvented roof assemblies. By installing spf insulation spray in place insulation in lieu of mineral fiber insulation in the joist cavity of a flat roof or between rafters of a cathedral ceiling the assembly may perform the same as or better than that of a typical one and venting can be shown to be unnecessary. Another common flat roof vent is the mushroom vent. Minimum 1 4 in.

Any roofing material can be used on an unvented roof assembly with ccspf. Control airflow install at least 1 in. The most common way to eliminate venting from a roof assembly is to use a spray foam spf insulation. In hurricane prone zones with frequent high wind events vented soffit collapse can pressurize a building which can cause windows to blow out and the roof to be blown off.