Unvented Flat Roof Assembly

This document provides a brief description of different types of unvented roof assemblies and the benefits of unvented roof construction.

Unvented flat roof assembly.

Martin s post focuses on the marketing of owens corning 039 s propink unvented attic insulation system. Unvented flat roof assembly. It s not and so code doesn t require r value ratios. However architects of these compact roof assemblies must consider vapor permeance and arrangement of the materials to produce acceptable hygrothermal performance and low risk of condensation in the resulting conditioned attic space.

An unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with insulation. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck. Recent building codes only allow unvented roof assemblies for pitches 3 12 and greater irc 2015 section r806 5 2 7. If you don t want to use sips or nailbase there are four basic approaches.

There is absolutely no room for error in the detailing and installation of roofing and air sealing in these assemblies. Four options for building an unvented 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. In some climates it requires a vapor barrier.

Read all of r806 5 carefully. This is a mis interpretation of the code. Figure 6 describes open cell spray foam as air permeable. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction.

Condensing surface temperature controlled the thickness of the rigid insulation at both the roof deck and perimeter is determined by a calculation procedure similar to figure 5. 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. Low slope and steep slope unvented roof systems i e. The colder the climate the thicker the rigid insulation required.

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. 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. In unvented flat or low slope roofs with parapets perform a full flood test to ensure leak free installation of roofing membrane. Drying of unvented roofing assemblies is by definition only provided to the interior.