New IBC Rules for Insulation in Unvented Enclosed Roof Framing Assemblies

Low-slope and steep-slope unvented roof systems (i.e. insulation and air barriers 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. 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. To this end, the 2015 version of the International Building Code (IBC) includes new provisions that relate to unvented enclosed rafter/truss assemblies and unvented attics. These rules, especially in section 1203.3, “Unvented Attic and Unvented Enclosed Rafter Assemblies,” dictate how design professionals specify vapor retarders and placement of insulation in compact roof assemblies for moisture control.

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