Monitoring of Unvented Roofs with Fibrous Insulation, Diffusion Vents, and Interior Vapor Control in a Cold Climate

Unvented roof assemblies have been accepted in North American building codes since 2007; code-compliant construction calls for a minimum thickness of “air-impermeable insulation” (based on climate zone) to avoid cold-weather condensation and moisture risks. This is typically implemented with polyurethane spray foam insulation; however, negatives of this material include high first cost and possible adverse environmental impacts. Implementing unvented roofs in a moisture-safe manner with air-permeable fibrous fill insulations (e.g., fiberglass or cellulose) could potentially reduce the first cost of unvented roof designs, allowing for more widespread use. Building unvented roofs in a cost-effective, airtight, and moisture-safe manner opens up options for high-performance house designers and builders. This research involved construction of a conditioned test hut in a cold climate (Climate Zone 5A) with multiple side-by-side instrumented roof rafter bays. This work examined seven experimental unvented roofs and one code-compliant control hybrid ccSPF-cellulose roof over three winters and following summers. Examined variables included insulation material (fiberglass and cellulose), the presence or absence of a ridge diffusion vent (vapor-open material at the roof ridge to promote drying), the effect of various interior vapor control membranes (fixed and variable permeance), the effect of interior relative humidity, and the effect of interstitial airflow (from the interior into the cavity). This experiment was run over three winters, changing variables of interior conditions, roof configurations/materials, and the addition of intentional air leakage.