Prototype Insulated and Ventilated Roof Deck Cools the Entire Roof System

A new roof system design is being studied that is usable with almost all roofing products. The heart of the design is a profiled and foil-faced expanded polystyrene (EPS) insulation that fits over and between rafters in new construction (Figure 1) or can be attached on top of an existing shingle roof system. The EPS insulation is profiled to form a 1-inch (0.0254-m) air space between rafters to promote thermally induced convective flows that carry some of the heat penetrating the deck toward a ridge vent and away from the attic. The top and bottom sides of the EPS are foil-faced; the top side acts as a radiation shield in the inclined air space and the bottom-side foil performs as a radiant barrier in the attic (Figure 1). Ventilation is enhanced by cutting a slot near the eave just above the soffit to provide a source of makeup air from the soffit vent and attic. Buoyant air moves up the inclined air space and creates a negative pressure at the eave. Cool makeup air is pulled from the soffit and attic, which further enhances the temperature driving potential for natural convection heat transfer in the inclined air space. A ridge vent expels the heated air back to the outdoors.

This ventilation scheme keeps the air intake internal to the attic, which eliminates the intrusion of pests and any threat of burning embers entering the inclined space. The 1- inch (0.0254-m) of EPS insulation also serves to reduce the conduction heat transfer trying to penetrate into the attic. The lower deck flux results in a cooler radiant barrier temperature compared to conventional construction having an oriented strand board (OSB) deck with or without a foil faced radiant barrier. The reduced foil temperature of the EPS therefore further drops the radiation exchange between the roof deck and the attic floor.

As mentioned, the roof assembly can also be installed in retrofit applications provided the existing roof system can bear the added load. Furring strips are attached to the existing shingle roof and the EPS insulation mounted on top of the old shingles with a new OSB deck, weather-resistant sheathing and new layer of shingles.

About the Author