Billy's Blatherings

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.

A Prototype Roof Deck Designed to Self-Regulate Deck Temperature and Reduce Heat Transfer

William (Bill) Miller, Ph.D.
Oak Ridge National Laboratory
Oak Ridge, TN, USA

Stan Atherton and Russell Graves
University of Tennessee MABE Department
Knoxville, TN, USA

Billy Ellis
Billy Ellis Roofing
Fort Worth, TX, USA

Abstract

A prototype roof and attic assembly exploits the use of radiation, convection and insulation controls to reduce its peak day heat transfer by almost 85 percent of the heat transfer crossing a conventional roof and attic assembly. The assembly exhibits attic air temperatures that do not exceed the maximum daily outdoor ambient temperature. The design includes a passive ventilation scheme that pulls air from the soffit and attic into an inclined air space above the roof deck. The design complies with fire protection codes because the air intake is internal and closed to the elements. Field data were benchmarked against an attic computer tool and simulations made for new and retrofit constructions in hot, moderate and cold climates to gauge the cost of energy savings and potential payback.

William (Bill) Miller: Dr. Miller is a specialist with 32 years of experience in building science, absorption heat and mass transfer and vapor compression refrigeration systems. He has a Ph.D. in Mechanical Engineering, and works for the Energy and Transportation Sciences Division of the Oak Ridge National Laboratory (ORNL).

Stan Atherton: Mr. Atherton is a graduate student in the Mechanical, Aeronautical and Biomedical (MABE) department of the University of Tennessee. He has an internship at ORNL to study attic ventilation as it applies to moisture management in attics.

Russell Graves: Mr. Graves is an undergraduate student in the MABE department of the University of Tennessee. He is a HERE1 scholar at ORNL and is assisting Dr. Miller with design guidelines for roofs and attics in hot and cold climates.

Billy Ellis: Mr. Ellis is the Chief Executive Officer of Billy Ellis Roofing, a limited liability company that installs roofs on homes and businesses. Mr. Ellis worked with ORNL under a User Agreement to test and verify performance of the insulated and ventilated roof deck described in this report.

1 HERE is an abbreviation for the Higher Education Research Experiences student program.