Another Type of Cool Roof

Tile roofing and metal-shingle products generally are installed on a batten/counter-batten frame system. This installation practice creates an air space between the underside of the roof product and the roof deck. Testing at Oak Ridge, Tenn.-based Oak Ridge National Laboratory; Berkeley, Calif.-based Lawrence Berkeley National Laboratory; Cocoa, Fla.-based Florida Solar Energy Center; and the European-based Lafarge Center has shown that natural convective airflow develops in the air space.

Buoyancy forces from heated air create airflow from the roof eave to the ridge. Hence, an “offset-mounted” roof with a ridge vent can dissipate heat buildup beneath the roof surface. Tests have shown the natural airflow reduces heat gain through the roof sheathing when compared to direct-nailed roof products with no air space. ORNL determined that a minimum 3/4-inch space is required to create adequate airflow to dissipate heat.

Another benefit of the offset-mounted roof systems is a reduction in heat loss that takes place in colder climates when compared to direct-to-deck installed roof products. This can negate any winter heating penalty associated with cool-roof assemblies.

Tile Roofing Research

ORNL investigated summer heat flows for the month of July 2005 integrated over the daylight and nighttime hours to show seasonal performance of various roof systems. The ridge vent was open from the attic for all test roofs. The best-performing roofs were the S mission clay tile and the S mission concrete tile spot adhered with foam. Both roofs showed a 50 percent reduction in the heat penetrating through the ceiling during the full month. The maximum attic air temperature for July shows attics below the tile roofs were about 10-15 F cooler than the attics with conventional shingle roofs. The lower temperatures are caused by the reduced heat flow crossing the roof deck when compared to shingle roofing.

During January 2005, winter exposure of tile roofing with above-sheathing ventilation, or ASV, reduced the heat loss from the tile roofs compared to the loss from the asphalt-shingle roof. The tile roofs are negating the heating penalty associated with cool roofs in cooler climates. The improved summer performance coupled with the reduced heat losses during the winter show the tile roofs can reduce energy usage in summer months while negating any heating penalty in cooler months or Northern climates.

Based on the data, ORNL concludes the reduction in night-sky radiation is primarily caused by the decoupling of conduction prevalent in the direct-nailed shingle roof rather than the thermal mass of the concrete- and clay-tile roofs. The air channel forces the heat flow from the roof deck to radiate across the air channel rather than conduct from the roof deck to the surface of the shingle roof.

These results were corroborated by the Florida Solar Energy Center in similar experiments and published in a report titled “The Measured Summer Performance of Tile Roof Systems and Attic Ventilation Strategies in Hot, Humid Climates.” Read the report at www.fsec.ucf.edu/en/publications/html/FSEC-PF-408-95.

Metal Roofing Research

The Glenview, Ill.-based Metal Construction Association and its affiliate members installed metal roofs with varying profiles and finishes on a fully instrumented attic test assembly at ORNL. Measurements were made of roof, deck, attic and ceiling temperatures; heat flows; solar reflectance; thermal emittance; and ambient weather data. An adjacent attic cavity covered with a conventional pigmented and direct-nailed asphalt-shingle roof also was monitored. The roof assemblies were monitored for one year.

The project’s objective was to document the potential energy savings of metal roofs with venting between the underside of the roof cover and roof deck. It was noted in this experimental study that dark-colored roofs with ASV had similar energy flows compared to their cool-colored counterparts.

Numerical simulations of the airflow in the air channel formed by metal-roof systems demonstrates that naturally induced airflow can be expected at very low roof slopes and very small temperature differences—well below those experienced in conventional roof systems. To generalize the experimental data, a computer model was validated against data from this experiment. Algorithms were developed for the computer model to predict the energy benefits of ASV and were added to the basic energy savings model. The model was then used to examine how these roofs would perform in a variety of climate zones.

Details of the methodology of this study are contained in the ORNL report, “The Effects of Infrared Blocking Pigments and Deck Venting on Stone-Coated Metal Residential Roofs,” which is available at www.metalconstruction.org/pubs/pdf/ORNL-TM-2006-9.pdf.

In another study, ORNL conducted research on stone-coated metal roofing exposed during February 2005. ORNL testing showed benefits of ASV with stone-coated-metal roof installations similar to the results from tile roofing.

For example, a light-gray stone-coated-metal shake product that is offset mounted with a batten and counter-batten frame system reduced the heat transfer penetrating the roof deck compared to the heat penetrating the deck of an attic covered with a direct-nailed asphalt-shingle roof. About 30 percent of the reduction in heat gain was caused by ASV. When a cool reflective roof surface was used, an additional 15 percent reduction in heat gain was achieved. (See Graph 1.)

Analysis and Results

Graph 2 summarizes the results of ORNL’s computer-modeled simulation exercise in all 16 climate zones in California. (The Washington, D.C.-based U.S. Department of Energy and Atlanta-based ASHRAE have assigned eight general climate zones in the U.S. The California climate zones are represented within the eight DOE/ASHRAE climate zones.) The California climate zones were used for this exercise as part of the information provided to the California Energy Commission for proposed changes to the 2008 Energy Efficiency Standards, Title 24. The graph depicts the annual energy consumption of attics in all California climate zones for three different roof systems (duct and ceiling losses).

One roof is a prescriptively compliant cool roof (per Energy Star criteria) having a solar reflectance of 25 percent, thermal emittance of 75 percent and no ASV. The second roof modeled is not a cool roof. It has a solar reflectance of 10 percent, thermal emittance of 75 percent and ASV. The third roof is a prescriptively compliant cool roof having a solar reflectance of 25 percent and thermal emittance of 75 percent. It was modified to add ASV. With the exception of Zone 15 in California, the noncool roof with ASV shows better or equivalent energy performance compared to the cool roofs.

The improved summer performance of the same three roof systems, coupled with the reduced heat losses during the winter, demonstrate that an offset-mounted metal or tile roof can negate the heat penalty associated with a cool roof in cooler climates.

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