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.