The Buildings Technology Center of the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) has installed two new test sections on its Roof Thermal Research Apparatus for measuring the long-term thermal performance of solar radiation control coatings on roofs.
One replaces a similar unit in service since 1991. The other allows side-by-side testing of two coatings on the same roof membrane or two different roof membranes with the same coating. Each test section is three 4 feet by 8 feet sheets of half-inch plywood instrumented with thermocouples to measure temperatures on the top and bottom of the assembly. There is also a heat flux transducer at the top of the middle sheet, which measures the rate at which heat flows through the plywood. A weather station on the top of the apparatus continuously reports air temperature and relative humidity, barometric pressure, wind speed and direction, as well as solar and infrared radiation.
The test sections have an R-value lower than normally insulated roofs. This low thermal resistance maximizes the effect of radiation control coatings, which are applied to one half of the interchangeable covers for the plywood. The other half of each cover is left uncoated to act as a control surface. When a particular cover is not used on the roof of the facility, it is kept on an adjacent, non-instrumented weatherization stand for continuous exposure to outdoor conditions in East Tennessee.
The new test sections have been covered by two new membranes made from modified bitumen roof membrane material. Half of one was coated with latex-based roof coating with ceramic beads added to increase solar reflectivity. Half of the other was coated with an elastomeric acrylic coating especially formulated for modified bitumen and with titanium oxide added to increase solar reflectivity.
In tests conducted June 22, 1996, results show a dramatic decrease in roof membrane temperatures that these coatings produce relative to uncoated control surfaces. Temperatures under the black roof surfaces rise as high as 160 degrees F. The coatings reflect the incoming solar radiation before it is absorbed in the structure. Consequently, temperatures under them are barely higher than the air temperature.
Time will tell how well these new coatings retain their high reflectivity for solar radiation. Experience with the former test section and several radiation control coatings reveal coatings became 15 to 25 percent less reflective after a year of weathering.
"Reflectivity seems to decrease less severely after the first year, but we do not have much data to support this conclusion," said Tom Petrie of the Buildings Technology Center.
The center is a national user facility of DOE and is funded by the Buildings Technology, State and Community Center. A way for manufacturers of radiation control coatings to test their products with the new test sections for the Roof Thermal Research Apparatus is to enter into a user agreement. Details are available from Petrie (865-574-9335) or Jeff Christian (865-574-5207).
ORNL, one of DOE's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Research Corp.
(Charts showing test results are available).