Reflective Roof Systems
We have all felt the effects of dark versus light clothing on a sunny summer day. Because dark colors reflect less solar energy, dark is hot and light is cool. The same principles govern roof temperatures. Light-colored roof materials make for a cool roof by reflecting more of the sun's rays. Because over 50 percent of the sun's energy received is in the visible range, the color of a roof system is often a good indicator as to what percentage of the incident solar energy is reflected by the surface. Light-colored roof materials have a high solar reflectance (as high as 85 percent), whereas dark-colored roof materials have a reflectance of less than 10 percent. The lower temperatures of light-colored roofs reduce the air-conditioning energy needed to maintain comfort cooling. Homeowners want the roofs on their homes to protect the underlying structure for a long time at an affordable cost. Since the roof is generally visible from ground level, appearance is important. It is a common perception that dark roofs look better than light roofs. Also, the attic under many pitched roofs shields ceilings from hot roof temperatures. However, in commercial businesses, the roofs are predominately low-slope and are not visible from ground level. Reduced air-conditioning costs and other potential savings from light-colored roofs can be weighed directly against cost premiums for the cool roofs.
Buildings with light-colored roofing materials better reflect sunlight and significantly reduce the peak roof temperatures. The lower roof temperature in turn improves the durability and life of the roof. Cool roof coatings and membranes can therefore be a critical component of a proactive roof maintenance program that results in lower lifetime roof temperatures during sunny periods. The lower temperatures, in turn, reduce the air conditioning loads of the building and potentially lengthen the service life of the roofing system. If the building is located where cooling loads predominate, peak load reductions and net annual energy savings are also realized. Our experience with cool roof membranes, and especially coatings that have high initial solar reflectance (over 0.75), shows that they age under field conditions. The reflectance of a roofing system changes with time as the weather degrades the surface or as dirt accumulates on the surface. Typical results are that reflectance is reduced by about 0.1 to 0.3 after the first several years and then levels out. As the solar reflectance changes, so do the potential energy savings. Washing helps; however, the membranes we have tested in the past have not maintained their high level of initial reflectivity even after washing. The roofing industry recognizes that a record of long-term performance is needed in order to develop realistic, defensible claims about annualized energy savings and affordable cost premiums. Testing is needed to document the long-term performance of reflective roofing systems, since the data accumulated to date show significant variation. Objectives
- Quantify the long-term energy and durability benefits of reflective roofing materials to the roofing industry
- Develop and promote fair and credible procedures to help consumers choose a coating or membrane for specific applications
For the past ten years, research has been focused on applying white radiation control coatings to existing low-slope roofs with non-reflecting membranes. The project has evolved into a three-year study of 24 different white and aluminum coatings. This research, begun in the summer of 1997, is conducted in partnership with the Roof Coating Manufacturers Association and several coating manufacturers. This project involves periodic measurement of the solar reflectance and infrared emittance of each coating and continuous measurement of the thermal performance of the coatings on instrumented test sections. Temperatures throughout insulated test sections (especially the temperature of the external surface) are measured as a function of weather conditions. As heat flows through the test section, it is monitored continuously to help calibrate models of each test section's thermal behavior. Calibrated models help to predict the behavior of similarly coated roofs with varying levels of insulation in different climates. Measurements of the mechanical performance of the coatings are possible with separate samples of the same coatings on uninstrumented weathering panels. Results from the test panels will be analyzed and compared with concurrent results from uncoated substrates and with results from other ORNL on-site tests. Long-term monitoring is needed to capture the effects of weathering on the coatings, especially on their solar reflectance. A similar test procedure is being followed in a 3-year study of 18 different low-slope roofing systems. This project began in the summer of 1998 and cooperates with several members of SPRI (sheet membrane and component suppliers to the commercial roofing industry) to study the energy efficiency and service life of single-ply low-slope roofing systems. A third project, started in the summer of 1999, involves five U.S. steel roof manufacturers who are keenly interested in documenting whether both high- and low-slope bare and painted metal roofs can reduce building heating / cooling loads.