Reflections on a Cool Roof
by Richard Fricklas, RIEI
|from the Spring 1999 issue of the RIEI Information Letter||In recent months, industry publications have
been addressing the "urban heat island" issue. In urban areas where there is a
high density of structures, roads and parking lots, these centers create their own
climate. Solar energy is absorbed and convected to ambient air, raising the temperature
into this "heat island". Air pollution apparently worsens as well.
Since roofing occupies such a large surface area in these urban centers, it becomes logical to consider ways to reduce the heat load contribution from the roofing. The Lawrence Berkeley National Laboratories has recommended the use of higher albedo surface materials for roofs and pavement (as well as planting trees to make the cities cooler). One only has to think about photographs of the Greek Islands, where whitewashed buildings dominate the landscape.
It comes as no surprise to those involved in the roofing business that white is cooler than black White roof coatings and granulated sheets have been very popular in the South and West. Whereas asphalt-based aluminum roof coatings have been successfully used to reduce heat load, their greater contribution has been to serve as an ultraviolet screen. These asphaltic products have excellent adhesion to asphaltic and modified-bitumen substrates, and typically last 5-7 years (for fibrated products) between applications. White products have had greater difficulty, especially where water ponds or the substrate is not especially clean.
The use of sprayed-in-place polyurethane foam, coated with an acrylic, urethane, or silicone reflective coating has been highly successful in the Southwest. Not only does the white coating provide high albedo (reflectance), but high emissivity as well. More importantly, the urethane foam is an excellent insulator and air barrier. The literature is replete with documented cases of quick pay-back and dramatic drop in mechanical air-conditioning demands.
Other white surfaces do not have quite as high an albedo. White Kynar on metal, or siliconized white polyester have only 67% and 59% reflectivity, compared to white paint coatings at 85%. However, their emissivity is 85% compared to only 4% for galvanized steel. White cap-sheets and white shingles at 25-28% are far worse. Should the granules become darkened with algae, typical of the more humid regions of the U.S., the reflectance approaches that of a black shingle at 5% to 10%.
Single ply membranes have a relatively high score, with white Hypalon reported at 76% and white EPDM at 69%. Chalking of Hypalon has helped keep the surface white, although algae at-tack and excessive erosion has been observed in some high water-absorption (>10%) sheets. Other white single ply sheets have had problems with static attraction to dirt and air-borne pollutants. Clear lacquers have helped some PVC sheets stay clean.
Other approaches that have not appeared regularly in the press include protected membrane roof systems which use light-colored concrete pavers, aggregate or composite mortar-polystyrene pavers. Back in the 1970's shortly after the oil embargo, many excellent studies were published on the virtues of PMR Systems. The focus was on cold climates, as PMRs indeed protect the membrane against thermal changes, ice, hail, UV, and more. But they can be equally successful in southern climates as well, especially where the roof was to be ballasted anyway. In many southern climates, the bigger problem has been handling the weight of a ballasted roof. However, the interlocking Lightguard pavers weigh no more than a BUR system.
The introduction of TPO systems increases the popularity of ballasted roofs once again. Whereas PVC systems embrittled when ballasted due to hydrolysis of the plasticizer, the TPO sheets are not plasticized. However, when unballasted, the TPOs require UV stabilizers and fire retardants, both of which increase cost and make recycling and/or disposal more complicated. The ballasted versions are already offered by most manufacturers.
Reflectivity has been enhanced on BURs in the past by using white marble chips instead of darker aggregate. Aggregate-surfaced roofs provide improved wind resistance due to stiffening of the membrane, and the associated flood-coat helps resist water-attack as well as increasing fire and UV resistance.
When discussing all of these options to reduce the heat-island effect, the one that is least addressed is keeping the reflectance! Wind-blown dirt and airborne pollutants are a fact of life. In the LA basin, the phenomenon of "tobacco juicing" is legend. This resin-like deposit is found not only on bituminous roofs but virtually all surfaces, especially near highways. In west Texas, surfaces match the wind-blown dust. In Mississippi, it's algae attack
If roof surfaces are truly going to meet the challenge, then they have to stay reflective. This becomes either a problem or an opportunity. Many savvy roofing contractors have expanded their maintenance business. Perhaps periodic cleaning becomes an additional value-added service. Roof designers have to take roof drainage more seriously. The State of Georgia is permitting a reduction of up to 10% of the installed 'R' value of a building if high albedo/emissivity coatings are used, but if the roof drains poorly, we know it will look (and perform) like mud shortly.
A landmark paper by Don Backenstow, then R&D Director for Carlisle SynTec Systems, studied white versus black surfaces for energy conservation.1 Mr. Backenstow cited reasons why accurate estimates of energy conservation are difficult:
An additional complicating factor was cited by George Courville of Oak Ridge Laboratories. ORNL has been studying the potential for self-drying of a roof system after repair or re-cover. If there is wet roof insulation, the best chance for dry-out is during the summer months when the roof membrane is hottest, and the vapor drive is downwards into the interior of the building. A light-colored roof would reduce the effective drying of the insulation, and of course wet insulation is a major energy waster, summer and winter.
Backenstow's analysis of cost performance is worth repeating. Refer to table 1 and table 2. Backenstow's data indicates that at R-values of 10 or greater, the advantages of white over black tend to be negligible. In any climate with significant heat as well as cooling the year-round advantage of additional thermal insulation will likely be superior to added cost of a reflecting coating which only contributes during solar loading. Table 2 suggests that at R-5 or greater the difference between a dirty or dark ballasted roof and white is also negligible.
More recent data2 where heat load is the major factor indicated energy savings averaging 20% in Florida, even where an R-19 insulation was installed.
It appears that we roofing professionals can best serve the industry by doing what we do best:
According to Hashem Akbari of Lawrence Berkeley Laboratories, there is equipment available to measure in situ the reflectivity of our roof systems. When this data is factored in, perhaps more accurate energy calculations can be made. Meanwhile, dirt happens, and let's not go crazy looking for a perfect white in an imperfect world.
Mr. Hashem Akbari, Chair, ASTM Committee on Cool Construction Materials, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkley, CA 94720. email@example.com
ASTM E198 (revision to be published in 1999) - Standard Test Method for Measuring Solar Reflectance of Horizontal and Low Sloped Surfaces.
ASTM E1980 (new in 1999) - Standard Practice for Calculating Solar Reflectance Index of Horizontal Low-Sloped Opaque Surfaces.
Continuing committee work includes impact of aging and weathering on solar reflectance and thermal emittance as well as cleanability of roof coatings.
NISTIR-6228. Analytical Study of Residential Buildings with Reflective Roofs. R. R. Zarr, October 1998.
© 1999, Roofing Industry Educational Institute
Roofing Industry Educational Institute,
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