Making Cool Coatings Work
A closer look at the potential benefits of roof coatings formulated to keep facilities cool
— By Carl G. Cash
Reflective roof coatings are receiving a great deal of attention within commercial and institutional facilities these days, largely through the efforts of a group funded by the U.S. Department of Energy at Lawrence Berkeley National Laboratory, called the Heat Island Group, and supported by work at Oak Ridge National Laboratory.
Recently, a newer organization — the Cool Roof Rating Council — was formed to rate energy savings related to roofing systems, much as an energy rating is provided for appliances. And the U.S. Environmental Protection Agency (EPA) has added roof products to its Energy Star program to heighten awareness of the benefits of reflective roofing products.
Managers who are looking into using roof coatings on their facilities are likely to find many claims made for these cool roofing systems. A thorough evaluation of the value of cool roofing can reveal how these benefits coordinate with a facility maintenance program.
The benefits of reflective coatings include decreases in peak and total energy loads, as well as increases in service life for the roofing system.
Service life claims
Table 1 lists the average black, gray and white surface temperatures for horizontal surfaces exposed at United States sites subjected to a span of typical exposures.
Reducing the average temperature of the roofing membrane will prolong its service life, but how much of an increase to the average service life is provided by the change from black to gray, or from black to white surfaces?
Table 2 attempts to answer this question for low-sloped systems, those with less than 25 percent slope.
These data — the result of calculations based on a 1996 survey The Relative Durability of Low-Sloped Roofing — show that the increase in service life due to an aluminum or white coating depends on the facility’s location and upon the roofing system.
For example, black EPDM and APP polymer modified asphalt would benefit from coating with a compatible white coating, but an aluminum coating — if a compatible and durable aluminum coating can be found — would be less beneficial. Also, the increase in the service life of both BUR and SBS membranes may not be improved enough to justify the expense of coating.
In addition to the percent increases in service life as shown in Table 2, managers making coatings decisions must know the basic average service life for a system at a given location (Table 3), the service life experienced to date — the age of the roof — and the fact that many available coating systems may not last more than three years.
Coating by example
Sample problems using these tables will help managers making coating decisions.
Problem 1: You have a five-year-old SBS polymer modified asphalt membrane in Atlanta. Estimate the expected increase in service life for coating the roof with a white acrylic roof coating.
Solution: In table 3, look up the mean life of the system, which 15.4 years. Determine the remaining life if left uncoated: 15.4 - 5 = 10.4 years. In Table 2, look up the increase in the expected service life, and multiply one plus the percent increase by the remaining life: 1.02 x 10.4 = 10.6 years. Since one coat lasts about three years, you will need 10.6/3, or at least three applications of coating during the balance of the life of the membrane.
Conclusion: Three coating applications to gain 0.2 additional years of service seems an inappropriate use of maintenance funds.
Problem 2: You have an eight-year-old APP polymer-modified asphalt membrane in Concord, N. H. Estimate the expected increased in service life from coating the roof with a white acrylic roof coating.
Solution: In Table 3, look up the mean life of the system, which is 16.5 years. Determine the remaining uncoated life: 16.5 - 8 = 8.5 years. In Table 2, look up the increase in service life you can expect and multiply one plus the percent increase by the remaining life: 1.16 x 8.5 = 9.9 years. Since one coat lasts about three years, the roofing system will need at least three applications of coating during the balance of the life of the membrane.
Conclusion: Managers will have to decide if three coating applications to gain about 18 months of service life are an appropriate maintenance expenditure.
Regardless of the type of the existing low sloped roofing system, the following cautionary steps apply:
Managers concerned with steep-sloped roofing should study the detailed work by Robert Zarr of the National Institute of Standards and Technology (NIST). In multi-factored experiments, he studied the effect of location (climate), roof reflectance, insulation thermal resistance, degree of ventilation, and the mass of the framing on the peak and annual heating and cooling loads. The study, in part, reached these conclusions:
Geographic location. Of the five factors studied, the geographic location had the greatest effect on the peak and annual heating loads. The colder the climate, the higher the peak and annual heating loads.
Ceiling thermal resistance. Increasing the thermal insulation in the attic had the greatest effect on the peak cooling loads. Increased insulation was beneficial in almost every case studied; it reduced both the heating and cooling loads.
Solar reflectance. Increasing solar reflectance reduced the peak cooling loads, had an insignificant effect on peak heating loads, and increased the annual heating load.
The bottom line for coating any kind of roofing system:
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