In recent years, white single-ply commercial roofs have become increasingly popular. However, it is understandable that some wonder if it’s just as good to stay with the traditional heat-absorbing black roof in colder climates.
Let’s see what Dr. Thomas Taylor, Executive Director of Building Science at GAF, says on the subject. Taylor suggests that there are sound reasons why white membrane roofs are advantageous in colder climates as well. The nature of solar radiation is a key factor. Heat energy arrives in our atmosphere as ultraviolet light. According to Dr. Taylor, about half of the sun’s radiation arrives as visible light and the other half as invisible ultraviolet rays that produce the heat.
Black Roof Winter Advantage Minimal
Although black roofs absorb a great deal of heat, the effect is not nearly as great in the winter. There are three mitigating factors that minimize the heat-producing capability of a black roof in the winter. First, the low angle of the sun reduces the impact of solar radiation. Second, the prevalence of cloudy days further reduces UV radiation levels. Third, winter days are far shorter, further reducing the time that sunlight is actually hitting the roof.
By contrast, in the warmer months the sun’s angle is high, sunny periods are more common and days are hours longer. Ultimately, this means that the heat-absorbing capacity of a black flat roof is highest when it is least desirable, during the hot summer months.
Traditional black flat roofs get very hot in the summer, whether they consist of EPDM, TPO, FTPO or PVC membranes. Surface temperatures above 190 degrees are not uncommon. Invariably, some of this heat warms a building’s interior, adding to air-conditioning loads.
Another factor increases the gap between black roof advantages in the winter and white roof advantages in the summer – the type of energy used. Most commercial buildings are heated with natural gas or oil, but they are cooled using electricity. Relatively speaking, electricity costs more than either natural gas or oil.
Furthermore, the way many utilities charge commercial customers for electricity increases the financial impact of peak-usage cooling costs. Therefore, high demand in the summer may adversely impact rates year-round. Because commercial rates are often governed by peak loads in the summer, the impact of a white cool roof on utility costs is that much greater.
Case Study
Dr. Taylor cites roofs installed by Minneapolis-based Target as a case study. For the last two decades, the retailer has used white PVC membranes for the vast majority of their store roofs. The nature of the company’s big-box stores is such that the roofs alone represent about three-fourths of the overall building envelope, so the choice of white over black membranes is even more significant.
The result? A study by Fenner, DiPietro and Graveline noted that Target enjoyed a net energy savings from cool roofs in almost all cases. Furthermore, the study contrasted the company’s dark roofs in northern climates with the white PVC roofs. Any marginal heat gain from a black roof did not lower heating costs to any appreciable degree.
Energy Star Cool Roof Grading System
Cool roofs typically possess reflectivity ratings of approximately 75 to 80 percent, which means that less than a quarter of the sun’s energy is absorbed into the membrane. The Energy Star rating system requires solar reflectivity of greater than or equal to 0.65 at installation and a value of greater than or equal to 0.50 after three years. The Cool Roof Rating Council (CRRC) does the Energy Star testing. Although compliance with these standards is currently voluntary, local codes may vary, and future requirements may change.
Retention of Reflectivity
To get gain maximum value from a white single-ply membrane roof, it is important that it remain clean. In many instances, the very smooth, semi-glossy surfaces get cleaned quite effectively by the occasional heavy rain. Nonetheless, it is important to monitor a white roof over time, and to arrange for periodic cleaning to maximize its reflective qualities.
Do reflective roofs in the North make sense? Yes, it is evident that there are important reasons why a highly reflective cool roof makes sense in the North as well, because the benefits in the summer far outweigh any minimal issues in the winter.
Minneapolis-based First Impressions Exteriors offers commercial roofing maintenance, repair and installation services. No job is too big or too small. To arrange for a complimentary visit by an experienced, professional roof inspector, please contact us today.