In full sun, the surface of a black roof may experience a temperature rise of as much as 90 degrees F, reaching midday temperatures of 150-190 degrees F. A white reflective roof typically increases only 10-25 degrees F above ambient temperature under the same conditions.
That’s why owners of large commercial, industrial and multi-family residential buildings are turning to white reflective vinyl roofing membranes and planted or green roofs to reduce peak energy demand by as much as 10 percent and control energy costs. Reflective single-ply roof systems also enhance the life expectancy of both the membrane and the building cooling equipment. A further benefit: improved thermal efficiency of whatever roof insulation is required. As the temperature increases, the thermal conductivity of the insulation increases.
When a roof can deliver high solar reflectance (or albedo) and thermal emittance values, it is known as a cool roof. A vinyl roof can reflect three-quarters of the sun’s rays – usually far more – and emit 70 or more percent of the solar radiation absorbed by the building envelope. Asphalt built-up roofs (BUR), by comparison, reflect between 6 percent and 26 percent of solar radiation.
Cool roofs also reduce the urban heat island effect, lowering surrounding air temperature and reducing smog formation.
Reflective roofing technologies are increasingly included in federal, state and local energy codes, and specifiers are consulting the Cool Roof Rating Council’s (CRRC) product rating database to help make purchasing decisions.
To reduce electric power generation and associated air emissions, more government entities and utilities are offering incentives and rebates when these systems are installed. In addition, credits associated with reflective vinyl roofing or planted roofs can help achieve certification under the U.S. Green Building Council’s LEED (Leadership in Energy and Environmental Design) Green Building Rating System®. Here, a third measure, the solar reflectance index (SRI), is evaluated prior to assigning credit. The Green Building Initiative’s Green Globes™ program gives points towards Globes ratings for 100 percent roof coverage with either vegetation or highly reflective materials or both.
Net annual energy savings are typical even in northern climates. Cool roofs can have more impact on energy cost than energy use, cutting consumption during peak power demand when the rates are the highest and offsetting any minimal wintertime increases in use when there is less sunlight to reflect.
