Roof Reflectivity and Good Design

05/30/2007 | By Carl De Leon

For more than 40 years, durable, highly engineered, light-colored thermoplastic vinyl roofing membranes have cooled and protected buildings in climates around the world. Cool vinyl roofs provide environmental and economic benefits, typically without an installed cost premium and without sacrificing any other roofing performance attribute. In addition, the vinyl resin feedstock comprises the least amount (typically less than half) of non-renewable raw materials of any roofing alternative.

Available in wide rolls or prefabricated panels, single-ply vinyl membranes are constructed of a flexible, tear- and water-resistant polymer reinforced with fiber glass non-woven mats or polyester woven scrims. These components give PVC the strength and durability to withstand wind loads, structural movement, temperature extremes, and thermal cycles.

Optimizing a Building’s Thermal Performance
One of many variables determining how a building performs over time is the thermal characteristics of the building envelope. Naturally, the choice of material that will be subjected most directly to the beating of the sun’s rays is especially critical to that outcome. Light-colored reflective roofing materials can (1) increase the thermal efficiency of the roof system by reducing the building’s heat load, and (2) increase the long-term performance and life expectancy of the roof system by reducing the temperature stress on those components. Of particular note is that minimizing heat load will increase the efficiency of a building’s insulation (more on that later).

White or light-colored single-ply vinyl membranes achieve some of the highest reflectance and emittance measures of which roofing materials are capable. Reflective materials can reflect three-quarters of the sun’s rays – usually far more – and emit 90 percent of the heat generated from solar radiation absorbed by the roofing system. By comparison, asphalt built-up roofs reflect between 6 percent and 26 percent of solar radiation, resulting in greater heat transfer to the building interior and greater demand for air-conditioning – a strain on both operating costs and the electric power grid.

Study Details Cool Roof Energy Savings
Cool roofs offer both immediate and long-term savings in building energy costs. In a 2001 federal study, “Measured Energy Savings and Demand Reduction from a Reflective Roof Membrane on a Large Retail Store in Austin,” the Lawrence Berkeley National Laboratory (LBNL) measured and calculated the reduction in peak energy demand resulting from the replacement of a black EPDM roof with a white vinyl roof on a retail building in Austin, TX.

Instruments measured weather conditions on the roof, temperatures inside the building and throughout the roof layers, and air-conditioning and total building power consumption. Measurements were taken with the original black rubber roofing membrane and then after replacement with a white vinyl roof with the same insulation and HVAC systems in place.

LBNL found that the average daily summertime temperature of the black roof surface was 168 degrees F., but once retrofitted with a white reflective surface, it averaged 125 degrees F. (a decrease of 43 degrees F.).

In conjunction with that, LBNL found that, compared to the original black membrane, the retrofitted vinyl membrane delivered an 11-percent decrease in aggregate air-conditioning energy consumption, and a corresponding 14-percent drop in peak hour demand. Without considering any tax benefits or other utility charges, annual energy expenditures were reduced by $7,200 or 7 cents per square foot.

Other studies show that net annual energy savings are typical even in northern climatesi. 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.

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