Roof Reflectivity, Good Design, and Decades of Cool

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.

Enhancing Investment in Insulation
From a design perspective, using a reflective material in the roofing system results in a more holistic approach. It’s a decision that works synergistically with other components of the system, particularly the insulation.

In some jurisdictions, local codes allow a reduction in insulation when cool roofing is employed. But, whether the building team chooses to reduce insulation or not, the performance of the insulation used will be optimized - meaning, thermal conductivity kept to the lowest level - with a reflective roof.

Thermal conductivities of roof insulation materials have been measured at different operating mean temperatures using computerized heat-flow meters. The results indicated that higher operating temperatures always lead to higher thermal conductivities in the insulation material. The effective temperature driving this effect in the roof systems is directly linked to the absorbed solar radiation of the surfaceⁱⁱ. So, when insulation temperature rises, such as occurs with dark-colored rooftops, the thermal conductivity of the insulation increases as well. This means that the R-value of the insulation is reduced and along with it, the effective performance of the insulation. According to one study, effective insulation performance below a black membrane can be as much as 30-percent lower than advertisedⁱⁱⁱ.

Cooling-load contributions of the roof system components have been modeled using theoretical nominal insulation thermal conductivities. These nominal values are typically measured at 75 degrees F. Modeled in comparison with these typically reported values were actual field temperature-impacted insulation thermal conductivities. The models indicated significant advantage on roof-induced cooling load through synergistic action of the insulation thermal conductivity, and lower roof surface operating temperature by the use of light-colored membranes^iv.

Preserving Reflectivity Over Time
Natural weathering and soiling can affect a roof’s ability to maintain its high-reflectance values. Depending on such variables as geographic location and climate; urban, agricultural, or industrial setting; the amount and type of discharge from the building and adjacent structures; and roof slope, particles and pollutants of all kinds can accumulate and diminish the roof surface’s inherent reflectivity.

Testing conducted by LBNL and the National Research Council (Canada) has shown that even simple cleaning techniques can restore most, if not all, of the original reflectivity^v. Washing a weathered cool-roof membrane can result in a practically complete restoration of solar reflectance. Studies and field experience have also found that most reductions in reflectivity occurred during the first year and then leveled off, with further reductions negligible by the sixth year; however, reflectivity levels will continue to be significantly higher than those of traditional dark-colored materials.

Application Methods Offer Choice
But, the vinyl material itself is not the only thing that’s flexible. With vinyl membranes, designers and installers also have flexibility in application methods. Taking into account building code and insurance requirements, vinyl membranes can be installed on the roof deck or substrate in several ways:

Mechanically attached with various fastening systems.
Loose laid in vegetative roofs or plaza decks, where the weight of the overburden holds the membrane in place.
Adhered with either adhesives or self-adhered membranes.

Since vinyl membranes offer a broad spectrum of application methods to accommodate varied building design criteria, the best possible application method for a given building can be employed.

The numerous variables involved make it important to take into account manufacturers’ guidance when designing and specifying a project. A full complement of manufacturer-authorized adhesives, insulation, sealants, plates, fasteners, flashing components, and other accessories are available to enhance the vinyl roofing system’s prospects for a long service life as a cool roof, along with a wide array of product and installation warranties.

REFERENCES:

Freund, S., Dettmers, D. and Reindl, D., "White Roofs in Northern Climates: Simulated Influence of the Roof Reflectance on the Building Energy Balance in Two Northern Cities," HVAC&R Center, University of Wisconsin-Madison, presented at ASHRAE Region VI Chapter Regional Conference, May 2007.
Budaiwi, I., Abdou, A. and Al-Homoud, M., "Variations of Thermal Conductivity of Insulation Materials Under Different Operating Temperatures: Impact on Envelope-Induced Cooling Load," J. Arch. Engrg., Vol. 8, Issue 4, December 2002.
Akbari, H., Berhe, A., Levinson, R., Graveline, S., Foley, K., Delgado, A., and Paroli, R., "Aging and Weathering of Cool Roofing Membranes," Proceedings of the Cool Roofing Symposium, Roofing Consultants Institute Foundation, Atlanta, May 2005.
Budaiwi, I., Abdou, A. and Al-Homoud, M., "Variations of Thermal Conductivity of Insulation Materials Under Different Operating Temperatures: Impact on Envelope-Induced Cooling Load," J. Arch. Engrg., Vol. 8, Issue 4, December 2002.
Akbari, H., Berhe, A., Levinson, R., Graveline, S., Foley, K., Delgado, A., and Paroli, R., "Aging and Weathering of Cool Roofing Membranes," Proceedings of the Cool Roofing Symposium, Roofing Consultants Institute Foundation, Atlanta, May 2005.