April 2, 2015
by Scott Moffatt
According to the U.S. Energy Information Administration (EIA), buildings account for almost 40 percent of energy consumed in the United States. With concerns over global warming and the associated impacts of energy consumption, energy efficiency has become a critical part of building design.
The U.S. Department of Energy (DOE) has a goal of achieving net-zero energy commercial buildings that will be marketable by 2025. Improving energy efficiency involves various approaches and design options, including:
In addition to these strategies, flat, white roofs have become standard practice in many applications. These assemblies feature high-reflectance values to reduce heat gain from solar radiation falling on the roof, and are widely used, particularly in warmer climates and on warehouse and big-box buildings.
To increase the aesthetic options available to architects and building owners, coatings manufacturers have developed reflective metal coatings for steep- and low-sloped roofs in a wide range of colors. These coatings reflect solar energy as effectively (or nearly so) as flat white roofs.
These so-called ‘cool’ coatings, which can also be applied to exterior walls and window frames, have been available through composite panel manufacturers for years. However, the U.S. Green Building Council (USGBC) does not yet allocate Leadership in Energy and Environmental Design (LEED) points for such applications. The American Architectural Manufacturers Association (AAMA) has developed a voluntary solar-reflective specification, but the aluminum extrusion market has yet to convert to this energy-saving technology.
This article describes and examines the results of an independent energy modeling study that quantifies the potential energy savings associated with high-reflectance coatings not just on roofs, but also on underutilized applications such as wall panels and aluminum extrusion window frames on mid-rise commercial buildings.
An independent energy and environmental analysis firm conducted a simulation analysis to evaluate the energy consumption effects of high-reflectance on a generic, eight-story office building using eQUEST/DOE 2.2 building energy modeling software.
The firm ran separate simulations for walls, window frames, and roofs on the generic building, incorporating metal coatings with solar reflectance values of 0.25, 0.35, 0.55, 0.65, and 0.70. The energy consumption for those models was compared to that for a baseline building with reflectance values of 0.05 on all three surface types—walls, window frames, and roofs.
To examine the impact of climate on the results, the simulations were also performed using weather data for 12 cities in a representative range of climates.
The results were quantified using three metrics:
The modeled building
The eight-story office building was modeled with 22,389 m2 (241,000 sf) of floor area and a slightly rectangular footprint, with five zones per floor. The wall insulation varied by the location, and was based on the minimum requirements of American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings. Roof insulation was R-20 continuous insulation (ci), as specified by ASHRAE 90.1, for all the modeled locations. Figure 1 provides additional details on the building envelope, while Figure 2 gives details on internal loads of the building. Figure 3 describes the heating and cooling systems.
Energy costs were found using U.S. average rates for commercial customers, as of September 2009, provided by EIA. Electricity cost was $0.1051/kWh and natural gas was $0.898/therm.
The study assumed the use of thermally broken frames to produce conservative savings estimates. Frames without a thermal break have default U-values double those of the frames modeled in this analysis. Thermal break frames are common in cold climates, but less so in warm climates, where heat gain reduction from low-reflectance coatings is most valuable.
Results for reflective wall coatings
The price difference between standard fluoropolymer coatings and heat-reflective coatings for metal wall, window frame, and roof applications is relatively minor, yet the study demonstrated the small premium will pay for itself many times over in savings. This finding was consistent even in cold climates—cities such as Boston, Chicago, and Ottawa—where the modeled energy savings are smaller than for warmer locales.
Figure 4 shows annual energy cost savings realized on the modeled building with metal wall coatings with reflectance rates of five to 70 percent in 12 major cities encompassing a broad spectrum of climates.
Figure 5 shows the design airflow reduction. Combined with other measures, reflective wall coatings may enable smaller air-handling unit (AHU) sizing, or reduced fan speed and fan brake horsepower (BHP) in the same AHU. As expected, the greatest capacity reductions are realized in hot, sunny climates.
Results for reflective window frames
Figure 6 illustrates annual energy cost savings realized on the modeled buildings with window frame coatings with reflectance rates of five to 70 percent.
Figure 7 shows the design airflow or fan-sizing reductions that potentially can be achieved by using reflective metal coatings on window frames. The savings may not be significant by themselves, but they can be substantial when combined with cost reductions provided by other reflective-coated building components.
Results for reflective roofs
Figure 8 shows annual energy cost savings realized on the modeled building with roof coatings with reflectance rates of five to 70 percent.
Figure 9 illustrates the modeled airflow reduction for buildings with reflective roof coatings. They are smaller than other air flow reductions for walls and window frames because the roof area on the modeled building is small in proportion to the wall and window areas.
Results for walls, window frames, and roofs with 70 percent reflectance
In addition to the analyses where high-reflectance coatings were applied to the walls, window frames, and roof individually, the modeled building also was analyzed with reflectance for all three surface types set to 70 percent. As expected, combined savings are higher than savings for a single low-reflectance surface (Figure 10).
The energy modeling analyses showed high-reflectance coatings reduce energy costs significantly in warm and hot climates, and less so in the coldest climates, as would be expected. Other findings include:
Cool roof savings are limited in comparison to other surface areas in this modeling study, because on tall buildings, a well-insulated roof only accounts for a small portion of its surface area. It should also be noted higher surface reflectances reduce design airflow in all cases.
Building configuration, HVAC system type, and local climate all are significant factors in determining a building’s potential energy savings. Nevertheless, the simulation analyses showed when designing a building for low energy consumption, even in cold climates, high-reflectance coatings should be specified for walls, window frames, and roofing given their small cost premium.
Scott Moffatt currently holds the position of market manager, building products for the coil, extrusion, and wood groups within PPG Industries. He has 36 years of experience within PPG’s industrial organization in various sales, sales management, and marketing functions covering multiple technologies and market segments. Moffatt’s current position involves getting PPG specified with architectural firms, glazing companies, and applicators. He can be contacted by e-mail at firstname.lastname@example.org.
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