Reducing Cooling Loads With External Shading
By Anne Cosgrove
Published in the November 2006 issue of Today’s Facility Manager
Facility managers who combat heat gain and glare caused by sunlight streaming into their buildings have a variety of options. Specially treated windows and window films, interior blinds, and external shading devices offer different approaches to this issue. In this age of energy conservation, keeping unwanted heat out can not only be a comfort issue but also an economic one. When the heat from the sun is effectively eliminated or controlled through the use of one or more of these strategies, the cooling load on mechanical systems is subsequently reduced.
External shading can be a good strategy to prevent occupants from having to lower shades or close blinds perpetually. This strategy can also help reduce heat gain and glare when a building situation does not allow for specialty windows or window films to be installed.
However, there are numerous factors to consider with such a system, and simply affixing a shade over a window does not guarantee reduced heat gain. The type of shading device, climate, and where on the building the device is used all contribute to the effectiveness of this approach.
A recent study conducted by the Center for Sustainable Building Research (CSBR) at the University of Minnesota (UMN), and sponsored by the Air Movement and Control Association International, focused on external shading devices and their impact on energy use and glare control in a commercial building. The study, titled “External Shading Devices in Commercial Buildings,” took into account five types of shading conditions—none, vertical fins, shallow overhangs, deep overhangs, and a combination of overhangs and fins. These constructions were then coupled with six types of performance glazings, six geographic climates, and windows facing east, south, and west. (The report states that north facing windows were not included, since the impacts of external shading devices are small in those situations.)
Researchers John Carmody, director of CSBR, and Kerry Haglund, information technology specialist at CSBR, produced a series of 36 matrices illustrating the impact of each type of shading device on energy usage and glare in the various scenarios. Using the U.S. Department of Energy DOE2.1 simulation software, Carmody and Haglund were able to report the estimated percentage of energy saved and percentage of glare reduced for each scenario.
As might be expected, the results varied widely by system and climate. However, across the board, the presence of shading devices on south facing windows showed the most promise in reducing energy usage. While many estimated savings were in the single digits, energy reductions were estimated as high as 38.5% with a south facing shading system in Phoenix, AZ.
Richard Basta, AIA, senior vice president, design, at EI Associates, an architectural, engineering, and construction firm in Cedar Knolls, NJ, has found this to be the case as well. “It is most effective to use shading devices on south facing windows. As far as north facing windows, shading will not have an effect. In fact, it can create a deficit, because the natural light does not get into the space.”
Once a facility manager has discovered what type of external shading will benefit his or her building most, there are cost factors to consider. The more sophisticated the system, the higher the price is likely to be.
Basta notes that operable systems are more costly. “They require power,” he says, “and that presents a maintenance issue as well. But if cost is not a limitation, the operable systems can be a good choice. For instance, users can minimize shading from the system on cloudy days, and there are benefits to that.”
In terms of maintenance, adding external shading to the building envelope means there is another element for the facility manager to oversee. Basta, whose firm does work in the Mid-Atlantic region of the United States, notes the impact of local climate on system maintenance.
“In a climate where it snows, there may be dripping or icing issues on the shades,” says Basta. “Safety and cost concerns would dictate using devices that shed rain and snow immediately.” This means choosing devices constructed of material that can unload these elements.
Physical structure of a shading device is key as well. A new elementary school that Basta’s firm completed earlier this year has been outfitted with angled shallow overhangs comprised of a series of elliptical tubes, rather than a solid mass.
“The tube construction lets the snow and ice through,” explains Basta. “Also, since there are slots in the overhangs, light can still get through to inside the building. Sunlight bounces off of the tubes and into the interior space; there is light gain, but not solar heat gain.”
When interiors are uncomfortably warm or too bright from invasive solar heat or glare, exterior shading may be an effective solution. Facility managers should work with experienced design professionals to explore potential benefits of this sustainable strategy.
Information for this column was compiled through an interview with Basta and from the CSBR/AMCA report. To learn more about the CSBR at UMN, visit www.csbr.umn.edu/index.html. For more about the Air Movement and Control Association International, visit www.amca.org.