by Gregory Franta, FAIA
In "The Magic of Windows, Part 1," I summarized a history of glazing, presented fundamental performance characteristics, and introduced integrated design (see RMI Solutions, Fall/Winter 2006). In this article, I'll describe the analysis and integration of fenestration's effects in building design.
Integrating optimal window design into the initial conception and engineering of building systems is a critical step in creating low-energy buildings. The design and analysis process for integrated glazing solutions includes considerations of orientation, area, application, thermal transfer (U-Value), and solar heat gain coefficient (SHGC). Other important factors include the transmittance of visible light, air leakage, cost (initial and life-cycle), energy, thermal comfort, and effective daylighting.
RMI's researchers have studied window design in hundreds of buildings around the world and we have perfected processes to optimize total building performance. The two examples presented here exemplify how window design influences energy use in buildings. Both are educational facilities with six side-by side classrooms and have windows along one side. However, they are located in two very different climates: Minneapolis and Atlanta.
In the Minneapolis example, our analysis found the building will use a total of 57.7 kBtu per square foot per year. And, predictably, heating is the largest single energy user, at 27 kBtu per square foot per year.
To fully illustrate the impact of glazing, the base case chosen for the analysis was a classroom that had clear doubleglass covering a little more than 20 percent of one west-facing wall, and windows with no overhangs. The room had a 1.3-watt-per-square-foot lighting power density, required a modest amount of energy for computers, and, overall, the construction was typical.
During the window optimization process, considerations included "tuned" glazing,* which has ideal performance characteristics (U-value, SHGC, visible light transmittance, infiltration), overhangs and light shelves for sun control, daylight controls on electric light fixtures, and a southern orientation. These strategies alone reduced the total energy use to 34.5 kBtu per square foot per year. In addition, the operating costs dropped by $0.55 per square foot per year, with the largest savings being in energy for heating
and lighting.
By comparison, in Atlanta the building needs much more cooling and less heating, and it has a total energy appetite of 37.0 kBtu per square foot per year. When the same kind of windows used in Minneapolis are optimized for the Atlanta climate, the results show the energy use dropping to 21.8 kBtu per square foot per year. This energy use reduction is less than that for the Minneapolis school, but because the cost of supplying electrical cooling in Atlanta is more expensive than providing gas heating in Minneapolis, the energy cost saving is still about $0.55 per square foot per year.
In both cases, the most important economic factor was the reduction in peak loads for big equipment in the heating, cooling, and ventilation systems. The peak heating loads dropped by more than 20 percent, and the cooling and ventilating loads dropped by more than 50 percent. These construction cost savings due to equipment down-sizing can be achieved in any kind of climate to off-set the cost of up-front efficiency measures to the point that the return on investment is more than 50 percent (a two-year simple payback period)—not bad compared to my IRA.
Ultimately, the bottom line in both cases is that the emissions of carbon dioxide, nitrous oxide, and sulfur hexafluoride caused by the burning of coal and gas to power the buildings were reduced by 40 percent. When we combine these efficiencies with a wholebuilding design approach that includes other building envelope measures, enhanced lighting efficiencies, reduced mechanical equipment, passive energy strategies, on-site renewable energy systems, and green power from the utility company, the goal of net zero emissions
can be realized.
Integrated design is critical for the cost-effective development of buildings. Optimizing window design within a whole-building approach will assure a high performance building. RMI continues to conduct research and consulting on glazing strategies to reduce greenhouse gas emissions and operating costs.
Gregory Franta, FAIA, leads RMI's Built Environment Team.
*"Tuned": when specific glazing characteristics are applied for each orientation, climate, and application.
