The Shape of Things
Feb 1, 2008 12:00 PM, By Joe Aliotta and Gerald Pde
A high-performance school relies first on form and materials, and then on systems.
Designed to the equivalent of LEED gold certification, the new Hamilton Avenue School, Greenwich, Conn., will feature a geothermal system, extensive daylighting and top-lighting of classrooms, and a super-insulated building envelope.
Many people narrowly focus on energy efficiency when defining a “high-performance” school — a school building that is economical with respect to heating, cooling and electric lighting. That is certainly true, but in the broadest terms, a high-performance school is designed to minimize reliance on fossil fuels — and provide a comfortable, healthful, productive and beautiful learning environment — by optimizing the efficiency of the building form and mechanical equipment.
Working together, architects, engineers and environmental designers can achieve this complex goal by integrating effective school planning, sustainable architecture and sophisticated building performance analyses. In the process, they use energy modeling early in the design process to analyze the building envelope, and mechanical and electrical loads. Daylighting, ventilation and mechanical/electrical strategies are used to develop and refine the design.
Laying the foundation
Design-stage analysis is possible through a range of modeling software, depending on the nature of analysis required. These include a program patented by the U.S. Department of Energy and Lawrence Berkeley Laboratory, as well as proprietary software developed by manufacturers of mechanical systems and independent researchers. Building-performance benchmarks based on a designer's experience with various types of buildings in various locations also serve as useful rule-of-thumb tools for qualitative and quantitative analysis. Ideally, a design team should work together as early as the conceptual design phase to identify the site and climate factors that affect the building envelope.
The thermal performance of a building envelope — roof, wall, window and floor systems — is critical in determining energy consumption for heating and cooling. Likewise, the design of an efficient building envelope lays the foundation for the design of a high-performance school. In an early design-stage building envelope study, a whole-building energy simulation of the school can provide a useful initial assessment of the annual heating and cooling requirements. This study can help determine the energy use that can be attributed to the building envelope. It focuses on massing and the insulating properties of the building envelope.
Building energy simulation typically uses a “base case” for comparison — that is, a building whose roof, wall and underfloor R-values and glazing U-values meet the minimum code requirements set by the ASHRAE 90.1 standard, which is used by the U.S. Green Building Council. The modeling process compares the predicted energy performance of the base case against the design alternatives, whose energy-efficiency measures may have individual or cumulative effects on building performance. Whole-building energy modeling typically focuses more on the sensitivity of the overall envelope than on the localized effects of the envelope on individual spaces.
Optimizing daylight
Effective use of daylight is critical in a high-performance school; lighting loads typically account for 20 to 30 percent of total energy costs. An effective daylighting strategy reduces dependency on artificial lighting and enhances the quality of the indoor environment.
Daylighting studies investigate glazing configurations required to achieve an average target daylight factor of at least 2 percent on the work-plane, which is a minimum recommended target to achieve adequate daylight in a space. Daylight factor measures the percentage of outside light that penetrates into the space under an average overcast sky condition, making it a good indicator of the daylight availability in the space.
But effective daylighting is dependent not only on the quantity, but also the quality of light. If the light level in one part of a space is substantially higher than levels in another part, then the space could appear dark and gloomy, and glare conditions are likely. Relatively uniform light distribution of daylight serves as an indicator of daylight quality.
A daylighting study looks at the effects of various window shading configurations and their effects on daylight in classrooms on all levels of the building. A typical study also considers the reflectance values of ceilings, walls and floors, as well as the light-transmission values of various types of glass. The results of the study typically include a graph mapping the percentage of hours throughout the year when diffused light levels meet or exceed certain target daylight levels, or 3D renderings showing light levels in a space.
The study also may include a human perception rendering representing what one actually would see in the classroom. Human perception images adjust the calculated light levels based on the characteristics of the human eye. The eye only can perceive a limited range of light levels: as the pupil contracts to focus on an area with high light levels, the areas with lower light levels are perceived as darker, and vice versa. The results are used as the basis to develop design strategies to achieve uniform lighting throughout each space using a combination of daylight, electric light and shading.
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