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.
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.
An effective ventilation strategy also is critical in a high-performance school because it improves indoor air quality and thermal comfort, and reduces reliance on conventional mechanical systems. Today's displacement ventilation strategies go well beyond the use of conventional mixed ventilation. Ventilation strategies also can be analyzed early in the design process to assess their energy efficiency and life-cycle costs. These strategies can include simple systems, such as side-wall displacement terminals, and more complex underfloor systems.
In addition to widely available raised-floor systems, non-conventional underfloor air systems can deliver air and improve the thermal comfort in a space. They reduce the need for ductwork and drop ceilings, and provide radiant and forced-air heating through a network of steel forms embedded in the concrete slab. Another alternative distributes ventilation air via a duct integrated with the partition walls between two classrooms. The supply vents can be on the wall or integrated with classroom furniture.
Displacement ventilation and envelope strategies can reduce cooling and heating loads in the building. Mechanical energy is required to meet the remainder of the building load and provide a comfortable environment. After the building envelope is optimized, the school and design team may choose to evaluate supplementary mechanical systems, such as chiller-boiler mechanical systems or advanced geothermal systems. The team, ideally, also will consider further strategies to optimize the chosen system, including careful selection of variable frequency drives, fan motors, heat-rejection equipment and building automation controls.
A geothermal heating and cooling system can be a good choice for a school that has relatively uniform heating and cooling loads, and that is on a site that offers adequate installation area, such as athletic fields, to serve as a geothermal well. In this system, a geothermal heat pump takes advantage of the relatively stable earth temperature (45°F to 58°F) deep in the ground. Water or another liquid is circulated through pipes buried in a continuous loop in the geothermal well. Depending on the temperature in a building, the water is used to transfer heat into or out of the building.
In addition to these key issues and systems, a high-performance school design also requires analyses of water-management issues — in particular, stormwater management, landscape design and maintenance, and indoor plumbing fixtures. For example, rainwater-harvesting strategies that employ the roof structure may be investigated during the building envelope study.
Aliotta is principal of Swanke Hayden Connell Architects, New York City. Pde is an associate, practice leader-building simulation and systems analysis, with Atelier Ten Consulting Environmental Designers, New York City. The firms collaborated on the Hamilton Avenue School (see sidebar).
45 to 58
The Earth's relatively stable temperature in degrees Fahrenheit. A geothermal heat-pump system takes advantage of this temperature.
Achieving the gold standard
The new Hamilton Avenue School, a pre-K to 5 facility under construction in Greenwich, Conn., was designed as a high-performance school.
The project includes 60,000 square feet of new construction, along with the partial restoration and reuse of a historic 1938 building, accounting for an additional 16,000 square feet of space. The contemporary design of the new portions of the building complements the historic fabric of Hamilton Avenue in Greenwich through the preservation of the civic front of the existing school, a brick and stone building with a slate roof and signature cupola. Classrooms are organized in clusters by age group. The new gymnasium and cafeteria will be designed to accommodate multiple functions and community use.
Designed to the equivalent of LEED gold certification, the school features a geothermal system, extensive daylighting and top-lighting of the classrooms, displacement ventilation and a super-insulated building envelope. The school chose not to pursue formal LEED certification because it would have incurred additional cost for registration and submittal, and certification is not a Connecticut state mandate.
The design incorporated energy and daylighting analysis; modeling studies and life-cycle cost analyses of certain aspects of the building envelope; natural ventilation strategies; and assessment of the costs and benefits of using a geothermal system over a chiller-boiler system.
For example, factoring in the results of the building envelope, ventilation and daylighting studies, the geothermal energy modeling study compared a ground-source heat-pump system to a four-pipe chiller boiler system. The results found that the geothermal system would achieve a simple payback in just 8.1 years, returning a net-present-value savings of more than a quarter million dollars over 25 years.
Overall, a life-cycle energy cost analysis projected a five- to seven-year payback for the upfront costs of high-performance design strategies compared with a conventionally designed school.