Fundamentally Green

June 1, 2005
Compact wraparound building design is worth a look as it relates to energy efficiency.

Lately, the people that design, build and operate education buildings have received an education themselves — in sustainable design. Architects, engineers, contractors and clients all have taken part, not always with the specific intent of education. The vehicle for this education has been LEED, the U.S. Green Building Council's program that seeks to define and encourage green building. Schools can indicate their interest in using LEED simply by including the line such as “must be experienced with LEED Version 2.0” in their RFPs.

With the spread of interest in LEED at schools and universities, much has been written about the program's components: sustainable site planning, water conservation and efficiency, green power and optimal energy performance, efficient mechanical systems, recycling older buildings, reusing and recycling materials, and improving indoor environmental quality.

What has been all but ignored in the LEED-driven educational process is the discussion of the age-old planning question: how can I get the most building for the least cost? Translated into thermodynamic language, how can I enclose the largest volume of usable space with the minimum quantity of negative heat exchange through its exterior surface? A spherical soap bubble is the poster child of this concept. It encloses the most volume with the least surface area. When a soap bubble lands on a flat surface, it becomes a perfect hemisphere.

Bubble wrap

The classic indigenous built example of a hemisphere is an igloo. The basic hemispherical shape is circular in plan and a half-circle in cross section. When a fire is built in an igloo, there is only minimal heat loss through the thick, curved walls. The body heat of the occupants contributes to the interior heating.

When soap bubbles combine with each other, they do so in a manner that continues to minimize surface area while maintaining maximum volume. Two equal soap bubbles join to form an “8” shape in plan. Three will form a figure resembling a three-leafed clover. Eventually, with more equal bubbles, a hexagonal “honeycomb” pattern is formed. The interior bubbles are surrounded by perimeter bubbles — just like a wraparound building plan. If nature were building a school out of soap, it would use a wraparound design.

The compact wraparound plan is not new. It has been used in many indigenous cultures for centuries. More important, the economics of building construction and operation support such a strategy even when other green strategies are too costly. An efficient plan often is the most profoundly green aspect of a building design.

The surface area and support systems of a building usually constitute a significant portion of the construction budget, with the MEP systems and interior finishes taking the rest — often more than 50 percent. Less building surface also means less built weight. Less weight requires less structure.

Prior to a commitment to energy conservation, structural efficiency was a primary cost goal. Increased depth of beams increases the floor-to-floor height and eventually the volume of the building — more volume, with more air to heat and move, results in larger equipment and higher energy cost for the life of the building. Today's efficient building designs often use more structure in favor of less energy.

So the general design task usually is twofold: maximize the built volume but minimize the building envelope. The size of the mechanical system is directly dependent upon the building volume to be heated or cooled and the ability of the building envelope (the roof and walls) to serve as a positive heat exchanger. On a cold day, a heated building can lose heat to the exterior environment. On a hot day, an air-conditioned building can gain heat from the exterior environment.

Unique challenges

Typical education buildings have additional problems. Classrooms and lecture halls full of people generate heat, regardless of the season. Areas such as administrative offices hold few people so they rarely overheat. Some large spaces, such as gymnasiums or auditoriums, are inactive much of the day and then need to be activated immediately for heavy use. Corridors are vacant most of the day as well, but are used regularly. By carefully organizing the plans of educational buildings, the various functions of the building can be optimized for seasonal heat gain and heat loss.

Thermal efficiency is manifested most obviously in the floor plans of buildings. A linear, sometimes “saw-toothed” plan maximizes the envelope of a building. Where cross-ventilation is possible and the climate justifies the strategy, such a plan can work for classroom buildings. However, in many colder U.S. climatic zones, “wrapping” large, infrequently used spaces such as auditoriums, gymnasiums and even libraries with classrooms or offices condenses the floor plan, reduces the quantity of envelope, structure and foundations, and saves operational energy for the life of the building.

The “little red schoolhouse” frequently had one room for all classes. All four inner walls also were outer walls. Heat gain or loss occurred when one side of the wall was hot and the other side was cold. In winter, heat often passed out of the building through the walls and roof. When the air temperature is the same on both sides of a wall, no heat exchange occurs. A two-room schoolhouse has one interior wall between classes. As the number of classrooms increases, side by side, linear corridors usually are the direct result. So each classroom needs a section of corridor area directly in front of it. The classroom does not lose heat to the corridor wall, but opening and closing the door can cause a direct heat exchange of air. The general idea usually is to reduce the corridor area of a building.

Reducing the perimeter enclosure of a building has an important additional result: it also usually reduces the length of footings around the edge of the building. This is especially important for low buildings where wall thickness can be governed by build-ability criteria, not absolute structural strength.

A disadvantage of reducing the perimeter of a building is that the perimeter is where the natural light usually enters the building (through windows). When a building approaches a square in plan, the innermost “captured” spaces can suffer from lack of light. As a result, skylights often are employed that allow light to enter through skylights or clerestories. A frequent partner to skylights in multistory buildings is a lightwell that extends down through the building. The general idea is to avoid losing the energy efficiency gained in the process through the skylights or clerestories.

Obvious candidates for interior enclosed areas are spaces that do not need windows for view — auditoriums and gymnasiums. Both of these areas normally are two-story spaces. A two-story building surrounding a two-story space is even more efficient than the one-story version. All buildings lose or gain heat through the roof. Placing one floor on the other minimizes the roof. Heat exchange through the ceiling of one space to another above is negligible.

There is a limit to the size of an auditorium or gymnasium that can be encircled effectively by other functions. As the floor area of the building increases, it often is desirable to reduce its apparent size. No one wants to have the feeling of being engulfed daily in a sea of people. As wraparound buildings do increase in floor area, it is possible to combine them with other neighboring wraparound buildings, or pods. Circulation paths then can connect two or three such pods to make a single, efficient building unit. The overall efficiency of the wraparound design is extended to a multiple pod building of higher occupancy, but at a much more human scale than the large population otherwise might imply.

Many important human criteria help determine the final form and proportions of a building. Thermal efficiency is just one. Security or fire safety may be a priority. The criteria may conflict with one another. For example, in the winter, a large window might be ideal to let light in, but a small window would reduce heat loss. For security reasons, the window should remain closed; for fire safety, it may need to be opened in order to escape through it. With a compact design solution, both the occupants of the building and the other occupants of the planet can be well-served.

Rush is a LEED-accredited architect with the Office of Michael Rosenfeld, Inc., Architects, West Acton, Mass.


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