Energy and the Building Envelope

11/21/2005 |

Learn how envelope design can impact mechanical system performance

In the very first hours of conceptual design, major decisions are made that impact the behavior of a building, including thermal behavior, energy efficiency, comfort, and natural light.

During that short time of decision-making, the architect most likely has little outside help from specialists. He/she makes decisions on basic principles, precedent designs, past experience, and, today, on objective and reliable information and simplified models provided by websites like (www.buildingenvelopes.org).

In the early design stages, it’s imperative to understand that the type of the building, its orientation, the skin of the building, and the mechanical systems are highly interdependent, and any decision about one will determine the design of the others. The design of the building should not work against the laws of nature and should not depend on the brutal force of technology to rectify the design. In general, the type and orientation of the building sets the stage for performance, and then, the skin and the mechanical systems are a zero sum game: A properly designed envelope will result in a more efficient mechanical system.

Thus, the starting point should be an extensive study of the climatic conditions of the site, and how vernacular architecture has handled those climatic conditions in the past. The techniques of the past may not be directly scalable to today’s larger and more complicated building programs, but the basic concepts could be used in alternative forms. Then, the conceptual design of the building should be based on selecting the proper site location, orientation, and shape to maximize the benefits and minimize the problems of the sun and the prevailing winds at the site.

At that point, the building envelope should be selected and could be different for each side of the building, responding to the sun and wind conditions in the different seasons of the year. Mechanically movable systems have shown little reliability in the past, while permanent installations on the other hand can serve satisfactorily, although not optimally in the different seasons.

As a principle, a skin made of highly insulated but permeable materials is ideal. However, architects often choose large glass surfaces and even all glass façades in today’s buildings, choices counter-intuitive for thermal performance. On the other hand, double- or triple-glass walls have shown promise by allowing a layer of air between the glass walls that can be either static, or be circulated mechanically, following different paths depending on the season. In the winter, the heated air circulates in the interior, while in the summer it is exhausted, transferring the accumulated solar energy outside the building. In a triple-wall system, two of the glass layers are separated by a static layer of air to increase the insulation. Double- and triple-glass walls are significantly more expensive than single façades. However, in some cases, they can be financially feasible.

The power and operating costs of the mechanical systems are less than those mechanical systems that would be required for single glass façades; thus, the combined cost of both the envelope and the mechanical systems and the operating expenses based on the projected costs of energy may be favorable for a more sophisticated envelope. In addition, the more sophisticated envelope provides more comfortable space in the perimeter of the building.

Spiro N. Pollalis is professor of design, technology, and management at the Graduate School of Design and director of the Center for Design Informatics, Harvard University, Cambridge, MA.


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