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The Building Envelope

05/20/2011 by James Armstrong

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The building envelope is a major determining factor in a building’s energy consumption. The envelope controls the impact of outside conditions on the interior spaces, while the HVAC systems strive to maintain the specified design conditions. An example is sunlight entering the space through the windows, doors, and/or skylights. Sunlight adds solar heat to the space. Depending on the climate, the design set point for temperature can be exceeded due to the thermal solar gains. The building systems will then strive to achieve set point by attempting to condition, or cool, the space.


Solar Heat Gain & Natural Light

In North America, a space with a south-facing façade with large window walls will require more cooling than a north-facing space with similar window walls. Heat must be added to the north-facing space and removed from the south-facing space. This is a normal dynamic for many office buildings in northern climates, even in the middle of the winter. Adding to this equation is the heat gain from building occupants, lighting, and computers or other equipment, which may require that the building be cooled year-round, and, in many southern climates, may never need heat at all. Solar gain through windows accounts for 32% of needed cooling energy in U.S. buildings. Another 41% is caused by heat from electric lighting, 17% from equipment, 7.5% from people, and the rest from heat conduction through roofs and walls.


The perimeters of these same buildings may at times have too much light, and yet the buildings’ lighting systems are still on. With green design, it is possible to shut off or dim the electrical lighting to maintain even lighting levels, and to incorporate skylight shafts, clerestory windows, glass interior walls, and other daylighting techniques, to bring natural light deeper into the building. A green design would also transfer heat from the overheated to the under-heated sections and use outside air economizer systems to cool whenever the outdoor ambient temperature meets the design conditions without the use of mechanical cooling. Green design also relies on trees to provide seasonal shade, thereby reducing the need for mechanical cooling. A deciduous, or leaf-bearing, tree planted on the south-facing wall will provide shade during the summer months, and natural cooling. These trees will lose their leaves in the fall and allow the space to receive natural lighting and solar heat in the winter months. Trees also reduce the water consumption of the lawn underneath by two-thirds. Trees are effective for shading west walls when the sun is low in the sky. This is true green design—using natural elements to reduce the consumable energy required to condition a space.


Insulation and vapor-retardants are other components of the building envelope that can have a tremendous impact on the consumable energy required for a space.


The Roof

Roof design has made major strides where sustainability is concerned. Because the roof is the most weather-impacted part of a building envelope, designers must consider long-term solutions to the conditions caused by weather exposure. Selection of roof materials for a green building involves health, environmental, and energy considerations. The available materials have pros and cons. Coal tar and gravel, for example, can be produced from recycled materials, whereas EPDM roofing is made from virgin materials. (LEED provides a listing of roofing materials with their environmental impacts that can be helpful in making selections.) Some green roofing approaches include using light-colored roofing materials to reduce the thermal gains by reflecting solar heat, using recycled or recyclable material (such as shingles manufactured from slate dust), and using material that has a longer life (such as metal or polymer roofing systems). A protected-membrane roofing system allows reuse of the rigid insulation during future reroofing, substantially reducing material use.


Some roofs have become a resource as well as a shield. Rain can be collected by a roofing system connected to a cistern (collection tank) so that the water can be used for other purposes. Living roofs become their own ecosystem, with plantings and grass contained within the membrane. This strategy provides insulation, alleviates runoff, and helps to meet the impervious surface requirements on some facilities so that more building can be accomplished with less land. The key to understanding the impact of the envelope is to be aware of the interactions and changes that occur within and through it. Changing the envelope to become a more dynamic component of the building is a critical part of green design.


James Armstrong, CPE, CEM, LEED AP, the author of this article, is senior energy engineer at Siemens Industry Inc., Building Technologies Division. This article was adapted from Green Building: Project Planning & Cost Estimating, 3rd Edition, available through RSMeans.

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