Designing an Energy Efficient Building Envelope
In the early stages of the green building movement, lighting, mechanical, control system designs, and upgrades received the most attention. Over the last several years, however, the focus has shifted towards designing and specifying energy efficient building envelopes. Building construction and operation have an enormous impact on the environment in terms of energy usage, atmospheric emission, use of raw materials and much more. The design and construction community faces increasingly tougher challenges to meet demands for commercial buildings that are accessible, healthy and productive, but also environmentally friendly.
Building scientists have indicated for years that designing an energy efficient building envelope can make a significant difference in overall energy use and contribute to a smaller carbon footprint. Because of the varied functions that building envelopes can have, an integrated and synergistic approach is warranted that results in an optimal balance of cost, environmental and human benefits, while meeting the facility’s intended function.
Creating high-performing building envelopes includes a number of essential components, beginning with the choice of framing. In place of traditional framing, viable alternatives include wall and cladding systems that can make significant strides to well-constructed building enclosures. These include structural insulated panels, insulating concrete forms, exterior insulated finish systems, or insulated concrete block.
The management of moisture migration, though, is by far the most important component in building envelope design. Moisture-related problems vary widely in their types and consequences, making selection and proper installation of a vapor membrane crucial to a properly designed building. The Whole Building Design Guide notes that proper specification and installation of vapor barrier membranes can retard or prevent problems such as: frost heaving and freezing, water leakage, wood rot, mold growth, spalling, efflorescence, corrosion, and degradation and staining of finishes. Vapor barriers also lighten the load of building HVAC systems that help remove excessive moisture from interior environments. In conjunction with proper framing and vapor barriers, deflection and drainage of water away from the building foundation can also result in a more secure and energy efficient building envelope.
The main purpose of vapor barriers is to prevent the envelope structure and insulation from condensation damage. Many types of wall materials are permeable and allow the flow of water vapor from inside to outside as well as the other way around. Water vapor that attempts to move from the building interior out through walls on cold days encounters progressively lower temperatures and will condense at that point where the temperature of the air meets the dew point.
Condensation can damage many different types of envelope structures, rusting steel masonry reinforcements, while also causing freeze cracking in the masonry itself. Specifying and installing vapor barriers on the warm side of the building envelope will prevent water vapor from traveling through the wall, thereby preventing condensation. It’s important to make sure vapor barriers are installed on the warm side as those installed on the cold side will trap moisture inside the envelope and can make moisture problems worse.
This situation can pose a dilemma in areas with hot and humid summers and severely cold winters, such as the northern Midwestern United States, however, the best practice continues to be specification of vapor barriers on the inside.
When water vapor condenses inside insulation, dampness reduces thermal resistance and may ultimately damage the insulation itself. In addition, installation of well-designed vapor barriers also helps reduce air leakage through the envelope, although they will not reduce airflow within the structure itself. At the same time, vapor barriers can also reduce the amount of energy and water in buildings where humidification is necessary.
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