PRINCIPLES/DESIGN AND CONSTRUCTlON
Requirements and Recommendations
These guidelines are not able to offer a redirection of the process by which office buildings are
designed and constructed. However, there are several essential design principles, stressed
throughout these guidelines, that need to be incorporated into the design and construction
processes. These include a modification of the rules stated by Brand for evaluating envelope
designs and all associated details:
Enclose the building in a continuous air barrier.
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Provide continuous support for the air barrier against wind loads.
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Ensure that the air barrier is flexible at joints where movement may occur.
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Provide continuous insulation.
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Design copings, parapets, sills and other projections with drips to shed water clear of the
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facade.
Provide the means for any water that does penetrate the facade to drain back to the outside.
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Thermal envelope design must also include a recognition that wall materials are not dimensionally
stable and will move differentially from each other and from the structural frame. The location and
extent of this movement must be anticipated. The air barrier element at these locations, whether it
is an elastomeric sealant or a flexible membrane, must be designed to accommodate the
anticipated degree of movement. If such movement is not adequately dealt with, the air barrier will
fail at these locations and the continuity of the air barrier system will be lost. The need for continuity
of the air barrier system can not be stressed enough. This continuity must also be maintained over
wall areas, including those that are not readily accessible such as above suspended ceilings and
behind convector cabinets.
The distinction between the control of water vapor diffusion and air leakage must be clearly
understood. By definition a vapor retarder controls water vapor transport by diffusion, but not water
vapor transport that occurs due to convection. An air barrier system is required to control
convective moisture transport due to air leakage. The amount of water vapor transferred by air
leakage is much larger than the amount transferred by diffusion, making the installation of an air
barrier essential to the control of water vapor movement.
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