PRINCIPLES/BUILDING ENVELOPE PERFORMANCE
2.1 BUILDING ENVELOPE PERFORMANCE
While these guidelines are concerned with the thermal performance of building envelopes, the
exterior envelope of a building must serve several functions. These functions, and the relationships
between the elements intended to perform them, must all be considered when designing and
constructing the envelope. Consideration of specific envelope requirements in isolation from one
another can be a source of design and performance problems. This section discusses the
performance requirements of the building envelope and establishes a context for the consideration
of thermal performance issues.
Performance Requirements
Hutcheon described the overall function of the exterior wall as providing "a barrier between indoor
and outdoor environments, so that the indoor environment can be adjusted and maintained within
acceptable limits." In achieving this general goal, the following requirements need to be considered:
.
Control heat flow
.
Control airflow
.
Control entry of outdoor pollutants
.
Control water vapor flow
.
Control rain penetration
.
Control light, solar and other radiation
.
Control noise
.
Control fire
.
Provide strength and rigidity
.
Be durable
.
Be aesthetically pleasing
.
Be economical
The first eight requirements relate to the wall as a barrier between inside and out, and they are met
by selecting elements that provide the appropriate resistance to each of the flows. In addition,
however, the arrangement of the elements meeting each requirement is important. This
arrangement determines the distribution of conditions within the wall, such as temperature and
water vapor pressure, and the environment under which the various elements must function. The
last four performance requirements are general requirements that must be satisfied while meeting
the others. The analysis and design techniques related to structural performance, fire safety,
aesthetics, noise and economics are well-established and covered elsewhere.
The durability of the envelope and its components describes their ability to maintain their function
over time. Durability is not an inherent material property, but depends on the environment to which
the element is exposed and the degrading effects of service. The arrangement of the elements
within the envelope can improve the durability of the elements, and the system as a whole, by
lessening the severity of exposure. The aesthetic appearance of the exterior envelope need not
conflict with the other performance requirements, but as is the case with other performance
requirements, aesthetic considerations should not be allowed to predominate over the achievement
of other requirements.
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