PRINCIPLES/BUILDING ENVELOPE PERFORMANCE
Arrangement of Envelope Elements
The arrangement of envelope elements is important to the fulfillment of envelope performance
requirements. This arrangement influences the conditions within the wall, and therefore the
environment under which the materials must function. For example, the location of the thermal
insulation determines the temperature distribution within the envelope, which in turn determines the
temperatures of the individual envelope elements. Temperature affects durability of materials,
impacts the degree of dimensional movement to which the elements will be subjected and the ability
of the materials to accommodate this movement. A considered arrangement of the envelope
elements will lessen the severity of exposure of these elements and can simplify issues of material
selection. Issues regarding the relative positioning of structural elements, thermal insulation, air
barriers and vapor retarders are discussed frequently in these guidelines. The positioning of these
elements and the impact of this positioning are complex issues, with every arrangement having both
advantages and disadvantages to consider.
Movement and Dimensional Change
The movement of envelope elements is an important issue related to the design of those elements
intended to control heat, air and moisture transfer. Envelope elements move and undergo
dimensional changes for a variety of reasons including thermally induced expansion and
contraction, changes in moisture content, aging, structural loading and movement of the building
frame. These movements must be anticipated and accounted for in the design of the envelope. If
these movements are not accounted for in the design, the driving forces will induce discontinuities
in the various barriers to flow or even result in more serious failures such as the cracking or
dislodging of facades. The accommodation of differential movement arises frequently in these
guidelines as a source of thermal performance failures and as a necessary consideration in
achieving good performance. Examples include the design of joints between precast concrete
panels and the interface between spandrel beams and concrete block infill walls. In both cases the
inevitability of differential movement must be recognized, and the intersection must be designed to
accommodate this movement in order to maintain the continuity of the air barrier and insulation
systems.
Terminology
This discussion of building envelope performance requirements provides the opportunity for
clarifying the use of several terms in this document. The building envelope, and at times simply the
envelope, refers to the barrier between the indoor and outdoor environments. This barrier includes
the walls, roof and foundation, though these guidelines are focussed on walls. While the building
envelope is composed of many elements and systems, sometimes performing very distinct
functions, there is a single building envelope that must meet all of the performance requirements
discussed above. The thermal envelope describes those envelope elements and systems intended
to meet the thermal performance requirements of the building envelope. The thermal envelope is
not in general a distinct portion of the envelope, since the same elements which perform thermal
functions may also serve other functions, e.g. windows. When using the term thermal envelope,
one must be careful not to consider those elements in isolation from other envelope performance
requirements or in isolation from other forces acting on the envelope.
References
Hutcheon, N.B., "Requirements for Exterior Walls," Canadian Building Digest No. 48, National Research
Council Canada, 1963.
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