Overview
Human physiology can only tolerate a narrow range of environmental conditions beyond which health and well-being are compromised. For relatively brief periods, extreme conditions are tolerable, but only by fit and healthy individuals. In regions of the earth where the climate is not ideal for habitation, this has necessitated the construction of enclosures to provide shelter from the external environment. Even in ideal climates, enclosures proved necessary as protection from insects and animals, including hostile human beings.

Our first skin, that which envelops our bodies, performs numerous functions, many of which are reflected in our technological extensions. Our second skin, in the form of clothing, keeps us warm, dry and protected against external phenomena. (In some cases, this second skin is highly specialized, as in the case of sporting equipment, astronauts' flight suits, etc.) Our third skin, in the form of building enclosures, does more than simply emulate the functions of its human counterpart. In practical terms, enclosures provide protection from the elements and ensure survival. More recent architectonic developments involve looking at the fourth skin in the form of double-skin facades to go beyond shelter into the realm of comfort and delight.

The performance requirements for enclosures have been well understood for several generations. Neil Hutcheon, one of the pioneers of Canadian building science, established a set of minimum requirements almost half a century ago.

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Since the time Hutcheon outlined these requirements, some of the terms used have taken on new meaning. Control of heat flow now fully considers solar radiation in terms of daylighting and passive solar heating. The stability and durability of materials includes issues such as off-gassing of contaminants, molds and embodied energy. Cost has been generalized to consider the environmental impacts of buildings. The biggest changes regard aesthetic considerations, in particular thermal comfort, indoor air quality, and auditory separation.

It is not surprising that Hutcheon defined a basic set of performance requirements which extend from the Vitruvian parameters of "firmness, commodity and delight" - the needs and desires of humans have essentially remained constant as have their reasons for, and methods of, constructing enclosures. One aspect that has changed to some degree is the imperative for sustainability. We have evolved to the point where survival deals less with protection from the natural elements, and more with managing our ecological footprint, and the health and well being of building occupants.

Despite these additional considerations, enclosure requirements are driven primarily by two intersecting factors: the natural phenomena which occur in the world around us; and the intended use or occupancy of the enclosure. The table below indicates some of the more critical physical phenomena impacting enclosures.

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Occupancy
Occupancy or use of a building causes physical influences on the enclosure and supporting building systems that must be reconciled with programmatic elements. In practice, it is not always possible to achieve an ideal balance, however, consideration of the influences at the conceptual design stage enhances the likelihood of a successful design.

From an architectural science perspective, it is important to correctly assess the occupant influences on the enclosure to ensure that the whole building system performs its intended function, not only to a minimum standard of health and safety, but to the level of client expectations. The definition of building occupancy ranges across both animate and inanimate occupants, and typical and special uses, as listed in the table below.

It is common to have enclosures address the sub-division of occupancy and use, as well as hybrid occupancy and use. For instance, a library consists primarily of two types of occupants, humans and books. The control of temperature and relative humidity to avoid mold growth in paper is often a more critical consideration than the comfort of the occupants. Similarly, the loads associated with the stacks of books are more critical to the structural system design than the loads imposed by human occupants.

In some cases, such as libraries, satisfying the requirements for one occupant also satisfies the requirements for all other occupants. In other cases, these conflict and one or several of the occupants will not have their requirements addressed. A good example of the latter case is an ice cream factory, where human workers must attain comfort through appropriate clothing rather through any element of the building enclosure or HVAC system.

Effective strategies for dealing with diverse and conflicting occupancies are: aggregate similar occupancies and confine to separate areas or zones; and/or provide a separate building system or sub-system for each occupancy or grouping of similar occupancies; and/or modify behaviour of an occupant or occupants to adapt to resulting conditions.

The latter strategy often goes beyond the conventional notions of architecture, however, it is important recognize that it is often more practical and economical to modify occupant behaviour (e.g., wearing protective clothing, etc.) than to satisfy occupant requirements through the building enclosure and environmental control system. This is particularly the case as buildings become more ecologically centred, and occupant behaviour must conform to the limitations of passive enclosures, or enclosures actively assisted by renewable energy sources.

Numerous criteria and requirements apply to legislated occupancy classifications, especially when they adjoin each other. Most legislation is based on damaging and/or tragic past precedents. In some cases, certain combinations of adjoining occupancies are prohibited for the sake of health and safety, underlining the fact that despite our technological sophistication, enclosures have inherent limitations.

In the enclosure design process, it is necessary to fully assess occupant/use factors to establish performance criteria for the building envelope, and for any supporting environmental control systems needed to supplement the passive role of the enclosure. The table below includes the more typical occupancy/use factors to be considered in design and construction.

It is important to note that these factors are intended to inform the larger and more sophisticated process of architectural design, and should not be confused as a convenient substitute for a holistic approach.

The next section on Enclosure Typologies deals with the tectonics of enclosures which are explicitly or incidentally intended to address the requirements that have outlined herein.

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