Building enclosures are human prostheses that represent the 'third skin' separating indoor environments from the outside world. Like our first skin which is a living, regenerating organ, and unlike our second skin, clothing, (which seldom outlives the vagaries of fashion cycles), the skins of buildings are ideally intended to last the life of the whole building, in particular its structure, or skeletal system. In traditional building forms employing loadbearing masonry, this relationship was axiomatic since the structure was also the skin. But as building technology evolved, and the structural and cladding functions became separated, the durability of the skin over the life cycle of the building increasingly challenged the architect. This challenge often focuses on the design of walls, which represent among the highest cost components of the building envelope system, and are also the most visible aspect of the building, its faade.

The structures of modern buildings are engineered to perform adequately for a long time, typically several hundred years as confirmed by numerous precedents that remain serviceable to this day. Mechanical and electrical systems are routinely upgraded or replaced in the life cycle of commercial and institutional buildings, along with the periodic renovation of interior finishes and furnishings. It is normally expected that the structure will remain serviceable for the useful life of the building, and that services, finishes and furnishings may come and go. This leaves architects and owners to ponder the relationship of the skin to the rest of the building.

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Many important questions remain unanswered regarding enclosure durability and its relationship to whole building sustainability. Where does the skin of a building begin and end? Are 'pure' unambiguous skins preferred to composite envelope assemblies with interdependent components? What degree of redundancy with respect to critical control functions is necessary for acceptable long-term performance? How is durability defined at the design stage and subsequently confirmed during mock-up testing and construction review? What are the appropriate means of transferring the answers to these questions, assuming we obtain them, to students and practitioners of architecture? These pivotal questions surrounding the quest for enclosure firmness have been largely obscured by deference to commercial commodity and fiscal delight.

Two important concepts may be explored to begin to address these critical questions, differential durability and functional obsolescence.

Key characteristics and relationships associated with differential durability concepts are depicted in the figure below. As discussed earlier, durability may be expressed as a function of service quality and service life. There are three critical service quality thresholds related to durability: 1) the specified quality, established by the designer and/or minimum codes and standards, representing the typical new service condition; 2) the minimum acceptable quality indicating the need for replacement or retrofit; and 3) failure, where the material or assembly is considered completely unserviceable.

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Failure may occur suddenly as in the case of a lamp, pump or similar type of equipment, or it may result after gradual deterioration. Maintenance or restoration taking place prior to failure can extend the service life, whereas deferred retrofit or replacement beyond the minimum acceptable quality threshold can accelerate total failure. It is important to note that in some cases, the initial service quality of the material or assembly may exceed the specified quality based on codes and standards.

Given these basic characteristics and relationships, it is possible to explore various aspects of differential durability. The next figure depicts the underutilization of durability in assemblies with interdependent components exhibiting differential durability.

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A practical example of interdependent durability is the case of bricks and brick ties, where the former deliver a longer service life than the latter. When the inferior durability component reaches the end of its useful service life, the superior durability component is often replaced at the same time, resulting in an underutilization of its durability. The lesser the degree of durability harmonization, and the greater the degree of difference in initial service quality between components, the greater the underutilized or wasted durability (embodied energy) of the assembly. This underutilization has a direct impact on the recurring embodied energy demand over the building life cycle.

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This type of accounting is not normally conducted in durability research related to the recurring energy content of buildings. At this time, it is difficult to accurately assess the magnitude of these compounding effects due to the scarce availability of verifiable data. However, a tour through any typical building demolition/reclaim yard indicates that many of the materials and components are serviceable. In the case of old windows where the glazing is serviceable long after the frames have deteriorated, the compound recurring energy for the glazing may easily approach 50%.

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Obsolescence

Another facet of differential durability is associated with the degree of flexibility and adaptability in buildings, commonly referred to as functional obsolescence.

Poor initial design leading to functional obsolescence is not normally considered in building durability, yet the recurring embodied energy implications may easily compare to those associated with physical deterioration. When the costs of retrofitting for adaptive re-use equal or exceed the construction cost of new facilities, the value of the original design is fairly questionable.

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