Durability
The durability of buildings lies at the core of sustainable architecture, yet it remains to be fully assessed as a measure of sustainability. As was noted in the discussion on recurring embodied energy, non-durable building components, especially the envelope, result in high life cycle costs due to maintenance, repair and premature replacement. Durability is often compromised when designers and owners confuse it with the issue of first costs.

There is great interest in durability by the international building research community, predominantly in the materials and assemblies comprising building envelopes. Building envelopes 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 façade. There are also many durability issues related to foundations, roofs and building services. Refer to the Related Resources + References page for further information on durability issues and research.

But what is the reasonably expected durability of sustainable buildings? In Canada, guidelines for building durability have attempted to define acceptable ranges of durability for buildings and their components according to the following parameters:

"The loads on components and the building that result from the operation of the systems and services should be considered along with environmental and structural loads."

[Source: CSA S478-95 Guideline on Durability in Buildings, CSA International, Rexdale, ON, 1995.]

Design for durability implies the need to contextualize the forces and phenomena impacting the building, and suggests that a building envelope in Vancouver should be different from one in Montreal for the same occupancy, and that in the same climatic zone, different occupancies may result in different envelopes. Advocates of bioclimatic design go one further and suggest that envelope design should also vary according to solar and wind orientation.

If structural designs vary according to occupancy, snow, wind and seismic loads, why has this thinking not been extended to other building components? North America's architectural science community is now coming to realize the need for limit states design of building envelopes according to the parameters outlined above, in response to the various climatic zones. This will lead to envelope durability which is as predictable as the integrity of building structures. Only then will other measures of sustainability truly reflect the environmental impacts of buildings by reliably assessing the useful service provided by all of the components, including walls. Until such time as the durability of building envelopes can be designed and predicted as accurately as that for structures, measures of sustainability remain highly questionable at the design stage.

As durability research advances, parallel developments in hygrothermal modeling tools for simulating heat, air and moisture interactions in building assemblies will gain in sophistication and accuracy. But it remains to be seen if these advances in architectural science will be used to enhance the durability of innovative assemblies, or to whittle away the factors of safety in order to reduce first costs?

Performance
The level of service provided by a building material, component or system, in relation to an intended, or expected, threshold or quality.

Durability
From a sustainability perspective, a material, component or system may be considered durable when its useful service life (performance) is fairly comparable to the time required for related impacts on the environment to be absorbed by the ecosystem.

Using sustainability parameters to define durability is derived from existing precedents. For example, some 100 years later, the shed depicted above continues its service long after the trees, now replacing those cut down to construct it, have grown back to maturity. Given sufficient service life even materials like aluminum, which is high in embodied energy and environmental impacts, can have their eco-sins absolved.

The acceptance of sustainability criteria to derive durability parameters requires careful consideration on the part of the architect. The building must be viewed at varying levels of resolution, from the detail through to the whole artifact. Failure of a minor detail, such as the attachment of stone cladding to the structure, could undermine the durability of the façade. Similarly, an inflexible building which is not adaptive to evolving use could face demolition even though all of its components are durable and performing adequately. To achieve a level of durability which fully utilizes natural resources within sustainable thresholds, idiosyncratic notions of design must be reconciled with proven precedents and typologies. The timeless desire by humans for shelter, health and well being must be balanced with material chemistry, statistical models of environmental loads, and ecological carrying capacities. Innovation so constrained represents the challenge of sustainable architecture.

The next section deals with Externalities.

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