Introduction
Durability, traditionally referred to as firmness, remains a cornerstone of sustainable architecture. It must now be reinterpreted within the context of the “total building performance” concept, which recognizes the environmental, economic, technical and social dimensions of buildings as cultural resources rather than real estate commodities.

In order to effectively apply this holistic concept, means of reconciling qualitative and quantitative data with incommensurable parameters must be incorporated into the architectural design process. Recent research has suggested that tools with this sort of sophistication are yet to be developed. It is also unclear how training on the use of these tools could be delivered to design professionals within current disciplinary structures. However, with respect to durability issues, the challenges associated with implementing the total building performance concept have been identified as:

Preparation of comprehensive guides on the performances of various building details;

Development of tools for durability analysis and life expectancy prediction of building elements and major building parts; and

Follow-up and monitoring of projects built under the performance concept for more practical and reliable feedback into the process.

The importance of addressing the durability challenge can be appreciated by considering the four key parameters governing total building performance: 1) user satisfaction; 2) organizational flexibility; 3) technological adaptability; and 4) environmental and energy effectiveness. Differential durability, when it is understood to include the service life of materials and assemblies, and the obsolescence of whole building systems, plays a significant role in the total building performance concept. It directly impacts three of the four key performance parameters, and may in some cases influence user satisfaction when differential durability affects aesthetics or ergonomics.

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Components
Enclosure components and assemblies must satisfy a number of requirements. Their deterioration is due to environmental loading and failure of the building envelope to address these physical phenomena. This occurs when environmental loads are overlooked, misunderstood or under-estimated in design. Some of these environmental loads include moisture, temperature, air movement, solar radiation, seismic forces, chemical exposure and a host of other agents that stress the integrity of the enclosure. [Refer to Principles of Enclosure and Enclosure Design Strategies for more detailed information]

Component durability may be determined through investigation of actual field performance, accelerated testing, and/or compliance with predetermined guidelines. Actual service life depends on the materials used and the environment to which they are exposed, as well as the suitability of the design and construction process. Proper design should take into consideration all factors, the role(s) of the building component, its interaction with the structure and whole building system, and its service environment. It is especially important to consider the implications of premature failure, the accessibility of the component for inspection, maintenance, repair or replacement, and the consequences of premature failure. While the architect may not be able to convince the owner to accept the recommended durability design strategy, documenting risks and consequences can serve to reduce liability when the chickens of first-cost myopia come home to roost.

Foundations represent inaccessible assemblies that require redundant control measures to assure acceptable performance. Designers must also consider weather conditions during construction and specify methods that are forgiving of less than perfect workmanship.	Corrosion of steel studs in exterior walls continues to be a problem waiting to be uncovered in many existing buildings. New design methods and materials have significantly improved the durability of steel stud walls.

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Systems Integration
After carefully selecting materials, and fashioning these into appropriate components, the real challenge remaining is to integrate these into a whole building system. Looking at the problem from this perspective, it is remarkable that most buildings tend to endure at all. The perpetual quest for progress in Western civilization demands that systems integration takes into account differential durability and functional obsolescence in order to avoid wasted embodied energy associated with buildings that are prematurely repaired, retrofit or demolished. It is not enough that the building functions technically, but it must also endure socially and culturally without stressing the environment (or the owner’s purse).

Several aspects influence the serviceability and thus the service life of buildings. The focus has traditionally been on technical aspects in service life predictions, even though technical aspects are of less importance in the overall assessment for many building types. For office blocks, commercial buildings, hospitals and other large and complex buildings, functional and economic aspects are often far more important than the technical factors. Based on this, there is a need for studying the influence of technical, functional and economic aspects on the service life of buildings, building components and materials. Further, there is need for developing prediction models for service life of buildings that may be used as support tools during the planning and management of buildings.

Differential durability affords a different perspective on the sustainability of buildings because it takes into account both physical deterioration and obsolescence. These two aspects of differential durability are not yet fully appreciated or understood in conventional approaches to durability design and assessment.

When environmental criteria are applied to physical deterioration, the minimum performance requirements for materials and components, or assemblies, differ from current normative standards. They become based on the time it takes for the environmental impacts associated with extraction, processing, transportation and installation (initial embodied energy), as well as the recurring embodied energy between replacement cycles of building elements, to be absorbed by the ecosystem. This implies more durable building elements with better harmonized durability incorporated into flexible and adaptive architectural design.

In order to advance differential durability research and practice, numerous barriers and opportunities have been identified in the recent literature. It must be recognized that a concerted research effort undertaken across a number of disciplines will be required to effectively address differential durability issues.

The acceptance of sustainability criteria to derive durability parameters will require 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, and beyond to its community interactions.

The next section looks at Related Resources supporting enclosure durability.

Buildings are abandoned not because they are not durable, but because their supporting community is not sustainable. This may someday be the fate of many of our suburban bedroom communities.

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