Enclosure Durability

The proper application of enclosure design strategies and tools, as discussed in previous editions of Architectural Science Forum, involves numerous performance objectives. Durability remains among these objectives but seldom takes precedence over first cost. Enclosure durability is often compromised by thinking that is oblivious to life cycle realities and the pivotal role of durability in sustainable building system performance.

Durability–The ability of a building, its parts, components and materials to resist the action of degrading agents over a period of time.

Service Life–The period of time during which all essential performance characteristics of a properly maintained item (product, component, assembly or construction) in service exceeds the minimum acceptable values.

Design Life–The service life that the designer intends an item to achieve when subject to the expected service conditions and maintained according to a prescribed maintenance plan.

T. Nireki, Construction and Building Materials, Vol.10, No. 5, pp. 403-406, 1996.

Durability is Only Skin Deep

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 that of 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.

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.

Service Life of Building Components

In order to effectively address durability issues, it is important to examine the service life of components. The service life estimates of building components are reported in numerous publications, and vary significantly between countries, climatic regions, and across building types. The table shown here lists recent service life estimates for wall elements in Canadian high-rise residential buildings. Based on this data most major building elements, except for the structure, tend not to survive much longer than 20 to 30 years. The incremental cost of providing greater durability should be closely considered within the building life cycle. For many components marginal improvements are highly cost effective. Consider metallic flashing, a vital element where about a 50% increase in service life would better harmonize its durability with exterior wall claddings.

The absolute durability of enclosure components may be addressed through material selection; however, the relative or differential durability between interdependent components requires careful consideration during the design process.

Differential Durability

Differential durability is a term used to describe how the useful service life of building components, such as structure, envelope, finishes and services, differs–both between components and within the materials, assemblies and systems comprising the components. The term may also be used to describe the whole building system by comparing between the service life of the building and its functional obsolescence.

A review of international research generally indicates that with the exception of structural elements, all other components require varying levels of maintenance, repair and replacement during the life cycle of the building. The extent and intensity of these recurring embodied energy demands vary significantly, depending on how appropriately the durability of materials, assemblies and systems are harmonized, and how accessible they are for periodic maintenance, repair and replacement.

Differential durability causes prematurely expended durability (embodied energy) when a higher durability component forming part of an assembly is replaced prior to its normal service life. This embodied energy is then compounded with the underutilized durability associated with the discarded component.

Common outcomes of differential durability include:

Superfluous upkeep–the staging of excessive maintenance, repair and replacement activities due to the differential service life of building components;

Deferral of upkeep–the staging of upkeep activities is costly and disruptive when activity cycles are not harmonized due to asynchronous differential durability, and when fewer than the required or recommended cycles are observed, accelerated deterioration may occur to neglected elements;

Prematurely expended upkeep–where staging is expensive, such as in the case of exterior elements on high-rise buildings, serviceable elements may be replaced at the same time as unserviceable elements to minimize staging expenses and disruptions, leading to prematurely expended durability.

“From a general point of view when the capacity of a property to perform the function for which it was intended declines, it becomes functionally obsolete. Functional obsolescence may originate from several sources following changes in the market, in equipment design or process or because of poor initial design.”

Evaluation of the functional obsolescence of building services in European office buildings. Dominique Allehaux, Philippe Tessier, Energy and Buildings 34, pp. 127-133, 2002.

Functional 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 questionable.

It is important to appreciate the difficulty inherent in reconciling these two aspects of differential durability–physical deterioration and functional obsolescence. Even when these are balanced, factors such as “locational obsolescence” owing to shifting market demand and land value patterns may result in enormous expenditures of emb
odied energy. The incentive to address architectural aspects of differential durability is strengthened when their implications are better understood.

Implications of Differential Durability

Differential durability causes significant economic impacts, and can also affect sustainability in terms of environmental degradation, resource depletion, greenhouse gas emissions, and reduction in bio-diversity–the four commonly recognized environmental impacts of buildings.

First, an economic perspective on differential durability. The total value of investment in the Canadian housing sector was $42.7 billion in 2000, up 3.9% from 1999. The biggest contributor to the increase was the renovations component, which rose 5.9% compared with 1999. The value of residential repairs and renovations for the year 2000 was $18.2 billion. The total number of housing units in Canada was 11,908,049 in 2000. This represents an average expenditure of a little over $1,500 per housing unit, roughly equivalent to the annual purchased household energy. Durability, measured both as physical deterioration and functional obsolescence, accounted for between 24% and 73% of these annual expenditures. Hence, it may be inferred that differential durability, in its larger sense, is roughly comparable to operating energy costs in housing, which accounts for 15% of Canada’s annual greenhouse gas emissions.

Second, the sustainability implications of differential durability are considered. Based on recurring embodied energy measures, the relationship between durability and sustainability is linear–the more durable, the more sustainable. When sustainability parameters are properly considered, current standards for building durability become questionable. For example, some 100 years later, the shed depicted at right remains serviceable long after the trees, now replacing those cut down to construct it, have grown back to maturity. From a sustainability perspective, a material, component or system can only be considered durable when its service life is fairly comparable to the time required for related impacts on the environment to be absorbed by the ecosystem.

The service life of a shed suggested by current durability guidelines, such as CSA S478-95 Guideline on Durability in Buildings, would fall far below any realistic threshold of sustainable resource yield.

Designing for Durability

Architects play an important role in determining the durability of building enclosures because they remain largely responsible for designing the skins of buildings. Moreover, architects continue to exert the most design influence regarding the functional obsolescence of buildings.

Buildings are cultural resources that should not be treated as disposable commodities. Pressures from building owners to minimize first costs are difficult to resist when so little authoritative evidence is available to properly balance first costs with life cycle costs and environmental impacts. Society has not yet reached a consensus on regulating durability issues with codes and standards. This leaves architects with the formidable challenge of ignoring image and style in favour of functional elegance that is socially and environmentally sustainable. Unless these latter precedents are widely acknowledged and celebrated by architecture and its allied disciplines, only beauty, but not durability, will remain skin deep.

Coming in November 2002

The next edition of Architectural Science Forum will look at Enclosure Detailing with the intent of more effectively closing the gap between the construed and the constructed to better assure durable, sustainable building enclosures within current practice.

Ted Kesik is associate professor in the Faculty of Architecture, Landscape and Design at the University of Toronto. The unabridged version of this article is available on the Web at www.canadianarchitect.com under Architectural Science Forum. This forum is intended to facilitate the exchange of knowledge and information pertaining to architectural science, among practitioners, researchers, academics and students of architecture and its allied disciplines. For further information on submissions to Architectural Science Forum, contact the editors.

Average Range

Building ElementTypeMin.Max.Avg.

Exterior WallsPrecast Concrete394441.5

Brick Veneer323734.5

Curtain Wall323835

Stucco202221

Avg.30.835.333

WindowsMetal Casement222523.5

Metal Double-Hung212322

Vinyl Casement182019

Vinyl Double-Hung161917.5

Metal Sliding212422.5

Avg.19.622.220.9

FlashingSheet Metal222523.5

Non-Metallic161917.5

Avg.192220.5

CaulkingAll Types101110.5

Typical service life of high-rise residential wall elements. [Service life of multi-unit residential building elements and equipment: final report. Prepared by IBI Group for Canada Mortgage and Housing Corporation, May 2000.]

Average Range
Building ElementTypeMin.Max.Avg.
Exterior WallsPrecast Concrete394441.5
Brick Veneer323734.5
Curtain Wall323835
Stucco202221
Avg.30.835.333
WindowsMetal Casement222523.5
Metal Double-Hung212322
Vinyl Casement182019
Vinyl Double-Hung161917.5
Metal Sliding212422.5
Avg.19.622.220.9
FlashingSheet Metal222523.5
Non-Metallic161917.5
Avg.192220.5
CaulkingAll Types101110.5

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