Enclosure Durability - Durbability Implications

Durbabiltity Implications
Introduction / Service LIfe / Economic Impacts/ Environmental Impacts

Introduction
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. Durability issues also affect quality of life when the aesthetic dimensions of the built environment remain deteriorated, as is so often the case in large urban centres. Living with decay is less than inspiring and generally promotes an indifferent or pessimistic attitude towards architecture and urban design.

Not all aspects of durability may be addressed through architectural intervention, but an examination of service life, economic and environmental impacts can provide valuable insights for architectural practice.

Service Life
In order to deal effectively with differential durability issues, it is important to examine the service life of components within the following context:

What is the acceptable amount of underutilized (wasted) and prematurely expended durability?

This is a difficult question to answer fully at this time, however, some insights may be gained by reviewing existing data. The service life of building components are reported in numerous publications, and vary significantly between countries, climatic regions, and among building types. The accompanying table lists excerpts of recent service life estimates for wall elements in Canadian high-rise residential buildings.

It is too often the case that modern buildings, with all of their supposed advantages in terms of material technology, fail to perform as well as their more traditional counterparts. The construction shown here from the University of Toronto campus is barely a teenager.

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

These estimates represent thresholds after which either repair/restoration, in the case of exterior walls, or replacement for the other elements is normally required. Walls exhibit the greatest variability in service life by almost a factor of two. The other elements exhibit relatively minor variability between types, particularly so for caulking. An interesting relationship may be noted between flashing and exterior walls where the durability of the flashing is not harmonized with three of the four wall types. Ideally, the flashing would remain serviceable until it was time to repair or restore the exterior walls.

This problem extends to many other building elements. The harmonization of durability, or rather the lack of it, has been identified in the area of building services for items such as piping. It has been advocated that the life cycle of building sub-systems be prudently selected so that multiples of the typically shorter service life of these elements fit wholly within the overall building life cycle (e.g., three 25-year sub-system life cycles within a 75-year building life cycle).
Common outcomes of differential durability include:

Superfluous upkeep - the staging of excessively numerous 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.

The question of whether or not the typical service life of building components is appropriate, or sustainable, also deserves consideration. Based on the Canadian data in the table, 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 as for many components the 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 incremental cost of harmonizing its durability only applies to the material quality, assuming manufacturing and installation are price neutral.

Harmonized durability and “just in time” facilities management represent ideal constructs. Acceptable margins for underutilized and prematurely expended durability clearly require further study, but a reasonable target should observe economic and practical realities. Damage associated with a leaky roof may far outweigh premature replacement, but few owners would tolerate replacement midway through the predicted service life of building components.

Despite the international development of durability standards for buildings, and supporting programs of collaborative research, a major problem encountered when designing for durability has been identified

The impacts associated with the difficult application of durability standards are significant and affect both the economy and the environment.

This building in Old Montreal is receiving a well-deserved roof retrofit after providing over a century of service. The cost of staging alone should reinforce the value of durability.

The building technology for barns is an excellent example of harmonized durability. The wooden superstructures do not require more upkeep than the stone foundations, and generally tend to fail together. Where this is not the case, the materials are highly reusable. How will our modern metal barns look after a century?

Economic Impacts
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 advance was the renovations component, which rose 5.9% compared with 1999. The cumulative 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, ranged between 24% and 73% of these annual expenditures, depending on how the data are interpreted. Hence, it is reasonable to assume that differential durability, in its larger sense, is not insignificant when compared to operating energy in housing, which accounts for 15% of Canada’s annual greenhouse gas emissions.

Renovation puts as much stress on the environment as it does on this do-it-yourselfer’s rear suspension. The home improvement industry continues to fuel ever shortening renovation cycles that urge consumers to scrap perfectly serviceable finishes and fixtures to keep up with style trends. Durable, universal design concepts are inimical to Big Box commercial interests.

Environmental Impacts
Second, the sustainability implications of differential durability are considered. Using durability as an indicator of sustainability is unavoidable because when other measures are employed, these typically attempt to quantify resource depletion and/or environmental degradation over the service life of the building. Interesting relationships have emerged when durability is considered in conjunction with other measures. For example, the sustainability of high embodied energy building components with a relatively long service life may be better than lower embodied energy alternatives with a shorter service life, especially if the former provide superior operating energy performance (e.g., thermal insulation, high performance glazing, etc.). Embodied energy and operating energy performance being equal, 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 above 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 standards would fall far below any realistic threshold of sustainable yield.

Durability precedent based on sustainable yield of natural resources. [Cedar shake-clad shed, Fruitvale BC, circa 1900.]

Category of Deisng Service Life for Buildings.
[Source: CSA S478-95 (R2001) Guideline on Durability in Buildings]

The embodied energy implications of differential durability provide another perspective on sustainability. The figure below is based on the work of Cole and Kernan, 1996. Their research included a comparison of initial embodied energy content to recurring embodied energy content (maintenance, repair and replacement), for a wood-structure building over a 100-year life cycle. Periods of 25 years were selected to quantify the recurring embodied energy associated with 6 major components of a building. The sustainability implications of building durability are significant notwithstanding the exclusion of underutilized and prematurely expended durability (embodied energy) in their analyses.

Comparison of Initial to Recurring Embodied Energy for Wood Structure Building Over a 100-Year Life Cycle. [Raymond J. Cole, Paul C. Kernan, Life-cycle energy use in office buildings. Building and Environment, Vol. 33, No. 4, pp. 307-317, 1996.]

First, to the credit of civil engineers, the structures of buildings normally do not expend recurring embodied energy, lasting the life of the building. By year 25, however, a typical office building will see an increase of almost 57% of its initial embodied energy due mostly to envelope, finishes and services. By year 50, recurring embodied energy will represent about 144% of the initial embodied energy, and it was projected that by year 100, this proportion would rise to almost 325%.

This relationship is a direct result of differential durability, where the service lives of the six major components comprising the building differ dramatically. Although difficult to quantify from available data, the significance of underutilized and prematurely expended durability cannot be ignored. The current preoccupation with lower first costs in buildings, coupled to misguided facilities management planning, reveals the widespread disregard for sustainability when viewed from a building life cycle perspective.

The revolution in building automated control systems has rendered many existing systems functionally obsolete. The typical life cycle relationship between building envelope and services favours investments in prudent architectural design.

Another reason that the sustainability implications of recurring embodied energy consumption are not given the serious attention they merit is due to dramatically higher levels of non-renewable operating consumption in contemporary buildings. The following figure depicts the relationship between initial, recurring and operating energy for a typical office building. The recurring embodied energy accounts for 8.3% of the total life cycle energy consumed by the building.

Compared to building fabric, building services exhibit dramatically differential durability. Mechanical services may easily be replaced 2 or 3 times within the useful service life of the building fabric.


Components of Energy Use During 50-Year Life Cycle of Typical Office Building with Underground Parking, Averaged Over Wood, Steel and Concrete Structures in Vancouver and Toronto [Cole and Kernan, 1996].

Recent analyses for single-unit housing in Sweden indicate that over a 50-year life cycle study period, operating energy accounts for 83%-85% of the building life cycle energy consumption, embodied energy represents between 11%-12%, and recurring embodied energy for maintenance and renovation ranged between 4%-5%. This compares favourably with the Canadian estimates for small office buildings.

Most building, however, tend to serve useful lives beyond 50 years and this is commonly identified in the current literature as a limitation in life cycle analyses. Potentially enormous recurring embodied energy expenditures can take place as buildings age beyond the 50-year horizon, especially when retrofit activities address both deterioration and obsolescence.

Further, as modern building technology improves upon the energy efficiency of buildings, and passive environmental control systems, and/or benign sources of renewable energy, increasingly displace non-renewable energy sources for the operation of buildings, the initial and recurring embodied energy content becomes more significant in the life cycle of buildings. Typically, recurring embodied energy surpasses the initial embodied energy of buildings, and as we approach “zero non-renewable energy” buildings, it is reasonable to expect that careful consideration of differential durability will grow in future importance.

The next section looks at Designing for Durability in terms of building materials, components and whole building systems.