Durability: A Key Component of Green Building
What is it that enables some structures to survive a hundred years or longer, while others don't even last a few decades? This article makes the case for durability and offers some strategies for achieving it. By Alex Wilson
The house I live in is 220 years old, having survived storms, fires, and droughts and having experienced the emergence of petroleum, electricity, automobiles, plastics, and nuclear power. It has been repaired, enlarged (around 1800), plumbed, wired, gutted, remodeled, insulated, and restored by generations of inhabitants -- adapting to meet changing technologies, needs, and fashions. Most recently, this house, which began life without electricity, central heat, or plumbing, was outfitted with wireless Internet access. Through its adaptation, the hewn beech frame, spruce rafters, wide sheathing boards, two chimneys, and dry-stone foundation have all survived, attesting to the durability of these systems and the house they collectively form. It is reasonable to expect that the house will last another hundred years -- surviving past the end of the petroleum age.
What is it about this house that has enabled it to last so long? How have the monasteries in France and the ancient temples in Japan survived as long as 1,000 years, and how have some buildings in other parts of the world survived millennia longer? And conversely, why are many of the tract homes and shopping malls going up today unlikely to make it to 50 years?
This article examines building durability -- what it means, why it's important, and some strategies for achieving it. While the article relates to all building types, many of the examples are drawn from light-frame construction, where durability is a particularly vexing issue. We take a look at durability issues relating both to building systems and to their component products and materials.
Why Durability Is Important
The environmental (and economic) benefits of durability would seem fairly obvious. A durable building -- one that lasts a long time -- provides a long period of time to amortize the environmental and economic costs that were incurred in building it. Notes Peter Yost, a building science expert with 3D Building Solutions, LLC, "if you double the life of a building, you halve the environmental impacts [of its construction]." The same argument goes for the products and materials going into those buildings. Durable products and materials will not need to be replaced or repaired as frequently, so the raw materials, energy, and environmental impacts invested in them can be spread out over more time.
A house or commercial office building designed and built to last 100 years offers significant resource advantages over a comparable building that will last just 50 years (assuming similar energy performance and indoor environmental quality). From a life-cycle standpoint, the longer-lasting building's higher economic and environmental costs can usually be justified by its durability. Investing in increased durability shouldn't be carried too far however. Buildings are often modified or replaced for reasons that have nothing to do with their structural integrity, so part of good durability planning involves picking a reasonable service-life target for the building and its constituent assemblies.
Durability often goes hand-in-hand with low maintenance. Usually -- but not always -- a durable material is also a low-maintenance material. In the GreenSpec Directory, durability and low maintenance are considered together as a criterion for product selection. Exterior siding that has to be painted every five years or vinyl composition tile (VCT) that has to be stripped and re-waxed regularly will not be durable unless it is maintained properly -- and that maintenance becomes part of the material's life-cycle environmental and economic burdens. By contrast, a pultruded fiberglass window frame and sash is considered highly durable, in part, because it will hold up well even with no maintenance.
Focusing on Durability in Buildings
Given the significance of durability, it is remarkable that this hasn't long been a top consideration of buyers, sellers, lenders, insurers, and others in the supply chain of buildings. Until the recent upsurge in lawsuits over mold, remarkably little attention was paid to building durability outside of a relatively small cadre of building science experts.
The lack of interest in durability has frustrated many, including Joe Lstiburek, Ph.D., president of Building Science Corporation, based in Westford, Mass. But things are looking brighter. Lstiburek sees the green building movement as a leading reason for the growing focus on durability. "I went from thinking the whole idea of green building was a boutique issue," he said, "to seeing how it is the leading driver of change. Durability and energy efficiency are the cornerstones of sustainability."
Durability is integral to the new LEED for Homes Rating System from the U.S. Green Building Council, which was released in a pilot version in August 2005. The rating system requires the preparation of a detailed durability plan. Implementation of the plan with third-party inspection can earn the project up to five points, depending on the region -- one point is available in regions with less than 20 inches (51 cm) of precipitation per year, three points in regions with 20-40 inches (51-102 cm), and five points in regions with more than 40 inches (102 cm). LEED for Homes also provides a point for humidity control in homes and mandates local exhaust of the most significant moisture sources (kitchens and bathrooms), with extra points for automated controls on exhaust fans and for performing third-party testing of the airflow rate from these systems.
While the LEED for New Construction (LEED-NC) Rating System, which applies to commercial buildings, does not address durability directly, the Canadian adaptation (LEED Canada-NC 1.0) addresses it explicitly as Credit 8 under Materials and Resources. One point is earned for developing and implementing a building durability plan in accordance with Canadian Standards Association (CSA) Guideline on Durability in Building.
The durability point in LEED Canada-NC is meeting some resistance, especially among insurance companies, according to Wayne Trusty, president of the Athena Sustainable Materials Institute, which is heavily focused on durability issues. "Insurers are advising developers against going after the credit," says Trusty, "because there's an implied warranty." In addition, the cost of producing the required durability plan is not insignificant. "To fully comply with the CSA standard, you really have to do a lot of paperwork," Trusty said. He has heard that the cost of creating that plan has run about CA$6,000.
As for U.S. government programs addressing durability of buildings, the Department of Energy (DOE) has taken the lead, largely through its Building America program and applied research at several national laboratories. Lstiburek believes that the Department of Housing and Urban Development (HUD) really should be the leader with residential durability initiatives, but it has not played that role.
A number of home warranty programs in North America address durability, with the Environments for Living program currently most active in this area. Environments for Living is a program of Masco Services Group, the largest insulation contractor in the world. Launched in 2000 and designed with the support of leading building science experts, including Building Science Corporation and Advanced Energy Corporation of Raleigh, N.C., the program establishes requirements for tight construction, improved thermal systems, right-sized HVAC systems, fresh-air ventilation, balanced air pressure, internal moisture management, and combustion safety. Homes are tested for performance, and participating builders have the option of providing homeowners with a heating and cooling energy-use guarantee and -- at the highest levels of certification -- a comfort guarantee.
To date more than 70,000 homes have been constructed under the Environments for Living program, and six of the nation’s ten largest builders currently participate. A new level of certification added in 2005, Diamond Class, adds green building elements, including water efficiency, to the program. By addressing moisture management, ventilation, and air pressure in new houses, Environments for Living helps ensure long building life with minimal problems.
Programs that seek to promote durability, whether in the form of LEED credits or homeowner warrantees, use one of three approaches, according to Yost: The simplest approach is to prescribe specific measures, but those must be customized for the building type and region to be meaningful. The next level is to set performance-based goals, but these are notoriously hard to verify until the building has been around (or not) for decades. Finally, a program can focus on the process. A process-based approach offers the best leverage for improvements over a wide range of users but doesn’t guarantee good results for any specific project. Both LEED for Homes and LEED Canada rely heavily on process requirements, while Environments for Living is more prescriptive and performance-based.
Elements of Durability
The durability of buildings depends on relatively few specific factors that can be addressed through design and construction. These are described below, followed by a checklist of actions for improving durability.
To a significant extent, durability is an issue of water management. Lstiburek estimates that fully 80% of durability problems in buildings have to do with moisture. Indeed, the publication "Durability by Design," published by HUD’s Partnership for Advanced Technology in Housing (PATH), devotes more than three-quarters of its space to moisture issues.
Thermal stress can reduce durability by causing materials to expand and contract. For example, this can affect long-term window performance; certain frame materials, including vinyl and aluminum, expand and contract at a higher rate than glass, making windows leakier over time. High roof temperatures contribute to the degradation of roofing materials (see also Sunlight, below), and this was an important factor in the highly publicized premature degradation of flame-retardant-treated plywood roof sheathing some years ago. With metal roofing, thermal expansion and contraction may loosen fasteners over time.
Ultraviolet (UV) light degrades many materials, including most plastics, wood, fabric, and paint. Along with heat, this is a major cause of the degradation of roofing materials -- and the reason vegetated (green) roofs can prolong the life of a roof membrane. Plastics that are used outdoors, including vinyl siding, are typically treated with UV stabilizers, but in some cases these stabilizers themselves carry environmental burdens. The toxic metals lead and cadmium were often added to PVC in the past to provide UV stability; today, less-toxic stabilizers are generally used, though disposal of older vinyl products is still a significant source of lead and cadmium in municipal waste incinerators. UV light also degrades interior materials, though UV-blocking glazings can reduce damage and prolong the life of interior finishes and furnishings.
Various atmospheric pollutants, especially ozone and acid rain, can degrade building materials. Ozone is formed at ground level through a chemical reaction between volatile organic compounds (VOCs) and nitrous oxides (NOx) in the presence of sunlight. While ozone is beneficial in the stratosphere because it blocks high-energy UV light, it is a pollutant at ground level. Along with causing a host of health problems, such as asthma and other respiratory illnesses, ozone also damages materials. Many synthetic materials, including rubber, polyester, nylon, dyes, and certain paints, are susceptible to ozone damage. It can also damage cotton textiles.
Acid rain, resulting primarily from sulfur dioxide pollutants in the atmosphere, corrodes various materials and is especially a problem with limestone building façades. Hundreds of cathedrals in Europe that had endured very well for many centuries began deteriorating with the dawn of the Industrial Age due to acid rain from coal and other hydrocarbon combustion. Acid rain also increases corrosion of copper and lead roofing, contributing to both shorter lifespans of those materials and the leaching of toxic materials into the environment. The low-emissivity coatings in most energy-efficient windows block most of the UV radiation.
A handful of insect families are responsible for more than $2.5 billion in damages to U.S. buildings each year, according to the National Pest Management Association. Most costly of these insects are several dozen species of termites, mostly in the subterranean termite family. The Formosan termite, a species of subterranean termite that was accidentally introduced from China, is becoming the most damaging species in North America. Other insects that can damage wooden buildings include carpenter ants and powderpost beetles. A wide range of design practices, specialized barrier products, and insect-resistant materials, such as treated wood, are used to protect buildings from termites and other insects.
Some materials and building components simply wear out. It is reasonable to expect that some materials have shorter lifespans than others. The challenge is to understand this and plan accordingly. In some cases, it makes sense to spend more to buy materials and products that are more durable than standard products. In other situations, it makes more sense to accept that a particular material has a relatively short lifespan and design the building assembly so that the shorter-life material can easily be replaced. What is illogical, according to Lstiburek, is to spend more to install a long-life wall cladding material, such as brick (which should last more than 50 years), over a material like plastic housewrap that is designed to last only 25 years. “It makes no sense,” says Lstiburek, yet this is common in many building assemblies.
Some buildings, even as designed, function poorly. While popular in the 1960s and ’70s, geodesic domes and yurts lost favor in part because homeowners found it difficult to position furniture along rounded or segmented walls. A building that is highly functional will be durable by virtue of the value its occupants place on it; that building is more likely to be restored or renovated as components wear out, while a less functional building is more likely to be replaced.
The function of a building also changes over time, and an inability to adapt to those changes can reduce its useful life, even if it is structurally sound. The recent interest in so-called “open building” systems is an attempt to keep components that need frequent updates, like wiring, separate from assemblies that should last a long time, like walls.
Residential architect Peter Pfeiffer, FAIA, worries that certain increasingly popular building materials, such as insulated concrete forms (ICFs) and structural insulated panels (SIPs), will not do well when it comes to adapting these homes to changing needs. “How easy will it be to remodel these homes in 25 years, much less rewire them,” asks Pfeiffer. “Durability means that you’ve got to be flexible,” he argues. “If it’s not flexible, it will become functionally obsolete.”
A recent study completed in 2004 by the Athena Institute for Forintek Canada Corp. adds credence to concerns about building longevity having more to do with functionality than durability. The Institute examined 227 commercial and residential buildings in St. Paul, Minnesota, that were demolished from 2000 to mid-2003, examining the age of the buildings, the structural materials, and the reasons for demolition. Only 31% of the buildings were torn down because of physical condition, while 57% were demolished because of area redevelopment or because the buildings weren’t suitable for anticipated use. Even though steel and concrete are considered highly durable materials, 63% of the structural concrete buildings and 80% of the structural steel buildings that were demolished during this period were 50 years old or younger, while just 14% of the wood-frame buildings were in that age category. The full report can be downloaded from the Athena Institute Web site.
The idea of “timeless architecture” has important bearing on durability. Attractive, aesthetically pleasing buildings are more likely to be maintained and repaired as components fail than are ugly, unloved buildings. In How Buildings Learn, author Stewart Brand describes two types of buildings that tend to last: “high-road” buildings that have that timeless quality and “low-road” buildings that are readily adapted and modified to suit changing needs.
John Abrams, of South Mountain Company on Martha’s Vineyard, Mass., makes timelessness a key component of his company’s house designs. “It’s the buildings that are loved -- and therefore cared for -- that endure,” says Abrams. “Make it beautiful and without trendy styling, and site it well, and you may ultimately be contributing to durability. In fact, that may be the most important thing you can do.”
Finally, in many areas, design for durability must also address natural disasters, including hurricanes, tornados, floods, fires, and earthquakes. (For certain building types, such as embassies and highrise towers, this category could be expanded to cover terrorism as well.) Design for survivability of natural disasters is highly dependent on the region. In the Gulf Coast region -- as we know all too well today -- as well as along much of the Atlantic coast, hurricanes and the resultant flooding are the prime concern. Along rivers and streams in virtually every state are areas where flooding is a risk during heavy rains -- whether accompanying a hurricane or not. Tornados are possible almost anywhere, but a large swath of the Central U.S., referred to as the Tornado Belt, is especially susceptible to the intense storm systems that spawn tornados. Fires are a major concern in many western states, particularly as development creeps out into the chaparral country that becomes tinder-dry most years in the summer and fall. And earthquakes are a concern not only along the well-publicized fault lines in California but throughout North America.
Some of our activities exacerbate natural disasters -- and the damage from natural disasters. On a macro scale, human contributions to global climate change are believed to be spawning more destructive hurricanes and rainfall events and contributing to droughts that are making western lands more susceptible to fire. On a more immediate level, population pressures and the desire to build close to scenic natural features are resulting in development along flood-prone rivers and streams and in places like the foothills of Southern California that are naturally managed by lightning-ignited fires.
Designing buildings to withstand natural disasters -- and protect occupants in the event of these disasters -- is a high priority but one that is fairly well addressed in our life-safety-based building codes. Other than moisture protection issues that relate to flood resistance, we do not address design for natural disaster resistance here.
Dealing with Durability
Addressing durability in buildings is an important priority, especially in any building we want to call green. For a long time, complacency was the rule in the building industry when it came to durability. Lstiburek believes that in the past we didn’t address durability because we didn’t need to. “The pain threshold for durability problems hasn’t been great enough for long enough,” he said. But that’s changing.
Lstiburek points to two reasons why we’re beginning to address durability. First, we’re experiencing more problems, according to Lstiburek, and we simply can’t ignore it. “We’re a reactive people,” he says. The legal profession is helping focus attention on this, especially moisture-induced mold problems. He calls lawyers “the single most effective mechanism of change.” Notes Lstiburek: “There’s nothing like a couple million dollars in liability settlements to focus attention on durability.” The second major driver of this new focus on durability, according to Lstiburek, is the green building movement. “It’s one of the great successes of green architecture,” he says. “People are now asking these questions.”
Creating durable buildings depends on the right knowledge and attention during design, specification, and installation, notes Yost. “If any one of these is lousy, the building fails,” he adds. Lstiburek, who has devoted his career to understanding and preventing building failures, puts most of the blame on designers. “I think it’s principally a design problem,” he said. More specifically, it’s a problem with the way architecture is taught. “We teach architects to be artists,” says Lstiburek, “we’re not teaching basic skill sets.” Lstiburek argues that the architect is supposed to be the generalist, yet the necessary training in technologies is not taught at architecture schools.
A checklist of the most important strategies for achieving durability in buildings is provided below. While not comprehensive, the list is a good starting point for addressing the critical issue of achieving durability. Note that this checklist addresses only the factors relating to the building and its materials -- not the issues of function and style, which can also be very important factors, as discussed above.
Design for durability has finally come out of the closet. It is being addressed in dozens of workshops across the country and by organizations such as the U.S. Green Building Council and the Energy and Environmental Building Association. Clearly, progress has been made, but we have a long way to go. Lstiburek believes that if everything is done right in the design and construction of wood-frame buildings, they should last a thousand years! He says the Norwegians and Swedes are the closest to this goal. And while the U.S. lags far behind, we appear to be moving in the right direction.
A fundamental need in the pursuit of more durable buildings is funding for research. The building industry in North America, and especially in the U.S., devotes just a tiny percentage of its revenues to research -- far less than in any other segment of industry. That needs to change. And it can change; there is growing awareness about the importance of understanding building and materials durability. This is one of the issues that the U.S. Green Building Council’s soon-to-be-formalized research committee hopes to take up in the years ahead.
This article has been reprinted courtesy of Environmental Building News. It first appeared in the November 2005 issue of that publication.