SYSTEMIC AND COMPONENT LONGEVITY

“Facade-related design decisions often come with tradeoffs,” comments Isabelle Hens, environmental designer at the San Francisco office of environmental design consultant Atelier 10. “The window-to-wall ratio will impact embodied carbon, since the glazing assembly will have a different embodied carbon than the opaque assembly; operational carbon and thermal comfort, since it will alter the solar heat gains; interior occupant experience, since the window-to-wall ratio determines how much daylight and direct sun enters the space; and exterior architectural expression, by changing the facade articulation.” Decisions about each of these factors are best taken holistically, she adds, rather than assessing components in isolation.

The lifespan of a complete system comprises the lifespans of its parts, which frequently differ. Vishwadeep Deo, facade consultant and vice president at Thornton Tomasetti, points out that once a curtain-wall system is installed, its enclosure infrastructure is “derived from multiple different components and pieces. Individually, those component pieces themselves have a very different lifespan; some could go away within 20 [to] 25 years and need to be replaced, while some of the others with metal in the enclosure could last up to 75 and beyond.”

Expectations for the durability of aluminum, glass, and other materials depend on multiple variables, Deo notes, including location, exposure to assorted destructive forces (weather, salinity, ultraviolet light, and pollutants), and maintenance cycles. A building in a marine environment will face high risks of corrosion, as will one exposed to acid rain. A system that includes sealants will need periodic inspections and replacements. A curtain wall system’s design can add to these variables, he continues; even if an owner performs regular maintenance and preserves the overall integrity of a facade, sections of it may be inaccessible and may fall into neglect.

Curtain-wall technology has progressed considerably over the decades, steadily improving in thermal performance while generating challenges in serviceability and durability. Brian McFarland, AIA, principal at CetraRuddy, traces the evolution of facade technology from early examples like SOM’s Lever House, the second curtain-wall building in New York City (after the United Nations Secretariat Building), to today’s unitized curtain walls and insulated glass units (IGUs). After “stick-built curtain wall, which was aluminum extrusions that you then applied glass to, and then you put a pressure plate on the outside of the glass,” came unitized curtain walls in four- to five-foot units going floor to floor, a less continuous skin than the previous generation’s “multi-floor continuous verticals.”

Further improvements included thermal breaks with nonconducting isolators at the pressure plate, then structurally glazed curtain walls with “no metal on the outside of the wall, so even though it’s not the greatest insulator in the world, you do have the IGU outside of the metal to create some thermal break between the exterior environment and the metal.” The unitized curtain wall improves speed of erection and reduces labor costs; it is “one step better than what we used to call thermally broken, but it does also have its issues,” McFarland continues, including thermal bridging from aluminum framing behind the glass and condensation from insulation on spandrel panels with a galvanized back pan for protection during shipping.

IGUs came to dominate curtain walls around the 1980s, replacing the early single glazing of the cheap-energy era predating the 1970s petroleum crisis, improving on early curtain walls’ poor insulation with a modular assembly: a frame, double (later triple or quadruple) glazing, spacers, hermetic sealants, thermal breaks, and optional components including interior thin-film coatings, fritting, and argon, krypton, or a vacuum to reduce heat conductivity in the cavity between the panes. Interior condensation is the bugbear of IGUs, since seals are the most common site of failure. Gaskets outperform wet seals, McFarland says, and unitized curtain walls require less maintenance than brick cavity walls, which have multiple intersections between components and “wet seals that have to be maintained continuously over the life of the building; [brick walls] may ultimately last longer, they may ultimately stand longer, but they require more recurring maintenance than a well-engineered glass curtain wall does.” Leaks lead to fogging, mold, and oxidation of aluminum and metal oxide coatings, compromising aesthetics and reducing service quality, sometimes leading to the replacement of an entire facade rather than changing out a single IGU, which is often challenging because of inaccessibility.

McFarland identifies several other reasons a modern curtain wall might deteriorate. “One is if you have any finished metal on the outside of the wall, and there are three different finishes you usually have. In the States, it’s usually a PVDF [polyvinylidene fluoride] coating; outside the States, it’s usually a powder coating.” The third finish, anodizing, used to be more common, but has waned in the U.S. as environmental regulations have increased, though is still more widely available in Europe, he notes. Finishing specialists generally estimate a life expectancy of 20 years for powder coating, 25 for PVDF, and 30 for anodizing. “We will have seen walls that have lasted longer than that,” McFarland says, and others that have not.

Recalling “one particularly bad example,” a midtown Manhattan building less than 30 years old with its finish visibly peeling off, McFarland cites inexperienced fabricators and installers as sources of preventable problems. The curtain-wall industry has expanded globally in recent years, he notes; many curtain walls “actually get pre-engineered systems from known companies, but there are still the people that are cutting the metal, fabricating it, and putting the glass on.” Architects’ specifications used to include language requiring use of contractors with experience in the field for a certain period, he says, often five years, but this is no longer the case; lab tests may be the chief form of quality control. With unfamiliar low bidders on a project, along with increasing sizes of IGUs beyond the 5-foot-by-12.5-foot modules that were customary on older commercial buildings, resulting in larger and heavier glass that adds stress to components, unwelcome surprises can occur. “When we get out in the field, we see things that are other than what was on shop drawings or engineering. And that’s when you start to worry.... Standardization, by default, should last longer, but it’s not the way the world is going. We architects are so spoiled now. There’s this temptation to have greater freedom with design, but then you have to be more diligent in your review of the engineering.”

Having recently worked on his firm’s first two Passivhaus projects, McFarland notes surprising differences in curtain-wall components’ contributions to thermal performance. “Triple glazing was really a very incremental improvement,” he reports; “other things like the better coatings, frits, photovoltaics (PVs), things that physically impede solar heat gain, are much more valuable than whether it be the argon in the cavity or the third piece of glass. You do get an incremental improvement on center-of-glass U value with triple glazing or argon, but if you have a frit or a PV, then you are actually impeding light energy from entering the building.”

The transition from double to triple glazing in IGUs, Deo notes, may also have downsides that offset triple glazing’s additional contribution to U value. “The moment we increase the number of interfaces, we are increasing the potential of failure at those interfaces, because in a curtain-wall system, failures happen where there is an interface between two different materials. So from a longevity standpoint, I think we are not increasing or decreasing the lifespan of the insulated glazing to go from double to triple, but we are increasing a little bit of risk.”

“In the almost 40 years I’ve been doing this, there has been a continuous evolution of curtain-wall technology,” McFarland says, crediting the sustainability movement that arose in the 1990s and now informs many cities’ building codes with driving steady improvements in components. “There are certain things they haven’t conquered yet, like completely isolating the aluminum framing, but I do think that things like warm-edge spacers and insulated panels on the inside of opaque panels will dramatically improve that.” A curtain wall by definition is lighter than a masonry wall; “it doesn’t overcome curtain walls’ sins, but there is a tradeoff in the reduced embodied carbon that goes into some other parts of the building: the primary building structure can be lighter.”

McFarland maintains optimism about progress in facade systems, contending that “actually, higher-performing curtain walls have a longer effective lifespan” provided IGUs are sized appropriately to avoid excess stress on seals and wall structures are isolated to reduce risks of localized condensation in fasteners for thermal breaks. Still, gradual component degradation strikes him as unavoidable. “There are curtain walls out there today that could last 75 years, there’s no doubt in my mind. But at 75 years, setting aside replacing IGUs, I can virtually guarantee you that the finish on the metal is going to be worn; I can virtually guarantee you that the gaskets and seals and sweeps are going to not be performing as well.” The fallibility of components implies not only that a maintenance-free curtain wall is a phantasm, but that a wise strategy to prolong a system’s life is to design it in ways that make it easier to replace its parts.

That modular approach, on the other hand, runs counter to a trend McFarland and others observe in today’s facade systems: “More and more, our curtain walls are bespoke from project to project. In some cases, they start from a basis of engineering. There are two German companies, Schüco and Wicona, who essentially sell their pre-engineered systems to fabricators in Italy or Germany or wherever. But on a lot of our projects we wind up slightly tweaking the details. So, in terms of service life, it goes back to being really diligent about what you’re asking for and the engineering that goes into it, because at the end of the day, most buildings are one-off. There, you can’t simply rely on a presumed warranty; the upfront understanding of what you’re asking for that may vary from the norm is something to pay attention to.” Product improvements, particularly in glazing, encourage architects to prefer original designs over standardization. “The genie’s out of the lamp now, and it’s going to be hard for us to humble ourselves and go back to saying that every curtain-wall grid is five feet. Because the capabilities just keep getting better and better. We can get the crispest, sharpest, flattest glass, and it’s much bigger than the glass ever was before. So the capabilities just keep encouraging us to stretch ourselves more.”