PLANNING FOR THE ADAPTIVE-REUSE OPTION

An increasingly prominent strategy in some locations is adaptive reuse, particularly commercial to residential, as needs for urban housing outweigh post-pandemic demand for office space. For adapting an existing building’s facade or long-range planning of a new one, such reprogrammings can be either a complication or an opportunity. “If the usage of the building does not change hands substantially,” Deo notes, “there’s a higher chance of reusing what you already have. But in my experience, I think there are more and more projects that are that forward-looking to at least understand, not from just from a capital-cost-investment standpoint but also from an embodied-carbon standpoint, how much of the existing building can be reused.”

Adaptive reuse implies that life-cycle assessments of energy and carbon metrics, applied to either the facade or the entire building, should be expanded to consider at least four phases: construction, operation (including maintenance), renovation/retrofitting/recladding (repeatable in some cases, depending on program changes and owners’ expectations), and end-of-life demolition or deconstruction (ideally including component reuse, at least in a down-cycling mode, since recycled architectural aluminum and glass usually go to less stressful uses because of purity questions).

“Working with an existing building, from the perspective of sustainability, you definitely are reducing the carbon footprint versus doing full demolition,” says Stanford Chan, senior principal at Socotec Group’s Vidaris and director of its Existing Buildings Division and Roofing and Waterproofing Division. “Also, if the conversion of that particular building can be done efficiently, then the schedule of reopening is much faster.” A facade system planned to anticipate improvements in high-performing technologies will ease transitions between the “Day One” design and a “Day Two” retrofit, Chan says. Reclads or overclads require structural foresight: “You first have to understand whether the existing curtain wall and building can accept the additional load. There are ways to come up with a system to replace the glass in a more efficient manner, as opposed to having to cut out the structural silicone from the scaffold 100 stories up in the air.”

Structural analysis, code compliance, and energy-usage studies are the pillars of performance in facade upgrades, Chan finds, though in practice, “the trigger for most building owners is not necessarily the improvement of performance, but to improve the value of the building: to make something old useful again, and also be able to compete with the new Class A office buildings.” Commercial-to-residential conversions must handle the transition from fixed glass (prevalent in commercial curtain walls) to operable windows that satisfy residential code requirements for natural light and air, since few mid-century buildings were designed with this scenario in mind.

Owners of Class B or C office buildings, often the prime candidates for conversion as occupancy rates languish in the 40 percent range, sometimes struggle with contemporary energy standards like New York’s Local Law 97; new office buildings, Chan suggests, may be planned with the flexibility to accommodate possible future conversion. “If you know you’re going to be upgrading the glass,” he says, “maybe you entertain an operable window within the opening, which can make it appealing if somebody does want to look at that building as a conversion” later in its life. For existing buildings, matching the programming to the building’s footprint is essential for a project to be financially feasible, since pre-conversion floorplans often have dimensions that do not work with existing glazing and may require removal of square footage or creation of new courtyards to meet residential code.

In his experience at Thornton Tomasetti and elsewhere, Deo has seen multiple projects where recladding an older, underperforming building, retaining its structural base, has breathed life, both aesthetically and commercially, into structures that might otherwise have been demolition candidates. As a caveat, he is also aware of a case (without identifying the parties involved) where material defects combined with subpar handling and inspection led to facade failure and eventual litigation. A specialized glass unit “relied heavily on a protected edge panel,” he recalls. “I think from a design standpoint, everything was great. It was during the execution, when those products and the panels were shipped, there was potentially some damage done to these edge tapes... and these glass panels happened to be super-sensitive to any moisture ingress.” Field personnel unfamiliar with the product failed to inspect the panels when they were mounted onto aluminum frames. The experience, he says, implies that when any type of new materials are used, risks are lower when contractors have extensive experience with shipping, handling, protection, installation, and maintenance of the component. “A curtain wall fails,” Deo summarizes, “when there is an oversight, whether in design or execution or quality control or fabrication.”

GLASS AS BOTH PROBLEM AND SOLUTION

Thinking of the principle that the service life of a building or a component assembly is “only as good as its weakest link,” Patterson recalls a startling realization during his research, one that he has passed along to audiences and firms, startling them in turn: that in buildings, unlike bottles, “glass is not recyclable, or at least not recycled. Everyone was a little shocked at that, and it was a dirty little secret of the glass industry, and it’s not an easy problem to solve.” Circular practices with glass components are more advanced in Europe, he acknowledges, and colleagues have described architectural glass recycling as a process the U.S. glass industry is ready to discuss; glass is durable enough to last centuries, provided it remains unbroken. From an embodied-carbon standpoint, Patterson of the Facade Tectonics Institute says, repeated recycling of glass is both desirable and feasible—“but when we make IGUs out of it, we compromise that service life down to a 20, 25-year time frame, and we make it not recyclable in the process, and then we call it high-performance glazing.”

When unprocessed float glass undergoes the secondary processes of coating, laminating, and insulation to become part of an IGU, Patterson and colleagues have argued, its durability is collapsed by “at least an order of magnitude” to 30 or 40 years. The gains in operational energy and carbon are offset by the embodied energy and carbon in the unrecyclable materials. Nickel sulfide and other contaminants in the glass mix can lead to spontaneous breakage in tempered glass and are undesirable for glass manufacturers, who routinely recycle cullet from in-house breakage but are reluctant to accept laminated or insulated materials with coatings and sealants for recycling. (Patterson is aware of a single exception, and not in a curtain-wall building: the Empire State Building’s window retrofit in 2009-2010, where “they took the IGUs out, set up a quasi-factory operation on one of the floors, stripped the glass off them, cleaned them rigorously, and made new IGUs out of them.”)

As an alternative, his group has proposed a “Millennium IGU” engineered for easy disassembly, either during maintenance or for end-of-life recycling (Patterson et al. 2014). Their Millennium IGU paper cites the Javits Center in New York, whose 1980s-vintage curtain wall was replaced in 2013; renovation was estimated to cost more than replacement, and the old IGUs ended up in a landfill, the fate of many subsequent IGUs as well. Yet “if I can take that thing apart,” he says, “I can clean up the inside of it, replace the seal, put it back in place, and it’s good to go for however much longer. The notion of the Millennium IGU is that at the end of 1,000 years, there may not be an original component in that assembly, but it’s had a continuous service life of 1,000 years.”

IGUs designed without maintenance and renovation in mind create a conflict between energy performance and thermal comfort (where these assemblies excel) and durability and recyclability; this conflict compromises the lifecycle carbon footprint of the IGU assembly. Patterson’s Millennium IGU concept strives to optimize the lifecycle carbon performance with no compromise to the thermal performance of the assembly. Its methods include using annealed, uncoated, unlaminated float glass and a removable cassette frame; placing low-emissivity film in a removable internal spacer cartridge instead of the surface of the glass lites; replacing wet-applied sealants with dry compression gaskets; using a vented IGU cavity to eliminate pressure differentials, stresses on seals, moisture buildup, and “pillowing” distortions in the exterior glass; and incorporating a removable filter cartridge into the assembly, allowing air passage while excluding moisture and particulates. Removable interior lites allow regular maintenance to be performed from within the building.

Though the Millennium IGU remains an aspirational concept, existing IGU systems continue evolving to improve performance and longevity. Chan and McFarland both cite warm-edge spacers, made of low-thermal-conductivity plastic or composite, as an improvement over stainless steel or aluminum spacers with polyisobutylene seals, which are vulnerable to compromise with condensation of the unit over time. Structural seals between two or three lites of glass, or between glass and the frame, make those components “contingent on the long-term performance of structural silicone,” Chan says. “We haven’t seen any case studies or evidence where there’s been any systemic failure of the structural silicone thus far,” though familiarity with its lifespan remains limited.

PROS AND CONS OF AN ALTERNATIVE

Timber is sometimes selected as mullion material in a stick-built curtain wall on account of its renewability, low thermal conductivity, and low embodied carbon. It may not last as long or insulate as well as aluminum, Deo notes, and may “increase the thickness of your walls, and that has additional implications for the project.” Its perviousness to vapor migration and tendency to expand, he adds, may make it unsuitable for humid environments, and its combustibility poses “some design limitations that you would need to work with, some fire-engineering consideration that you have to bring in to honor the fire separation between spaces.” Architects considering timber in a curtain-wall system, he says, should ask several questions: “What is the space usage on the inside? What is the expected movement that we will see in the timber frames? And how is everything connected back and composed and built?”

In academic research, Hens has found that the use-stage U-value of both timber-based and aluminum-based curtain walls “complies with ASHRAE 90.1 standards for New York and San Francisco... resulting in a negligible difference in energy use and operational emissions” (Hens 2021). When transportation impact is included in comparative calculations of the materials’ Global Warming Potential (GWP), however, transport-related GWP in long-distance sourcing scenarios mitigates the advantages of timber in production-stage GWP. The studies emphasize the importance of full life-cycle assessment, noting different advantages for the two materials according to different green metrics (Hens et al. 2022).

Raffles Boston Back Bay Hotel and Residences: Elegance in a Difficult Shadow

Photo: Ed Wonsek; courtesy of The Architectural Team

Figure 8. Raffles Boston viewed from west along Stuart Street, with Hancock Tower in partial view at left.

The first North American mixed-use building for the Singapore-based Raffles hotel chain relies on its advanced facade to turn the unusual constraints of its location into opportunities to create distinctive living and gathering spaces. The 35-story, 430,000-square-foot tower, hosting 147 guest rooms and 146 residential units, occupies a tight site (about 10,000 square feet, says project manager Alexander Donovan of The Architectural Team, a firm based in Chelsea, Mass.) just 65 feet away from Boston’s tallest building, the John Hancock Tower (a.k.a. 200 Clarendon), notorious in the 1970s for shedding glass panes due to thermal and wind stresses and engineering errors. Proximity to the Hancock (Fig. 8) creates a challenge more tangible than a memory of that civic and professional embarrassment: wind loads around the parallelogram-shaped Hancock are substantial, as prevailing winds strike it broadside and then become particularly strong on the Raffles’s northern facade.

Another neighboring structure, the 100-foot-tall University Club building, occupies a space on Stuart Street close enough that the Raffles obtained its air rights to cantilever itself some 30 feet over the UClub’s property line, establishing enough floorplate area for the units, corridors, and circulation spaces to be financially feasible, notes Donovan. The cantilevers use two sets of large trusses, one located at floors 4 and 5 and one at floors 17 and 18 supporting the upper segment of the building (Figs 9, 10). Though the ground-floor lobby area at Raffles is distinctive, its three-story sky lobby at the 17th floor, site of the second cantilever, is the signature space. “The real magic,” Donovan says, “is, as a hotel guest, when you take the express elevator and get off at level 17, you’re open to a view of the Boston skyline that not a lot of hotel lobbies have the opportunity to provide... accented by the floor-to-ceiling glass and a three-story stair atrium [looking] out onto the Charles, a nice view of Back Bay, and almost over to Cambridge.”

Image courtesy of The Architectural Team

Figure 9. Structural diagram of Raffles Boston shows trusses for cantilevers at floors 4-5 and 17-18.

Image courtesy of The Architectural Team

Figure 10. Structural diagram of level 4, Raffles Boston.

The Raffles has a unitized curtain-wall system, which Donovan calls “basically a curtain-wall version of a rain screen... you’re anticipating taking on a certain amount of water, and the system itself, the extrusions and the configuration of the seals, manages the water to get it outside rather than depending on one full seal.” It includes three seals; the outermost seal deflects about 85-90 percent of water between the glass and the beauty caps, and the innermost seal includes weeps and channels to remove condensation and bleed-through (Fig. 11). “We’re not based entirely on an exterior seal, which could degrade,” he says; the inner seals are not exposed to direct sunlight or the elements and are not prone to ultraviolet breakdown.

Image courtesy of The Architectural Team

Figure 11. Diagram of Raffles Boston’s curtain wall system with rain-screen features.

“Wind, obviously, was one of our largest constraints on the project,” Donovan continues, and the team put a scale model of the airfoil-shaped tower through a battery of wind-tunnel tests, examining wind effects on the overall base building structure, a cladding wind-load study for individual pressures on different areas, and a pedestrian comfort study. “Tests showed that as wind is coming across Stuart Street, which is the strip between us and Hancock, it hits the face of the building and wants to travel straight down to the sidewalk. So one of the requirements the city put on us, which is good practice anyway, [was] a fairly robust canopy over the entry facade of the building to make sure that pedestrians wouldn’t be exposed to undue wind conditions.” Another study aimed at long-term maintenance was a field reglazing test, where they removed a panel from the mockup, then reglazed it as though it were being executed in the field from the swing stage of the roof-mounted building maintenance unit (BMU), the cranelike system used for window washing and routine services including glass replacement.

Energy modeling indicated that the southern, eastern, and southwestern facades would need the most resistance to solar heat gain, achieved in part with increased low-E coatings, balanced at different areas to maintain a homogenous appearance. Donovan attributes the tower’s wind and energy management, along with certain amenities, in part to its tapered form (with the narrowest dimension facing the Hancock) and in part to how its programming follows its orientation, which he describes in nautical terms. Outdoor spaces used by residents and guests, including a terrace connected with the Long Bar on floor 17, are “biased towards the south of the building, which is really in the lee of the building.... Even though you’ve got a 28-degree day, because you’re in the lee of the building, we’ve got the curtain wall wrapped around the corner, so there’s not a lot of wind in that area, and you get the benefit of the morning sun. That’s a really pleasant space, and we’ve duplicated that on other floors: the level 21 residential amenity space has a similar balcony, it gets the morning sun, and there’s no wind.”

Donovan acknowledges that replacing curtain-wall panels would be arduous, since replacement would have to occur in the reverse sequence from construction. To replace a unit on a high floor, “you couldn’t just pop that one piece out and replace it with another; you’d have to remove everything above it [and] around it to either side. So it’s a little bit like building Legos.... When your building’s almost entirely glass, you want to get the maximum service life.” The team anticipates a lifespan of at least 60 to 70 years for “full service of the entire system”; for components such as gasketing with shorter expectations than the aluminum and glass, he estimates 40 years. A key question in the design stage was balancing aesthetics and serviceability: “How do you service this while the system while it’s on the building, because obviously, it’s not financially feasible to remove large components of the curtain wall, service them, and then put them back on the building.... In a perfect world, if you want the most easily serviced building, you’d have almost no windows.” In an 85 percent glass building, the combination of the unitized curtain wall, a robust BMU, thorough testing, and rigorous construction-stage quality control aims to keep the Raffles in shipshape.