If architects, planners, and public officials in Boston mean everything they say about sustainability and climate readiness, why is the city’s latest construction boom filling the skyline with so much glass? From the shimmering height of the Millennium Tower to the waterfront views of 22 Liberty, and a boxy office and condo complex going up at Pier 4, glass exteriors have become a major feature of today’s urban landscape. Just as we associate periods in Boston’s history with specific materials and styles — like 19th-century brick apartment blocks and 20th-century monumental concrete forms — glass is the material of the moment. The new buildings mimic others being erected in New York, London, Dubai, Singapore, and other cities around the world. Glass walls have become a shortcut for architecture that is sleek, cosmopolitan, and of-the-moment.

Yet glass buildings also take a lot of energy to heat and cool. When New York started tracking energy use by skyscrapers, the gleaming 7 World Trade Center — one of that city’s more efficient glass towers — scored worse than the 1930s-era Empire State Building. Oddly, glass buildings are proliferating even as cities like Boston set ambitious goals to deal with climate change. Former mayor Thomas Menino vowed to cultivate “the most sustainable city in the United States”; his successor, Martin Walsh, has called Boston “America’s climate champion” and set a goal of being carbon neutral by 2050.

Such rhetoric from City Hall resonates within an architecture profession that has embraced climate awareness in a big way. The “green” building industry has exploded in the past decade; green building conferences now draw tens of thousands of attendees every year. Sustainability is at the forefront of architecture curricula, and hundreds of thousands of architects get certified in sustainable design. In specialty publications, architects and other building experts have been fretting about the popularity of glass exteriors for years.


But all the talk about sustainability among architects hasn’t actually translated into lots of sustainable buildings in the real world. In reality, the industry faces a massive problem: By some estimates, the building sector consumes nearly half of the energy and produces 45 percent of greenhouse gas emissions in the United States. Many architects have signed on to an industry challenge to become carbon neutral by 2030, but new buildings are already slipping behind the targets to get there. Permissive building codes, industry inertia, and market demands — like clients clamoring for floor-to-ceiling views — have widened the discrepancy between the kind of buildings cities say they want and what they actually allow. So while the industry inches towards better environmental performance, buildings in Boston and other cities still fall short of the sustainability goals that everyone claims to embrace.


The debate over glass buildings is one example of a larger fault line in architecture, a profession where the dreams of social and environmental visionaries collide with the harsh realities of getting building projects financed.

Sustainability-minded architects are trying to wean colleagues and clients from all-glass buildings, which they see as a relic of the past rather than a vision of the future. “Our goal is not to demonize glass as a material,” says Blake Jackson, an architect at Tsoi/Kobus & Associates in Cambridge. But he says glass can be used judiciously in a way that’s responsive to the environment.


Others describe the issue more starkly. “Glass is like sugar,” says Ilana Judah, director of sustainability at FXFOWLE Architects in New York. It’s inherently appealing to the senses and was once a luxury. Now, as a commodity that’s both appealing and plentiful, it creates problems. “Sugar is an incredibly commonplace item now,” she says, “and we have an obesity issue.” Judah says that glass, like sugar, has negative consequences when used in excess. “My perspective is that we’re overdosing on glass,” she says.

Architecture has been in a love affair and struggle with glass buildings for nearly a century, since floor-to-ceiling glass walls became possible around the 1920s. “The big fight in all traditional buildings up to that time was how to get natural light into spaces,” says Blake Middleton, a partner at New York-based Handel Architects.

Glass walls were seen as a liberation, and became a key part of the modernist aesthetic. “It’s sleek. It feels like the future,” says Z Smith, an architect at the New Orleans-based firm Eskew+Dumez+Ripple. German Bauhaus architects who emigrated to the United States helped to popularize a glass-heavy international style that still resonates today. The iconic transparent glass walls of Mies van der Rohe’s Farnsworth House in Plano, Ill., are echoed today in the nearly invisible walls of an Apple Store.

The Apple Store on Boylston Street. David L. Ryan/Globe Staff/Boston Globe

The visual qualities of glass — transparency, reflectivity, smoothness — still captivate designers. Smith’s architecture students at Tulane University quickly learn that glass is a shortcut to a good-looking design: “When they’re not sure what do with it, they enclose it with a glass curtain wall.” Lit up at night, glass buildings make great marketing photos. Reflecting daylight, they appear jewel-like in the skyline.


For developers, the main appeal of glass is financial. Glass is pricier than other materials, but not exorbitant, and a simple all-glass wall can actually streamline costs over a wall with large windows. And the investment in glass yields a payoff. “It’s always about the view,” says Middleton, who designed Boston’s Millennium Tower. “In real estate, it’s location, location, location, and the third location is where you are in elevation.” A floor-to-ceiling window helps to maximize the value of that height.

In residential buildings, a glass wall “is only for the day the potential buyer of the condo walks in. They feel like they can fly,” says Smith. But what makes an apartment sell is different from what makes it livable. Glass walls are often touted as a way to feel connected to nature and the outdoors, but that illusion, paradoxically, comes at an environmental cost.

What’s so problematic about glass walls? In Boston’s climate, the biggest problem is a lack of insulation. Unlike opaque walls, glass allows heat to pass in and out easily. A 2014 report from the Urban Green Council in New York found that glass buildings have insulation values equivalent to medieval half-timber houses. “You have to now put more heat in your building to make up for that glass,” says Andrea Love, director of building science at Boston architecture firm Payette. On a cold day, glass walls will make you feel chilly, even if the air temperature in the room is comfortable, because your body loses heat to the cold surface. And as Love explains, they create a chill-inducing draft, as warmed air hits the top of the glass wall and falls. Perimeter heating systems are often needed to make up for these discomforts. In the summer, solar energy heats up surfaces inside, requiring more air conditioning. All-glass buildings often need constant heating or cooling to maintain comfortable temperatures. In an extended power failure, temperatures in a glass high-rise could quickly rise or fall to dangerous levels.


Transparent walls also limit privacy, and sunlight can create glare. Reflections on glass buildings can also be a problem; one London skyscraper infamously melted cars parked outside. The Urban Green Council has found that occupants of glass buildings often cover their views with shades and curtains, negating the effect of transparent walls. And a study by Love’s team found that floor-to-ceiling glass doesn’t bring in significantly more daylight than windows covering half the wall.

Downtown buildings reflected on the side of Millennium Tower. David L. Ryan/Globe Staff

Some glass towers earn LEED ratings for sustainability. But those ratings are in spite of the glass, not because of it, Love says. “Having an all-glass façade is never the more sustainable option,” she says. In fact, building codes dictate that buildings have less than 30 to 40 percent of the walls as glass, because of its energy impact. But buildings can sidestep those requirements if they reduce energy use in other ways. Developers can meet building codes and even win LEED ratings by investing in fancy, super-efficient heating and cooling systems. “The emphasis on efficiency has gone to parts we can replace,” says Russell Unger, executive director of the Urban Green Council. While he says there’s nothing inherently wrong with glass as a material, he says, there’s a problem when we use it to create inefficient building envelopes, which are harder to upgrade in the future. “We’ve kind of locked ourselves in to a certain kind of energy use,” he says.

For speculative developers looking for a quick sale — or even for those trying to make the numbers work in a high-cost market — the long-term energy drain isn’t part of the equation.

Glass has come a long way since Boston’s John Hancock building became notorious in the 1970s for panes popping out onto the streets below. It’s more durable and insulating. “You’re able to get more glass and have a high-performing building,” says David Nagahiro, a principal at CBT Architects in Boston. High-tech coatings, opaque glass sections, “smart glass” that can change properties, and automated shades can modulate temperature and glare. Nagahiro designed the glass tower at 121 Seaport with an elliptical shape that reduces solar heat gain. Some high-end buildings employ double glass walls that trap insulating air between them.

Black-painted plywood boards covered the windows of the John Hancock Tower on Aug. 8, 1973. High winds had broken and damaged much of the glass facade. Joe Dennehy/Globe Staff

But while technologies can make a glass building better, not every developer will pay for them. “You can make it a better bad decision, but it’s still, at the root of it, a bad decision,” says Love. A better strategy, she says, is to deploy glass on certain sides of the building but not others, or in a central atrium to bring in light to key spaces.

Beyond the environmental issues, some architects object to glass buildings for looks; they convey a globalist style that many find generic. Judah of FXFOWLE says that architects are “still sort of stuck in this modernist aesthetic.” But she adds that building codes and green-building standards like LEED are getting stricter about energy use, which will make it harder to deploy so much glass. “It’s going to help drive an aesthetic change,” she says. While FXFOWLE has designed glass buildings — including Boston’s 888 Boylston — Judah says that some new projects are starting to use glass more sparingly. “There’s a trend now toward a more homey, cozy experience,” she says.

New buildings represent just a slice of the city’s energy challenge — there’s also the daunting problem of retrofitting inefficient older buildings. But the difficulty of upgrading a building already in use only underscores the concern that today’s towers are far less energy-efficient than they could be. Making greener buildings is possible, but it requires developers, designers, and buyers alike to look beyond shiny surfaces and prioritize designs that put energy first.

Courtney Humphries is a freelance journalist in Boston.