How modern architecture’s love of windows created a comfort problem that nobody planned for
Floor-to-ceiling windows have become the signature of modern architecture. Glass stood for openness, daylight, and a clean break from heavy masonry. When the Museum of Modern Art’s landmark 1932 exhibition put a name to the International Style, the glass tower became its most recognizable form.
What gets less attention is that the glass box was never a passive object. From the start, it was a bargain with machinery.
The machines behind the glass
Glass curtain walls existed before World War II but were rare. Britannica dates the earliest, on a single street facade, to the Hallidie Building in San Francisco in 1918. Only after 1945 did air conditioning, fluorescent lighting, and synthetic rubber sealants make the glass prism practical. The United Nations Secretariat, with its green-tinted glass walls and air-conditioning system, helped set the standard for tall buildings. Lever House, completed in 1952, brought a fully sealed glass facade to Park Avenue. Domus magazine describes that facade as part of an engineered environmental machine.
NREL researchers add that highly glazed facades of that era were a symbol of recovery and economic wealth. The symbolism was real. So was the dependence on mechanical systems to make the glass livable.
The house that sent the bill early
Before the towers, there was a house. Mies van der Rohe designed the Farnsworth House in Plano, Illinois, in 1945, and it was completed in 1951, with floor-to-ceiling glass on every side. A building-technology paper from UMass describes a design that put concept ahead of comfort. Large expanses of unshaded glass and minimal insulation make the house hard, and expensive, to heat and cool. It was very cold in winter, and in summer it was sweltering, with one door and very small openable windows. Edith Farnsworth’s relationship with Mies soured as costs piled up, and she sued him.
The Farnsworth House is an extreme case: single glazing, no shading, little insulation. But it showed the pattern early. Glass trades thermal control for openness, and someone pays the difference.
What the numbers say
A 2022 study from the National Renewable Energy Laboratory, published in One Earth, traces the history in generations. Early buildings, made of concrete with single-pane windows, had window-to-wall ratios of 20 to 40 percent. After the war, the all-glass curtain wall pushed that above 50 percent. The paper cites earlier research finding that the move to highly glazed facades in the 1960s more than doubled primary energy requirements compared with buildings from the early 1950s. NREL’s summary of the work compares New York’s Equitable Building, at 25 percent glass, with Bank of America Tower, which opened in 2016 at 71 percent.
The oil crises of 1973 and 1979 did not reverse the trend. Designers kept ratios above 50 percent and answered with double-pane insulating glass, and low-e coatings followed in the 1980s. The authors call the U-factor the primary deficiency of glazing compared with opaque materials. They also note that the cooling energy needed to overcome solar heat gain is made worse by highly glazed envelopes.
Where is the sweet spot? Building Science Corporation, summarizing the research, puts the energy optimum for most buildings at 25 to 40 percent glass and finds no daylighting or energy benefit above 60 percent. Environmental Building News gave a lower threshold in 2010, saying most large commercial buildings pay an energy penalty beyond 20 to 30 percent. The ranges differ, but every one of them sits below the 71 percent figure above.
Energy codes have taken notice. According to a continuing-education article for architects, ASHRAE Standard 90.1 steers commercial buildings toward window area of 40 percent of wall area or less on its prescriptive path. The code still leaves room for designers to exceed it.
Why it did not stop
In a 2021 interview with Dezeen, Kohn Pedersen Fox president James von Klemperer said architects had collectively built “too many large expanses of glass in our cities.” Dezeen revisited predictions of the glass tower’s decline in March 2025 and reported that the shift had not happened. Most buildings on the Council on Tall Buildings and Urban Habitat’s list of the 100 tallest under construction were all-glass, steel, and concrete-framed skyscrapers.
The appetite for glass shows up at home as well. A January 2026 survey of industry experts by the home-cost site Fixr found 54 percent saying homeowners are asking for floor-to-ceiling or wall-to-wall glass. That is expert opinion rather than sales data, but it points the same direction.
What it feels like inside
Occupants meet all of this at the perimeter, which is where floor-to-ceiling windows put them. Building Science Corporation observes that solar heat gain through glazing is the reason many buildings need air conditioning. It adds that sunlit spaces need cooling while shaded ones need heat, so unless the controls are tuned very well, too many occupants end up uncomfortable. In 2015, Commercial Observer reported architect Stephen Byrns of BKSK Architects pointing out the irony that people in all-glass buildings are often the ones who block the glass with curtains.
At the scale of the housing stock, the U.S. Department of Energy attributes 25 to 30 percent of residential heating and cooling energy use to heat gain and loss through windows.
The glass that is already there
Nobody is going to strip the glass off existing buildings. The practical question for owners is what to do with the glass already installed. The options run from re-cladding a facade, to adding exterior shading, to replacing the glazing, to treating the glass that is there. Building Science Corporation notes that operable exterior shading can remove the need to choose a low solar-heat-gain window in the first place. That is a point in its favor wherever it can be installed.
Window film sits at the least disruptive end of that list. It goes on existing glass, it reduces the share of solar energy that passes through, and it cuts glare. Its job is the solar side of the problem. Winter heat loss is a separate matter that depends on the specific film. No film seals a leaky frame or repairs a failed window.
Glass gave buildings light and views. It also handed part of the bill to the people inside and the HVAC systems serving them. Some of that bill can be reduced without removing a single window.
Start With the Glass You Have
Ask your local CoolVu installer to evaluate your existing glass, whether it is a curtain wall, a storefront, or a house with a wall of windows. They can explain what a film would and would not change.
Find your local CoolVu installer: www.coolvu.com
