Green roofs and solar chimneys are here – experts say it’s time to use them

Builders already have the tools needed to build cooler homes for an increasingly hotter world

The US sweltered under record-breaking heat this year, with new research suggesting that air conditioning is no longer enough to keep homes cool. Spiraling energy demands and costs of indoor cooling now have planners looking to alternative ways to keep buildings cool – some fresh out of the lab, others centuries old.

“The amount of buildings we expect to go up in the next couple decades is just staggering,” says Alexi Miller, director of building innovation at the non-profit New Buildings Institute (NBI). “If we build them the way we built them yesterday, we’re going to use a phenomenal amount of energy. There are lots of ways we could be doing this better. It’s not all fancy, emerging technology – there’s some basic stuff we don’t do nearly enough.”

The Guardian spoke with architects, designers and developers about how they are building homes for a warmer planet.

Affordable passive housing

Homes built to passive housing standards – with airtight enclosures and thick insulation – are widely considered the gold standard for energy efficiency and indoor comfort, Miller said: “Basic efficiency stuff can get us nearly all the way there in terms of decarbonization.”

Passive home design got its start in Germany in the 1980s and has caught on in the US in recent decades, with more than 16,000 units built or under construction as of last year, according to the Passive House Network. In April, developers in New York City completed what is now the country’s largest residential passive building: a 709-unit affordable housing complex.

Passive homes typically use 80% to 90% less energy, Miller said. Though this type of building is growing in popularity, it is not yet mainstream.

“One way that we have seen a lot of growth is through policy and financial levers, like energy codes” with stricter efficiency standards, and incentive programs that offer benefits like better zoning treatment or attractive finance rates, Miller said. He added that passive housing is especially promising as a way to lower energy costs for families struggling to pay their bills.

‘Living’ exteriors

Green exteriors have been used to insulate buildings going back to ancient Mesopotamia, and can lower indoor temperatures by as much as 7F (3.8C).

“The more green we can bring back, whether it’s on a roof or on a wall, it helps keep cities cool,” said Bruce Dvorak, a landscape architect at Texas A&M, who built his first green roof on Chicago’s city hall back in 2001. A typical green roof doesn’t absorb heat the way black, tarry shingles of conventional roofs do, and can help cool not just the buildings they cover but surrounding areas. They also absorb rainwater, preventing runoff that contributes to flooding.

An ideal green roof is covered in lightweight soil and local vegetation, Dvorak said. In arid Texas, that might mean planting prickly pear and red yucca, whereas in woodsy Massachusetts you might use a native meadow. Similar planning goes for living walls: a frame is arranged against an exterior wall and covered with climbing plants, like yarrow and lavender, which can reduce energy costs by 23%.

Green exteriors are growing in popularity as cities including New York and Chicago offer tax incentives for transforming asphalt and other non-porous surfaces into green spaces. Even the retail giant Walmart has built green roofs in Portland, Oregon, and Chicago.

Dvorak sees enormous untapped potential: “You think of schools, shopping centers, buildings that have massive rooftops,” he said. “The federal government can really help support these initiatives … It’s the kind of thing that drives us crazy, because there is no reason not to do it.”

Solar chimneys and chilled beams

Some builders are looking to built-in features to cool homes. Solar chimneys are dark-painted shafts attached to the sides of buildings. The chimney absorbs heat, and when that hot air rises, the suction forces cooler air to ventilate through the house. Combined with other passive cooling strategies, solar chimneys can reduce indoor temperatures up to 14F.

Solar chimneys have been used for centuries in the Middle East, and started being used in the US around the 1960s, said Corey Saft, an architecture professor at the University of Louisiana at Lafayette. In 2018, Harvard’s Center for Green Buildings and Cities installed a solar chimney on its headquarters as part of retrofit aimed at making the building require almost no energy for heating, cooling, ventilation or daytime electric lighting.

Solar chimneys build on more widely used strategies of increasing airflow: “Before we had real careful building science,” says Saft, referring to hot places like Louisiana, “they would build leaky, because that would allow ventilation.”

Another feature growing in popularity are chilled beams, which circulate cool water through ducts that are usually attached to the ceiling, which then radiate cool air. First developed in the 1970s, they are quiet, inexpensive to operate and easy to maintain, and can reduce energy use by about 30%.

Miller, the NBI director, said that because the financial benefits of chilled beams only accrue over time, local and state governments need to create incentives to include these features to encourage widespread adoption. “We need to make the people planning the [building] project think about things in the longer term,” he said.

Light-deflecting fabrics

This summer, researchers at the University of Chicago unveiled a new fabric that reflects sunlight as well as thermal radiation emanating from buildings, asphalt and other surfaces.

Professor Po-Chun Hsu said that in tests, a wearable version of the fabric stayed 16F cooler than silk, and 3F cooler than cotton. He and his team are working on making the material more durable, so it can be used on building exteriors.

“When you put [this] material under the sun, it won’t be heated by the sun, but it will also keep radiating, or losing heat, to the sky,” he said. “That’s how we can do the cooling.” He said his team was working on bringing the material to market and that it can be engineered to be eye-catching – which could be an added bonus as the material becomes a part of the cityscape.

“By playing with the optical properties, at certain angles you’ll see different colors,” he said.

Cover photo: Knickerbocker Commons in Brooklyn, New York, is the state’s first 100% affordable multi-family building to be built and certified to the Passive House standard. Photograph: Danielle Amy/The Guardian