World’s largest energy-efficient ‘passive house’ office building opens in Boston

World’s largest energy-efficient ‘passive house’ office building opens in Boston

By David Abel

From a distance, the new skyscraper looks like any other generic glass tower looming over downtown Boston.

But it’s the bowels of the building — in its curtains of glass, in its thickly insulated walls, and in its specially designed ventilation systems — that make it distinct from the others.

The $1.4 billion, 691-foot Winthrop Center on Federal Street is the world’s largest office building certified as “passive house,” a designation that means it’s among the world’s most energy-efficient buildings. To be certified, passive house buildings must meet strict standards for heating and cooling, insulation, and natural lighting.

“This building is important because it paves a path for new development,” said Brad Mahoney, director of sustainable development at Millennium Partners, the project’s developer.

But the newly opened tower — the fourth tallest building in Boston — is already something of an anachronism.

Even though its developers say it reduces by nearly two-thirds the emissions of a typical office building of its scale, Winthrop Center still runs on natural gas, a fossil fuel that emits greenhouse gases, for heating and cooling.

As new city and state rules take effect in the coming years, similarly sized buildings will be required to operate on electricity, rather than gas or oil. By 2050, they’ll be prohibited from producing any carbon emissions.

Winthrop Center, which replaced an old parking garage, is still expected to produce about 1,000 metrictons of carbon every year, about a quarter of what a similarly sized conventional building would produce, Mahoney said.

By contrast, Boston University’s new Center for Computing & Data Sciences, also known as the Jenga building, uses geothermal energy rather than fossil fuels and is the city’s largest carbon-neutral building.

“If we were to build today, we’d be electrifying,” Mahoney said.

John Dalzell, a senior architect for sustainable development at the Boston Planning & Development Agency, called Winthrop Center “an exceptional building.” But he said newer buildings in Boston — such as WS Development’s 17-story glass tower for Amazon in the Seaport, which is scheduled to open next year — are even more efficient, relying entirely on electricity for heating and hot water.

“Best practice five years ago is not best practice today,” Dalzell said of Winthrop Center, which broke ground in 2018.

Still, even with its gas-fired heating and cooling systems, Winthrop Center shows how more office buildings could be designed in ways that significantly reduce the amount of power needed to operate them. Buildings and other structures account for more than a third of global carbon emissions, according to the United Nations.

In its 21 stories of office space, Winthrop Center reduces its carbon footprint with features meant to maintain a steady interior temperature: triple-pane windows designed to limit the amount of air leaking from the building; greater amounts of mineral wool insulation between exterior and interior walls; specially sealed ventilation ducts; and highly efficient chillers and boilers for air conditioning and heating.

“We hope Winthrop is an inspiration to commercial developers,” said Hank Keating, president of Passive House Massachusetts, which promotes such development.

Passive house, an energy efficiency standard that started in the 1980s in Germany, has been slow to be adopted in the United States. Less than 6 percent of all such buildings are in North America or Latin America, according to the Passive House Institute in Germany, which certifies buildings that meet its standards.

It’s estimated there are more than 120,000 passive house buildings worldwide, the vast majority of which are residential homes. Constructing commercial buildings to the certification has proved more difficult due to economics. Building to higher standards increases upfront costs, but tenants, not developers, recoup the long-term savings through lower electricity bills.

The amount of passive house construction in the United States has been steadily increasing, especially in Massachusetts, which now has hundreds of homes and other buildings completed, under construction, or permitted, Keating said.

And those numbers are likely to increase in the coming years, as a growing number of communities in the state have adopted an optional “stretch code,” which requires all multifamily projects larger than 12,000 square feet to be certified as passive house construction.

“This will further incentivize [passive house] projects,” Keating said.

Among those is a planned 800-unit housing development in Newton Upper Falls, which its developers say will be the largest residential housing complex built to passive house standards in the United States. That project, expected to break ground next year, aims to reduce energy costs by as much as 70 percent below the expense of similar buildings.

In recent years, especially as government incentives for energy-efficient buildings have increased, construction costs for passive house buildings have also fallen.

A 2023 report by the Passive House Network found that such construction in Massachusetts and New York costs on average just 3.5 percent more than a standard building. With incentives from utilities, affordable-housing programs, and federal tax credits and rebates from the Inflation Reduction Act, their report found that some multifamily passive housing is now actually cheaper to build than standard projects.

“Going forward, we expect teams to regularly deliver projects at cost parity, or for less than conventional code construction,” said Ken Levenson, executive director of the Passive House Network, a New York-based nonprofit group that trains developers and others in how to build passive housing. “Cost is definitely not an excuse for avoiding passive house new buildings any longer.”

With many buildings in Boston unable to use solar, geothermal, or other forms of renewable energy because of their design or location, passive housing is likely to play a growing role in helping the state meet its legal requirements of cutting emissions 50 percent below 1990 levels by the end of the decade and effectively eliminating them by 2050.

Owen Woolcock, a research associate at Harvard’s School of Architecture, called passive house construction a “very credible path” to reducing the state’s reliance on fossil fuels. “In essence, it’s bringing down the energy demand as far as you can reasonably do so, before offsetting the remaining demand with renewables,” he said.

Officials at the Passive House Institute also say they have introduced new certifications in recent years that require the use of all-electric systems.

“We need to be moving fully away from fossil fuels,” said Jessica Grove-Smith, joint managing director of the Passive House Institute in Darmstadt, Germany.

On a recent tour of Winthrop Center, where the smell of fresh paint still suffuses the lobby and most of the floors remain empty, Brad Mahoney said their construction costs were about 3 percent more than if they had built in a conventional way, adding about $15 million more to their bottom line.

But he said the additional cost for the energy-efficient building will be covered by a 25 percent premium that they’re charging tenants, who can expect to save as much as $2 million in utility costs over 10 years compared to a similar lease of 100,000 square feet in a conventional building. Mahoney said Millennium Partners would also purchase renewable energy andcarbon offsets to make the building effectively carbon neutral.

“This is the way we’re going to build going forward, because we have to,” he said.

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