The city of Boston is in the midst of a building boom, but environmental advocates say much of this new construction is a missed opportunity.

One out of every five buildings constructed here since 2012 falls in the country’s bottom half for energy efficiency, according to city records.

The data comes from the city’s Building Energy Reporting and Disclosure Ordinance, or BERDO, which requires medium and large buildings in Boston to report their energy use to the city.

One of the metrics used to analyze energy performance is the Energy Star Score, which grades a building on its performance compared to similar properties across the country.

About 20 percent of buildings constructed in Boston since 2012 get Energy Star Scores below 50, meaning they score in the bottom 50 percent of similar buildings across the country when it comes to energy efficiency.

“Buildings account for the majority of the greenhouse gas emissions for the city of Boston,” said City Councilor Matt O’Malley. “It’s about 52 or 53 percent.”

O'Malley said the city of Boston has a stated goal to be carbon-neutral by 2050 — eliminating or offsetting all of its carbon dioxide emissions in just 32 years.

“Boston is in the third biggest building boom of our nearly 400-year history,” he said. “We recognize that. We recognize we can’t put the toothpaste back in the tube, so to speak, with a lot of these buildings.”

O’Malley is developing legislation to address the city’s construction carbon footprint. He's convened three working sessions to discuss the issue with architects, real estate lawyers, developers and other stakeholders.

Some experts say the city’s zoning discourages some energy-efficient design features.

For example, buildings need thick insulation to keep air conditioning and heat inside. Real estate lawyer and former City Councilor Mike Ross said the zoning essentially requires developers to build any insulation into a building’s livable space.

“The dimensions of a property are measured by the outside wall, so if you are buying very expensive real estate … the thought of just killing two to three inches for the sake of better insulation, I mean, that adds up,” Ross said.

The state also recently updated its building code, but architect Stephanie Horowitz said that bar has been set too low.

“What incentives are we offering for doing the right thing?” she said. “The building code and the energy code is always going to be the stick.”

Since Horowitz specializes in high-performing buildings, she says she finds it frustrating to see all the inefficient design features in Boston.

“It would obviously be nice if they were choosing to design buildings that are using less energy than the median,” she said, “so it’s disappointing.”

The city has to make choices about its priorities for new construction, Horowitz said.

“We’re going to be saddled with the energy use of these buildings for decades to come,” she said. “It’s the new construction that’s really that low-hanging fruit.”

Boston's housing shortage might also be impacting the energy efficiency of new buildings, according to Andy Bean, campaign coordinator for the Boston Climate Action Network.

Boston hasn’t focused on energy efficiency because the city really needs new housing, he said. Plus, faster development means more development, and that means more construction jobs.

“We’ve prioritized building and economic development as quickly as possible, as opposed to building smart and building 21st century buildings,” Bean said.

Millennium Tower, built in 2016, operates less efficiently than 93 percent of similar buildings across the U.S.

O’Malley says he plans to propose legislation to help focus the city’s construction efforts around energy efficiency. He’s hoping to eliminate disincentives in the city's zoning and encourage municipal buildings to be ultra-efficient.