Standing in front of her three-level house in Somerville, Lena Sheehan looks down at the construction of a new high school and transportation hub just a block away. “I can’t get over it, I haven’t been here in so long,” she says. “This is the new T – isn’t that brilliant, right beside the house.”

After immigrating from Ireland in the early 1990s, Sheehan and her husband bought this property on School Street in 1998 against the advice of friends. “It was a crazy idea really, because Somerville wasn’t the place it is now,” she says. “We were very young and naive; it was such a big risk.”

Then one day, she saw a good omen. Her husband found an album of old Irish music a previous tenant had left behind and played her a song about his tiny home village.

“It’s not a very nice song,” she says, laughing and humming the tune. “But he played it and the two of us were dancing around in the living room. And we were laughing. And I remember thinking, okay, that has to be a good sign — maybe this wasn’t a terrible mistake after all.”

Their bet on the house paid off. The city began to gentrify, and more than 20 years of rent from the three-story structure allowed them to buy two more rental properties, and their family home in Newton.

Three-Deckers for the planet

Besides being an economic engine for families like Sheehans, the thousands of triple-deckers (or three-deckers, depending on who is speaking) across Massachusetts have a new significance – for the climate. State officials say if we’re going to reach carbon neutrality by 2050 – as Gov. Baker has set out to do — triple-deckers have to play a role.

State Secretary of Energy and Environmental Affairs Kathleen Theoharides says the houses of today are here to stay, and they need to be updated. About 80 percent of the state’s current building stock will still be here in 2050, she says.

“Being able to reshape that building stock and make sure it’s in a condition where it can save people money through energy efficiency and really be a comfortable place to live, is a key part of this work,” she says.

Buildings are responsible for more than a quarter of the state’s greenhouse gas emissions. Residential buildings with fewer than four units, including single-family homes, make up the bulk of those emissions.

Stephen Pike, CEO of the Massachusetts Clean Energy Center (MassCEC), says the clock is ticking toward 2050, and the pace of retrofitting needs to pick up in a major way.

“At this point, we’re probably doing a couple of thousand a year to a 2050 standard,” he says. “You can do the quick math: we need to be doing roughly 100,000 homes a year [over the next three decades]. And so, where do you start with that?”

One place to start is with our old friend the three-decker, and the solution du jour is known as a “deep energy retrofit.” That means insulating an entire building, sealing off air leaks and installing more sophisticated HVAC systems — ideally powered by solar panels.

That’s easier said than done. Triple-deckers weren’t built for energy efficiency, and making them so is a challenge. That’s why the state recently hosted a design competition to generate ideas.

One entrant was Travis Anderson, a local designer with the design-build firm Placetailor, which specializes in energy efficient buildings. (Full disclosure: two years ago, I hired Placetailor for help with a renovation.)

Anderson says retrofitting a triple-decker is like turning an old station wagon into a modern electric vehicle. “Instead of just trading the car in for a brand new one, we’re kind of stuck with having to figure this one out,” he says.

Anderson worked with experts at UMass Amherst on a submission called “(re)Facing the Future.” The proposal looks like a cross between 21st century modern and the classic three-decker we all know — featuring still-under-development window panels that let in light and collect solar energy.

The proposal uses a thermoelectric “smart façade,” creating an air-tight seal around the building, insulating, harnessing solar power — and doing all the work on the exterior so as not to disrupt tenants. The price tag: $329,952, plus $34,000 for solar equipment.

That was just one of more than a dozen solutions to transform triple-deckers into buildings fit for a carbon-free future.

But one question looms above them all: How economically feasible is it to retrofit the state’s entire housing stock?

Who’s going to foot the bill?

Out in Worcester, Taylor Bearden thought he had found a way to make it work. Bearden is a co-owner of Civico Development, a firm that has acquired 18 three-deckers in Worcester with the goal of making them as energy efficient as possible.

From the outside, the fresh paint, siding, and windows make Civico’s houses stand out from the rest of the neighborhood, showing how good these old buildings can look with a bit of investment.

Bearden walks into the basement of a three-decker in the Bell Hill neighborhood, where his company owns 10 of them, clustered together for a sort of “campus feel.”

He lists off the improvements: “All the old wiring is gone. The heating systems and the hot water, everything is brand new, all the plumbing is brand new. We’re not going to have a very expensive water leak that’s going to displace tenants for three months as we repair. We don’t leave any of those hidden issues.”

Civico isn’t your typical property owner. Its business model is based on what he calls the “triple bottom line” of people, planet, and profit. And up until now, it has worked — the company found a way to bring triple-deckers into the future while seeing returns for investors.

But there’s a wrinkle.

Civico started buying and renovating in Worcester in 2017. At that time, Bearden says, it was feasible to buy a building and do a deep energy retrofit. Now, just four years later, the numbers don’t “pencil out.” Housing prices have increased so much, there’s little left to pay for improvements.

Before housing prices shot up,”50 cents of every dollar went to acquisition and 50 cents went to renovation,” Bearden estimates. “In current markets, maybe 10 cents of every dollar could go to renovation. And that’s not a recipe for making better housing.”

Civico is now finishing the company’s 18th, and final, three-decker in Worcester. The total cost for the eight-unit building — for a gut rehab and deep energy retrofit — is estimated at $1 million.

Bearden thinks that as many as 6,000 three-deckers in Worcester alone would benefit from the same kind of investment. Boston has nearly 10,000 of them, and communities across the state have many more.

Bearden says his company has managed to convert its 18 building without subsidies. But to continue and expand this type of work, he says, builders will likely need government assistance.

Despite an array of incentives aimed at helping builders build green, experts say that if the state’s housing stock is going to reach carbon neutrality by 2050, it’s going to take billions in state and federal subsidies.

One bill in the Massachusetts Legislature seeks to retrofit a million homes over 10 years; President Joe Biden said during his campaign that he wants to upgrade four million buildings in four years, and weatherize two million homes.

But those are just proposals. Now that we know how to retrofit a triple-decker — we just have to figure out how to pay for it.

This story was published by WBUR 90.9FM on March 12. The Reporter and WBUR share content through a media partnership.