“Both are worthy goals and deserve our attention. The question is how can we achieve that most effectively when resources are limited,” said Darien Crimmin, vice president of energy and sustainability at Boston-based WinnDevelopment, which retrofitted the Lawrence plant. “And how can we effectively combine the two so that one plus one equals three?”

Eighty percent of the units at Stone Mill Lofts are for residents earning less than 60 percent of the area median income. In Lawrence, that income is just under $128,000 for a four-person household, according to MassHousing, a quasi-public agency that helps finance affordable housing across the state.

As cities and towns across the state grapple with a shortage of affordable housing, green redevelopment efforts offer a way to address this problem while meeting urgent climate goals.

“Eighty-five percent of the buildings that will exist in 2050 are already built,” said Joe Backer, Boston's senior project manager for neighborhood housing. “So it's extremely important to address the issue of decarbonizing the existing affordable housing stock.”

Whether it's converting a historic building into affordable housing units that don't rely on fossil fuels or remodeling existing affordable housing to be more energy efficient, green renovations have created opportunities for multifamily designers and developers to get creative and spurred them to work with what they have rather than start from scratch. And many of the recent sustainability innovations for multifamily housing can be applied to single-family homes as well.

“Most people … think that renovating an energy efficient home must cost more money than if they were starting from scratch. The reality is that's not true,” said Timothy McDonald, architect and co-partner of Philadelphia-based Onion Flats Architecture. “We find that new construction projects can be up to twice as expensive as renovating a building.”

McDonald, whose team recently began a sustainable retrofit of an existing public housing project in Salem owned by the nonprofit Preservation of Affordable Housing, emphasized, “The most sustainable building is one that is already standing.”

“All the carbon that went into making those bricks, that concrete, those mechanical systems is already there,” he added. By demolishing and rebuilding, “you're throwing away that carbon just like you're throwing away the building and the money. [it took to build] away.”

Best practices for sustainable home retrofitting have continued to evolve over the past decade as new technologies have emerged and certain existing technologies, such as solar panels, have become more affordable.

In Lawrence, for example, where the first tenants are expected to move into the Stone Mill Lofts, This month, developers at Winn are testing a new device that uses a single energy source to power both hot water and the HVAC (heating, ventilation and air conditioning) system, “virtually eliminating the need for gas on the property,” Crimmin said.

Additionally, because the mill is a historic site, the architects had to balance preserving the building's original look and feel with their focus on sustainability. The granite facade was left relatively untouched, and wood floors and ceilings were either restored or replaced to replicate the originals (the same goes for the windows, which are much better insulated than the original single-pane windows but are nearly identical in color, shape, and style). The builders were only allowed to increase the thickness of the interior walls by about 4 inches, so the designers were careful to select insulation that would minimize heat transfer and energy loss in the tight space.

But newer buildings bring their own challenges. Adapting buildings so that tenants can continue to live there during construction is a top priority, especially when renovating existing public housing, says Owen Woolcock, a researcher at the Harvard Graduate School of Design.

“The opportunity to retrofit this with existing tenants is really the only fair way to do it,” said Woolcock. “Because ultimately, tenants of affordable housing [often] do not choose the conditions in which they live.”

Renovations that benefit local tenants can be achieved using what Woolcock calls a “panel system,” in which an architect can design a new, better-insulated facade for a building on a computer and then have it manufactured in panels that are then attached to the outside of the existing building.

McDonald says when it comes to eco-friendly renovations, insulation is the best place to start.

“The first rule of thumb with a building is to give it a warm coat… that's the thermal envelope of the building,” he said. “Don't say, 'Oh, I'm going to replace my gas boiler with electric heat pumps just because they're all the rage.' No! First the coat. Put your coat on before you go outside.”

That means keeping drafty entryways, poorly sealed doors and thin single-pane windows to a minimum. In Salem, McDonald said, his company plans to use the panel process to create a “super airtight” shell that can be attached to the building. It also plans to have conversations with tenants to explain the changes they will see in their homes.

“People [will be] staring at their windows as you put this new panel on the outside of the building, and for a moment they're staring through two windows because the new window is outside the existing window,” he explained. “So the premise is to complete 98 percent of this renovation [without disrupting tenants]so you can schedule a day here and there to come by, remove the old window and then put in the new one.”

Through better insulation, solar power, and electrification of the heating, ventilation, air conditioning, and hot water systems, Salem's renovation promises to reduce energy consumption by 88 percent, providing tenants with a more comfortable living environment and dramatically reducing utility costs.

Green renovations aren't just for big developers. Mass Save, the government's energy efficiency program, offers rebates and other financial incentives to homeowners who want to install electric heating and cooling equipment, solar panels and better insulation (also known as weatherproofing).

In Boston, the city launched a Healthy and Green Retrofit pilot program last fall that works with about 30 multifamily property owners across the city to fully decarbonize their buildings over the next decade. Program participants can receive up to $50,000 per unit to help with construction costs and must commit to not raising rent over the 10-year period above what the city considers “affordable for households earning less than 80% of the area median income.”

Kristen Simmons, director of the city's Housing Decarbonization Program, said that in addition to reducing energy costs, the program will help homeowners take climate resilience measures. That could mean installing electric air conditioners in homes that don't already have air conditioning, while ensuring that renters don't see an unfair increase in rates.

“There is absolutely no expectation from tenants to contribute in any way,” Simmons said. “Our goal is to reduce the energy burden on tenants and homeowners.”

Ivy Scott can be reached at [email protected]. Follow her @itsivyscott.