Rubbing their hands and breathing dragon smoke into the cold air, residents rush inside the bright green building. The temperature outside has barely hit double digits, so the warm lobby inside Bayside Anchor, a low-income apartment building in Portland, is a happy reprieve.

The lobby is also an architectural feat, as Bayside Anchor has no centralized heating system. It is a certified “passive house,” which means the building has airtight insulation and thick windows to keep the interior warm and heating costs low.

This energy-efficient design has been gaining ground in American architecture among wealthy homeowners. But some cities like Portland, Maine, have realized this energy-efficient design for the affordable housing sector — for residents who can really benefit from lower heating costs.