"We do a lot of things that are green, but people don't know about it," Hannigan said. "We didn't change our construction methods to conform to LEED standards. We pushed for the certification to showcase our work."
Houses that meet LEED standards, developed by the U.S. Green Building Council, use sustainable construction practices, maximize fresh air indoors, minimize exposure to airborne toxins and pollutants, and save energy and water, among other goals.
For a Habitat home, sustainable construction starts below ground with an insulated concrete form (ICF) foundation. Styrofoam and metal grids resembling large Legos are set into an eight-foot-deep excavation and filled with concrete.
Typically, frames for the concrete would be made of wood and discarded afterward. In this case, the foam acts as insulation and prevents waste. Emphasis on insulation extends from the hot-water lines in the basement to the roof.
On Wilt Street, Hannigan said, an advanced framing technique was employed, requiring special methods, such as using 2-by-6 boards instead of 2-by-4s to allow space for extra insulation. The furnace installed could probably be smaller because the house is so airtight, he added.
The houses' extra-thick oak floors will last longer and are green, because wood is a renewable product. (Carpeting absorbs dirt and can emit chemicals that affect air quality.) All the appliances installed are Energy Star-rated for efficiency.
The houses' fiber cement siding is more durable than wood or vinyl, Hannigan said.
Their narrow backyards have gravel, rather than grass, which he said is not green in the environmental sense: "It's an invasive, and gravel provides better drainage." Out front, redbud trees will grow to provide shade.
The completed Wilt Street house has four bedrooms and two baths, and is handicap-accessible. Its neighbor has three bedrooms and 11/2 baths.
To build affordable housing, Habitat relies on fund-raising and the generosity of vendors and subcontractors, who donate or discount materials and services. Most of the construction labor is provided by volunteers.
Buyers, who purchase the houses with 30-year, interest-free mortgages, contribute 350 hours of sweat equity at their homes and other Habitat sites.
On Wilt Street, supervised by Habitat staff, hundreds of volunteers and four members of AmeriCorps - a federal program whose participants commit to a year of community service and receive a modest living stipend - worked 10 months to construct the two houses.
On weekdays, volunteers included retirees and graduate students; on Saturdays, church groups, high school students, corporate employees, and others helped out. The staff of Temple University's Liacouras Center, for example, primed interior walls with low-VOC (volatile organic compounds) paint.
To qualify for LEED certification, Habitat for Humanity had to submit to regular inspections and diagnostic tests by building consulting and engineering firm MaGrann Associates, a designated LEED "green rater" that explained the point system required and performed the on-site verifications.
The biggest challenge presented by the LEED process, Hannigan said, was keeping track of documentation such as product information.
In mid-January, with the second Wilt Street house nearing completion, site supervisor George Buckmann hung kitchen cabinets while AmeriCorps members Sara Mussoline and Julia Von Holt caulked wood trim around doors and windows.
Buckmann said he was confident that house also would be LEED Gold-certified.
On learning that a Habitat house in Charlotte, N.C., had earned LEED Platinum certification because a donor had provided solar panels, Buckmann said, "I'd love to have solar panels." He pointed to the sun streaming through double-paned windows and hitting the roof.
Even absent photovoltaic extras, the Wilt Street houses have assets that LEED prizes.
With access to parks, recreational facilities, and public transit - a bus stop is around the corner, and the Broad Street Subway is just blocks away - the houses were rated "outstanding" for community resources.