It became clear that nonprofit developer Rural Development Inc. was really on to something when one of the homes in its 20-unit Wisdom Way Solar Village project took third place in a 2009 utility-sponsored building competition called the Zero Energy Challenge.RDI’s entry in the Challenge, which was limited to builders in Massachusetts, was a 1,392-sq.-ft. three-bedroom attached two-story in Greenfield. Designed and priced as an affordable home, it earned a HERS Index rating of 18.7 and was equipped with a 3.42-kW photovoltaic system and solar hot water system.The remaining homes in the Wisdom Way project were built to similar energy-efficiency standards. Eleven were offered to low-income buyers, five were designated for moderate-income customers, two were sold on the open market, and two were designed for full wheelchair access and would be rented to people with physical disabilities. The design team predicted that the energy efficiency of all 20 homes would be about the same, however, with electricity and gas bills averaging less than $500 annually.It’s great to be rightThe buildings have been occupied for well over a year, and utility bills for eight of them do in fact show they are performing as expected. In a recent newsletter, building consultancy Steven Winter Associates noted that the average annual gas and electric bills for the eight units ranged from $324 to $485. SWA engineer Robb Aldrich collaborated with RDI and other partners on the project, including Bill Austin and Chris Farley of Austin Design.RDI and its team made building tight, well-insulated envelopes a top priority. Exterior walls are insulated to R-40 with 12 in. of blown-in cellulose, the roof to R-52 with 14 in. of cellulose, and the floor to R-38 with 11 in. of cellulose. Triple-pane windows were used throughout. Heat is provided by a Monitor Products GFI1800 gas space heater located in the central area of the first floor of each house. Each home also is equipped with an exhaust-only ventilation system and an additional interior fan that draws air from the ceiling of the first floor and redistributes it to each bedroom.In an article for the March/April 2009 edition of Home Energy magazine, Aldrich explained that the airtight and well-insulated building envelopes keep the design heat loads of the houses at less than 12,000 Btu per hour. The gas heater capacity ranges from 10,200 Btu on low fire to 16,000 Btu on high fire.Prices for the houses ranged from $110,000 (for a two-bedroom) to $170,000 (four-bedroom) in the low-income category and $150,000 to $210,000 for moderate-income buyers. A two-bedroom market rate home was priced at $210,000 and a three-bedroom was listed at $240,000.