For builder Christian Corson, low numbers are good numbers. His current project, a 1,600-sq.-ft. two-bedroom Passivhaus in Knox, Maine, has been testing well and not costing much. A few months ago, for example, with just the rough shell in place – oriented-strand board sheathing on a timber frame, all taping done, and windows installed and sealed in place – the building showed 0.545 air changes per hour at 50 Pascals pressure difference.There have been four blower-door tests since. The most recent, with construction essentially complete, Corson says, showed 0.286 ach50 – notably below the 0.6 ach50 Passivhaus requirement. In addition, the building’s annual heat demand is now at 3.1 Btu per sq. ft. – again well below the Passivhaus requirement, which is 4.75 Btu per sq. ft. (Modeling is based on 7,345 heating degree days.)But the other encouraging low number, Corson adds, is $130 – the cost per square foot, including all site work on the 10-acre lot.“This is the first step of an evolution for me to go beyond Passivhaus and do it affordably,” Corson, owner of EcoCor Design/Build, told GBA recently. “By exceeding the Passivhaus standard, it affords us some design flexibility down the road, so maybe we can deviate from the usual design, put more windows on the north side, and not be so committed to a single look.”Push to NZEThe slab for this project sits on a platform of expanded polystyrene that is 12 in. thick and insulates to R-54. Corson avoided using spray polyurethane foam.The exterior walls are insulated to R-58 with dense-packed cellulose; the ceiling is insulated to R-80 with loose-fill cellulose. Air conditioning and heat will be supplied by a Mitsubishi minisplit air-source heat pump, although the house also is equipped with electric baseboard heaters as backup.The home’s triple-glazed vinyl-frame windows, made by a Lithuanian company, Intus Windows, are rated at just under R-9. Corson became a product representative for Intus because, he says, the company offers an exceptional combination of quality and price.The project also includes a 1.4-kW photovoltaic system: six panels mounted on power rails that will affixed by brackets over the lower windows on the south side of the house. The PV system will bring the house to net-positive-energy operation – or very close to it.