It’s Christmas week in the Maine woods, and Chris Corson’s Passive House project is starting to wrap up. The cedar clapboards and cypress trim on the home’s exterior are complete, and sheetrockers are working inside. “We are just finishing the install of the kitchen cabinets, and we’re about to start the finished floor upstairs, and doors and trim,” Corson reported on Tuesday morning. “We have everything commissioned, and are balancing the ERV tomorrow.” A drywall contractor applies joint compound (top). Even with heating and ventilation systems turned off, the house has been able to maintain a comfortable temperature and humidity (bottom). With winter here to apply a real test for the concept, Corson is already jubilant about the building’s energy performance. The home’s heating system — a 12,000-BTU Mitusbishi Mr. Slim mini-split heat pump (model MUZ-FE12NA) — was hooked up a few weeks ago. In a conventional house, the heat pump would be sized to heat or cool one room; but in Corson’s super-insulated design, the heat pump is the whole building’s only heat (the house also has a few feet of electric baseboard as an emergency backup in case the heat pump breaks). The heat pump is rated at 22 SEER as an air conditioner; as heat, the unit has a coefficient of performance (COP) above 4 at 47°F. It maintains 92 percent of its rated capacity even at 17°F, and will heat the house until outdoor temperatures hit 13 degrees below zero. Last week, with outdoor temps in the twenties, Corson measured supply air temperatures at the heating registers at 105°F. Last week, Corson hosted a tour and meeting of his Maine Passive House group, including architects, builders, and energy experts from around the state. “Over and over again, different people would ask the same question,” he says: “Is the ERV running? Is the heat pump running? They’re so quiet, you can’t tell. You have to hold your hand right in front of the diffusers to feel the heat coming out, and if you want to know if the ERV is operating, you have to go into the mechanical room and put your hand against it, and you can barely feel it vibrating.” Best of all, however, is the envelope insulation performance — which, combined with a moderate amount of high-performance glazing on the east, south, and west sides of the home, makes the heating system almost an afterthought. “Last week, it was 25°F outside, and we turned the heat off,” says Corson. “We were working in T-shirts. It was 66°F inside. And two days later, the temperature inside had dropped to 64°F. Two degrees of temperature drop in two days — with no heat.” The home’s west wall showing the mini-split heat pump elevated on a mounting bracket (top); the south wall with glazing (middle), and the east wall with the entry door. The white vent at right is the air intake for the energy recovery ventilator (bottom). Once the new homeowners move in, Corson hopes to monitor the building’s energy performance for several years. “I know my first year will be the worst, because it will take some energy just to dry the building materials out — the drywall, the concrete, and so forth,” he explains. “I’m expecting the house to actually perform better and better as time goes on. But I want to measure it to be sure.” Meanwhile, Corson is already thinking ahead to his next Passive House project. “I have two projects on the drawing board,” he says, “and I already know there are a few things I’d like to do differently next time. People tell me that you have to build at least three Passive House buildings before you get your head wrapped around the idea. I figure by the time I finish my third one, I’ll really understand what I’m doing.”