Homebuilder Chris Senior wants to leave a legacy by creating homes of beauty and super-efficiency that will last for centuries. He’s steering his company, Anchorage Building, toward specializing in homes that meet Passivhaus energy-efficiency requirements and use about one-tenth of the heating and cooling energy of conventionally built U.S. homes.
Passivhaus is a construction standard that focuses on minimizing heat loss with an incredibly tight building envelope and insulation levels several times greater than conventional construction. While its German name translates to “passive house,” Passivhaus-designed homes go far beyond passive solar, with the result that conventional heating and cooling equipment is unnecessary in most circumstances—even in frigid climates.
A tight building envelope has two additional advantages over previous passive solar designs. It allows useful heat gain from smaller, “passive” sources of energy like human bodies (equivalent to about a 100-watt incandescent bulb) and cooking, and also helps keep buildings cool in summer with minimal effort.
Like a conventional passive solar home, many of the windows in a Passivhaus are still south-facing, but they are fewer in number than previous passive solar incarnations, and are of highly insulating (R-7 or greater) triple-pane construction. Many Passivhaus homes, especially in Europe, are also multi-unit; reducing the number of exterior walls further reduces heat loss.
In 2011, Chris and Leigh Ann Senior built the second Passivhaus home in North Carolina. Designed by architect Jay Fulkerson and located on a picturesque lake in Chapel Hill, it is the couple’s primary residence and was built after completing the first one for a client just a few miles away.
The walls of Chris and Leigh Ann’s 2,155-square-foot home are built of 2-inch-thick concrete, which helps regulate interior temperatures. Five-inch-thick concrete vertical ribs provide additional structural support. This high-psi concrete is waterproof and has a lifespan of hundreds, if not thousands, of years, which helps justify its high embodied energy. Eight and a half inches of rigid foam insulation provide an insulation value of R-32. The inside of the wall has conventional 2-by-4 wood framing with 1/2 inch of air space between it and the concrete structural wall. The interior stud walls are attached to the floor and ceiling with metal plates but otherwise float free of the concrete structural wall.
Plumbing and electrical were run through these interior walls, which were then filled with dense-pack, blown-in cellulose insulation for an additional R-10. This gives the walls a total of R-42—the Passivhaus standard for this climate. The resultant 15-inch-thick walls give the home a fortlike feel and buffer sound transmission from outside.
The majority of the windows (100 square feet of glazing) are clustered on the home’s south side to optimize solar heat gain. Chris and Leigh Ann chose Thermo-Tech triple-pane, argon-filled windows with a solar heat gain coefficient of at least 0.5 and a U-factor of about 0.15.
While insulating the walls and floor was fairly straightforward, super-insulating the roof proved to be challenging. Although somewhat distrustful of the long-term stability of oriented-strand board (OSB), using wooden I-joists (made, in part, of OSB) gave the 14-inch depth necessary for insulating to Passivhaus standards. Since wood is a fairly good conductor (relative to insulation), reducing the width of the roof rafters to 1/2 inch from a conventional framing lumber dimension of 11/2 inches greatly reduces thermal bridging.