Northshore Home magazine highlights the best in architectural design, new construction and renovations, interiors, and landscape design.
Issue link: http://read.uberflip.com/i/1206063
55 their values, which include a low carbon footprint," Howell explains. "So they decided to spend their money on the shell of the house and the insulation envelope." The foundation, wall, and roof assemblies were paramount to the success of this project. Given the site's ledge rock—as well as the client's desire to limit the use of carbon-intensive concrete—an insulated slab-on-grade foundation made good sense. Sitting just 18 inches below grade, the concrete is cast into a tray of rigid foam; because there's no basement or crawl space, thermal bridging, which leads to heat loss, is greatly re- duced. (Eight inches of insulation under the slab results in a thermal resistance of R-30, and the slab edges have six inches of insulation for R-24.) This approach uses significantly less concrete than a conventional full- basement foundation, and the exposed slab doubles as the floor surface. The 2- by 8-foot walls are filled with dense-packed cellulose, and exterior rigid foam plus a ventilated Har- die board siding system heighten energy efficiency. At 14 inches thick, the exterior walls have an R-value of 50, and 24 inches of loose-packed insulation in the scissors- truss vented roof yields R-80. Key to the build is a contiguous air barrier layer running under the foundation, up the walls, and into the roof system. "The detailing of the air barrier was a challenge," Howell says. "It sits in the middle of the wall, so there are six inches of foam to the exterior and eight inches of cellulose to the interior, so you are allowing the wall to dry from both sides.… It's not treated as rigorously in traditional construction. We've done high- performance houses before, but never near-net-zero. It takes a lot of forethought and planning and care." Additional energy-saving measures include a high- efficiency electric mini-split heating and cooling system and a Zehnder HRV system for balanced ventilation and indoor air quality. Four 6-foot by 6-foot imported Ger- man triple-pane windows by Kneer-Südfenster allow for solar heat gain, which is absorbed by the concrete slab foundation, helping to moderate temperatures in the house by acting as a heat sink. Howell notes, too, that the shape of the house con- tributes to the energy savings. "The cube is the most