The late 2000s and early 2010s saw a surge of interest in deep-energy retrofits. Around that time, many of us in the green building community started coming to terms with the magnitude of emissions reductions needed to avoid catastrophic climate change. Inspired by the ultra-low-energy new homes of the Passive House movement and a President who assured us that “insulation is sexy stuff,” we sought ways to achieve deep-energy reductions in existing buildings. Our goal was to move beyond the approximately 30% savings on space heating and cooling energy typical of conventional retrofits toward the 50% to 90% we believed was needed to achieve climate protection goals. This kind of performance improvement required us to evaluate all of a building’s surfaces to look for opportunities to reduce heat transfer.
Above-grade walls are usually the most technically challenging—and expensive—component of a deep-energy retrofit. In existing homes, wall insulation is limited by the depth of the existing cavity, and the wall’s performance is further diminished by thermal bridging through studs and other framing members. To go beyond about R-20, it’s necessary to add insulation to either the inside or the outside of the wall. Most deep-energy retrofits involve removing siding and adding one or more layers of foam board to the exterior of the house. Windows and doors may or may not be replaced as part of the project; in either case, close attention needs to be paid to flashing these openings. Once the foam is in place, new siding is installed, usually over furring strips. Window and door trim and sometimes roof overhangs may need to be modified to accommodate the extra wall thickness.
Some early deep-energy retrofits, like those done by Paul Eldrenkamp’s team in Massachusetts, provided inspiration and proof of concept. However, the high cost ($140,000+) and…
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