The “perfect wall” concept was developed by Joseph Lstiburek of Building Science Corporation, a consulting and architectural services firm based in Westford, Mass. The idea is simple: Place cladding, insulation, and air/water/vapor barriers outside the structure to protect it and the interior from moisture and drastic temperature changes. If done correctly, a perfect wall system should provide superior thermal performance and increase the lifetime of the building (500 years is one quoted figure).
A new house in East Austin — designed by Eric Rauser, AIA, and Rebekah Rauser of Rauser Design and built by Risinger Homes — is putting the system to the test. Designed for a bachelor, the two-story, 1,450-sf residence relates to the changing landscape of the neighborhood, where single-family homes are giving way to multifamily infill developments whose size is constrained only by a profusion of protected heritage trees.
The wall assembly, from the timber framing out, is 1-in-by-6-in pine boards, 1/2-in OSB, a 40-mil peel-and-stick air/water/vapor barrier, 4-in silver foil-faced polyisocyanurate insulation, 1-in-by-4-in pressure-treated lathing, and 24-gauge corrugated metal siding pre-painted white. The roof has a similar makeup, the only difference being that it has six inches of insulation and a thicker weather barrier. Since the house is designed without overhangs, this barrier is able to wrap the entire house without breaks.
Inside, the architects left the wood framing — standard 2-by-4s on 16-in centers — exposed and painted white, giving the interior a rustic quality and making it much easier to spot leaks, should any occur, as well as for the structure to dry out if it gets wet. Resting on a concrete pier-and-beam foundation, it is also easy to access every part of the house for repairs. A ductless 3-head mini-split mechanical system — one head upstairs and two downstairs — delivers hot and cold air with a heat exchanger for exhaust and intake, and a separate dehumidification system to keep everything nice and dry. The house earned a HERS rating of 39, much better than the HERS 100 rating of today’s standard residential construction, putting it a stone’s throw from net zero status.