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).
With the cladding, insulation, and weather seal all outside the structure, the architects were able to leave the framing exposed on the interior. Photo by Casey Dunn.
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.
The lack of large overhangs allowed the house to be completely wrapped with a water, air, and vapor barrier, making for a very tightly sealed envelope. The corrugated metal siding and standing seam roof are typical of East Austin. Photo by Casey Dunn.
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.