Introducing the Polyhaus – a new metal-clad cross-laminated timber system that could be a game changer for building low-cost Californian houses out of mass timber. That is according to Daniel López-Pére – professor at the University of San Diego (USC) – who, with wife Celine Vargas – is cofounder of a start-up looking beyond box-shaped accessory dwelling units (ADUs) to make quality housing using the smallest footprint possible.
López-Pérez, the program director for the USC architecture program, developed the Polyhaus system by starting with a simple cube and repeatedly truncating the edges until he optimized the form for the largest volume and smallest footprint. The 440-square-foot ground floor includes a living room, a kitchen, a bathroom, and a nook for a desk and a washer/dryer, with the bedroom on a 100-square-foot mezzanine.
“Polyhaus is a design and construction-technology company,” according to López-Pérez, who built a prototype of the system in their backyard in suburban San Diego late last year – viewing the 17-foot, 4-inch-tall Polyhaus as an R&D investment to bring the timber innovations down to smaller scales.
64 CLT panels significantly reduce redundancy compared to stud-frame houses.
Wood Central understands that the house’s structure and envelope consist of 64 individually cut Douglas fir cross-laminated-timber (CLT) panels produced by Vaagen Timbers in Colville, Washington. According to López-Pérez, the structural engineer Fast + Epp—a leader in the push to build more using wood—estimated that the nonlinear structure of the CLT panels provides significant redundancy compared to a simple stud-framed house. The floor panels are five-ply, 7½ inches thick, while the rest are three-ply, 4½ inches.
“The virtue of this truncated polyhedron shape is that we have 25-panel shapes that repeat themselves, reducing details immensely,” López-Pérez said. In his view, the project has only two architectural details—”the structural connections fastened with bolted metal plates, and the simpler splines between panels that slot together.”
Site preparation took just one week, including pouring a concrete foundation whilst electrical wiring and plumbing were installed under the floor and extended to two conventionally framed walls serving the kitchen and the bathroom. With the installed platform floor, work could begin piecing the walls together until they formed a self-supporting diaphragm. Vaagen stacked each panel on the truck in the order in which it would be unloaded and set into place with a crane. Installation of the panels took a crew of three workers only three days on-site. Following that, a vapour barrier was applied, seams sealed, and Kingspan insulated metal panels were screwed directly into the CLT.
The Polyhaus could be less than half the price of a single-family home
A predictable, kit-of-parts supply line helped the couple reduce the price significantly. The smallest model costs approximately $300,000—including San Diego permitting and other development fees—with a construction schedule of 12 weeks, from site work to turning the key in the door.
Up to 40% of that cost is expected to be labour.
“We scheduled each trade separately for this project, but we were also testing things as we went through construction,” Vargas said, adding that one way to save time would be to eliminate the concrete foundation requirement, which the couple allowed for by also pre-permitting a version of the house that sits on 26 earth screws. With this, Vargas estimates they could reduce the construction time to eight weeks or less. Building a single-family home can easily cost between $600 and $1,200 per square foot in California – making the state one of the most expensive places to build in North America, compared to $550 per square foot for Polyhaus, comparable to more conventional ADUs.
The couple recently secured a patent for the Polyhaus system and plans to offer a business model for licensing the technology to builders this year. According to Vargas, given San Diego’s lack of housing supply, the licensing model would allow them to accelerate production. They also envision small-scale developers, who could build six or more on a lot, as the primary market. And since Californian land costs are so high, López-Pérez suggests that infill lots could be developed along a bungalow court, providing a “missing middle” with a density between apartment buildings and houses. “It would open up the possibility of more flexible houses for multigenerational families,” he says, “with people at different stages in life.”
- For more information about the rise of mass timber housing in California, including the rise of the “Super Bungalow” click here for Wood Central’s special feature.
