When you picture modern manufacturing in the United States, you probably envision computer-driven assembly lines, lasers, and robots—but you’d be wrong about factory housing. Factory home builders in the United States remain far behind their counterparts abroad. Few or none employ systems like the computerized numerical control machines common to European manufacturers, such as BoKlok, a division of IKEA, or Toyota Homes in Asia. Most U.S. factory housing is built in a basic warehouse with tradesmen wielding hammers and toolbelts. But they also have jigs and cranes, and, despite lagging technology, there are some powerful advantages to even low-tech factories over even lower-tech building on-site.
To start, the manufacturers we spoke to had sophisticated, professional management and building science expertise, including architects, engineers, and LEED-certified project managers on staff. “Running a factory on established practices of lean manufacturing and total quality management gives you several advantages over most builders: less waste, lower costs, and better quality control,” says Shilpa Sankaran, co-founder of ZETA Communities, a San Francisco-based manufacturer of net-zero modular buildings.
Once a factory focuses on sustainability, the combination of manufacturing efficiency with an ecologically motivated waste-reduction program can yield results approaching near-zero waste to landfill.
“We have 90% less waste than any well-run construction site,” says Sankaran. “We reuse and recycle all of our wood, drywall, and even paint.” While according to Brandon O’Connor at Clayton Homes, the nation’s largest manufactured and modular home builder, “There’s one 55-gallon drum full of waste when the house is done.”
Every manufacturer we spoke to also described redundant quality-control systems, averaging 20 or more inspection points per home, plus multiple inspections by third-party state and green certification auditors.
“We have had several highly detailed projects built and found no defects—these modular guys try very hard to avoid mistakes,” says Phil Kaplan of Kaplan Thompson Architects, a Portland, Maine, sustainable design firm that created the Modular Zero Home series for Keiser Industries in Oxford, Maine.
Perhaps the most important precision assembly in a high-performance structure is the exterior envelope. Manufacturers work from the inside out, installing the drywall first and the sheathing and siding last. With the drywall in place, technicians work from the back side, caulking and insulating behind electrical boxes and mechanical penetrations to create an airtight seal. And with full lengths of drywall running continuously along the inside face of exterior walls, the unbroken sheets create an airtight barrier at wall intersections and corners. Insulators working from the exterior can detail the intersections and corners without leaving accidental voids.
Drywall comes first even under the roof trusses, prior to the finished roof assembly being lowered onto the walls, so that the drywall lid runs to the exterior edge of the exterior walls (with a plywood crush-guard along the edge), leaving no air gap at the difficult-to-seal attic ceiling perimeter. Structurally, modular homes are built to a higher standard simply because they have to be craned on and off a truck and endure travel at highway speeds.
“By the time one of our houses arrives on-site, with nary a crack in the drywall, it has already gone through several minor tremors and a Category 1 hurricane,” says David Hunt, director of compliance and certification at Atlanta-based EcoClassic Homes, LLC. “It’s a 200-year structure,” he adds.
There are intangible benefits, too, which come with reduced cycle time and off-site construction, such as minimal neighborhood disturbance, and reduced construction-related noise, dust, erosion, and damage to existing site landscape. And although we found no research to substantiate the claim, factory builders assert that the transporting impact of delivering modules to the jobsite is offset by the factory’s centralized operations, reducing overall vehicle miles and emissions per project when compared with the daily deliveries and subcontractor and employee miles driven to a remote jobsite.