Passive House Building in the Digital Age
A Maine factory begins production of components for panelized Passive House buildings
The first components for prefabricated Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. homes are now rolling out the door of a small factory in Searsmont, Maine, where house building is sailing into the digital age.
RPA-Ecocor, the joint venture between Ecocor’s Christian Corson and architect Richard Pedranti, launched in June 2016 with 11 model home designs. It’s now producing wall sections for a Passive House that will be erected in the Albany, New York, area later this year.
It’s the beginning of what both Corson and Pedranti hope will be a turning point for high-performance building in the U.S. Houses will be produced in the controlled environment of a factory, much like modular and panelized buildings already are — but in this case they’ll be able to meet the energy-efficiency and airtightness requirements of Passive House construction.
Houses use a wall section that Pedranti says is difficult and time-consuming to build in the field, but that comes together quickly with the help of digitized modeling tools and state-of-the-art equipment from Sweden.
In time, Corson hopes to produce a house per month. If all goes well, carpenters will be able to leave their beloved worm-drive saws and tape measures on the truck.
Working toward paperless production
Ecocor’s Passive House designs start as digital files that Pedranti produces with Autodesk software called Revit. The Building Information Modeling (BIM) files allow Pedranti to produce 3-D models of houses, realistic enough to let clients wander through for a virtual tour and suggest changes based on what they see. The model homes the company offers are only a starting point. "We look at it as mass customization," Pedranti says, "not mass production."
Alterations such as larger or smaller windows, reworked door placement, or a different roof slope are easy to make, and when a plan is revised the changes are instantly reflected everywhere in the database. For example, a change that Pedranti makes in the window schedule is updated everywhere else — on the floor plans, the 3-D renderings, and the elevations — in real time.
To cut pieces for wall or roof panels, a technician uses a touch screen to extract digital instructions. The Swedish-made saw clamps the stock in place and cuts it to the right length and angle with a tolerance of about 4 thousands of an inch.
“It’s like a virtual world,” Pedranti said recently as he and Corson led a tour of the Searsmont plant. “We’re looking at a real life-size virtual model of this building.”
When plans are complete, Pedranti’s files are exported to Ecocor where they are translated into another digital language as Cadwork files. Sitting before two computer monitors in an office above the production floor, Greg Buzzell can open a file and add or subtract layers at will — with a single keystroke, away goes the claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. . With another, the rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. magically disappears, then the I-joists, the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , and the insulation. Changes can be made at any step.
At the same time, the software generates cost information that is precise down to the penny. More important, the digitized information becomes cutting and assembly instructions, and this is where Ecocor and conventional building part company.
“Normally, I produce a set of paper drawings, and I give them to contractors and they price it out and we pick somebody and we build the house,” says Pedranti. “This is where the path really changes.”
Pedranti won’t have to produce a set of construction drawings because carpenters using the digitally driven equipment on Ecocor’s assembly floor don’t need them.
"This generates the instructions which would typically be a permitting set, and a set of construction drawings," Corson says. "The goal moving forward is to try to eliminate some of that stuff and definitely eliminate the paper stream in our business."
Digital files become cut lumber
Construction starts at the Swedish Randek saw installed along one wall of Ecocor’s main assembly floor. The saw is expensive — Corson won't say exactly how much it cost — and has been in place for only a week. But George Reefer, the carpenter running the machine, doesn’t show any hesitation as he queues up a stack of 1x3 strapping and selects a cutting option on the saw’s eye-level console.
He doesn't have to measure anything or check a drawing because the instructions have already been sent downstairs from the Buzzell’s computer. A computer-controlled stop adjusts the length of the stock. The saw clamps the wood in place, racks it tightly against the fence, and seconds after a curved guard lowers into place the 28-inch (700 mm) saw blade emerges from the table and makes the cut.
Reefer uses a black marker to label the pieces and slides them onto a portable rack, then moves onto the next layer of material. Larger pieces get an adhesive label produced by the saw. He’s building the wall section from the outside in — strapping for the rainscreen first, then the I-joists, then the studs. The saw, driven by a 3-phase, 480-volt motor, cuts to within 1/10th of 1 millimeter (about 4 thousands of an inch). Every cut looks perfect.
Ultra-precision: The white line on the left side of the green Zip sheathing is a chalk line to guide placement of the tape that air-seals the sheathing. It's emblematic of the precision that becomes possible in a digitized factory.
Reefer racks up the pieces one section at a time, and when he’s done the rack goes to the assembly area where two other carpenters are putting the pieces together. The “framing table” where the components are assembled will soon be equipped with multicolored lasers projecting lines on the work surface to guide placement of the pieces precisely.
Layer by layer, the pair builds a wall component, beginning with the 2x4 structural wall, followed by the taped sheathing, the I-joists, the Mento Plus membrane, and finally the rainscreen. Framing tables come in pairs, called butterfly tables, that allow carpenters to flip an assembly over by simultaneously tilting two tables together like leaves of a book (or butterfly wings) until nearly upright. A wall assembly can then be transferred onto the facing table and lowered, allowing carpenters to work on the opposite face.
On the day of the tour, Ecocor wasn't producing roof panels, but the "Super Roof" panels are made essentially the same way as walls. They start with rafters made of dimensional lumber, followed by sheathing, and I-joists filled with cellulose. R-values vary depending on requirements of the climate zone where the house will be assembled.
Windows and insulation are next
Triple-glazed windows are installed in wall sections after they've been flipped over. Ecocor currently uses M Sora windows but plans to switch to CDM windows, which are manufactured in Poland. After window installation, the wall assemblies are moved, with the help of a hydraulic lift, to another staging area. There, a technician bores a series of 4-inch holes in the top of the I-joist cavity and packs it with cellulose.
Stopping at one of these components during the tour, Corson pointed to where the insulation equipment had so powerfully packed in the cellulose that it stretched the Mento membrane and cracked a piece of strapping. The insulation is typically packed to a density of between 4 lbs. and 4 1/4 lbs. per cubic foot.
The R-58.6 wall sections at this point are nearly ready to be loaded onto a truck and moved to the building site.
Wall sections have been certified by Germany's Passivhaus Institut. (The Passive House Institute U.S. does not have a corresponding certification program for prefabricated walls assemblies, Corson says.) But Corson is "agnostic" about whether the houses are ultimately certified under the PHI or PHIUS standard, and he's happy to work with consultants from either organization.
Shorter production times
It's early yet for the RPA-Ecocor venture, but Corson has been building Passive Houses since 2009 and panelizing them for the last three years. He estimates that components for the 2,100-square-foot house bound for Albany can be made in about a month. It will take some 20 to 25 wall sections and another eight to 10 roof sections to make the house — all of it fitting on two or three trailers that can be loaded by two workers in a half-hour each.
Not much waste: The small pile of offcuts is the sum total of framing and strapping waste with about 70% of a 2,100-square-foot house complete.
“Sweden panelizes 96% to 97% of all buildings,” Corson says. “The factories in Sweden are like brewing beer in Germany. Each town in Germany has its own little brewery, right? In Sweden, each town has its own house-building panelization factory.”
He said that a small plant he visited across the street from the Swedish saw factory builds 25 to 30 houses per year. The space is smaller than Ecocor’s 14,000 square feet. He added, “There are companies in Sweden in shops this size that are building 100 houses a year, and we can do that here.”
The building model perfected in Sweden can chop months off the production schedule for a site-built house of this complexity. Corson offered the rough timeline for the Albany house:
- Ecocor sends three people to do the site work for the foundation and run the sub-slab plumbing. Once that’s done, others follow to put in the sub-slab insulation and set the reinforcing steel for the foundation. A local contractor places the concrete.
- In another three to four weeks, with the house components finished, Ecocor loads everything on trucks, delivers the components, and hires a crane to set the panels. “Within 10 business days, working 40-hours weeks, we’ll have the house dried in, metal roof on, cladding on and the house will be blower-door tested to less than 0.6 ach50,” Corson says.
- What’s left is roughly 12 weeks of work to install plumbing, electrical, mechanicals, interior trim, cabinets, and finish.
Including the four weeks Pedranti needs to complete all the plans, it adds up to about 22 weeks, start to finish.
Another advantage of this manufacturing process is a dramatic reduction in the amount of construction waste over conventional building. Carpenters working on a site-built house just chuck the off-cuts and other refuse into a dumpster. Here, the surprisingly small amount of waste is stacked in a corner or put on a shelf in one of the rolling racks where it can be picked over for re-use.
Corson pointed to one small bundle of strapping and dimensional lumber and said it was all the waste from some 70% of a house. It would fit in the trunk of a small car.
Ecocor is small but could get bigger
Corson has less than a dozen employees at the Searsmont factory, but that number could certainly grow. He and Pedranti have had plenty of inquiries about their houses, including one call from a woman in Palo Alto, California, who wants to order one.
One limiting factor is likely to be labor.
“The more houses we build, the more Passive Houses we build, the more we get out there as a company, the less expensive they become because our volume will increase,” Corson says. “The real limiting factor here, in a real rural state, is labor. We could run multiple shifts. We could run two shifts a day. There’s no reason the shop couldn’t be open from 7 a.m. until 9 p.m.”
But where will he find workers? In a “technologically deficient” state like Maine, he says, the brightest kids go to Boston or New York after graduating from high school, or they go to college just to get out of Maine. By building a “state of the art” facility, he adds, more young people may be tempted to stick around.
Cost is another issue, not just for RPA-Ecocor but for high-performance buildings in general. Corson and Pedranti have published a price list for all 11 model homes, listing costs by region for turnkey or shell-only options. Prices for delivery and construction of a completed house in Maine run from $205 to $230 a square foot.
In Palo Alto, and other parts of the country, that may be pretty cheap. But it’s still a barrier for many buyers.
“Essentially, the early adopters pave the way for the future from a financial sense,” Corson says. “I wish there were another way to put it. I’ve always been mindful of cost and bringing this superinsulated assembly to the market at parity of what a normal custom builder would charge for a 2x6 wall with cellulose or fiberglass batts or whatever the insulation du jour is. But I’ll get there. It’ll take a little time.”
- Scott Gibson
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