There are stories behind every community member, program and insight we uncover, and here is where you’ll find them. Read on to learn more about how we think, what we’re up to and what drives us.
Net Zero With Panels
We wrote in a recent blog about our visit to Unity Homes in New Hampshire. We wanted to learn how they're using offsite manufacturing to build Net Zero ready homes, and how their pricing stacks up to comparable homes in the local market.
We visited the Unity plant in Keene, NH then drove over to Guilford, VT to tour a just-completed "Xyla," a 3-bedroom bungalow style cottage.
The Xyla we saw has a 10-kilowatt rooftop PV array that should meet all the home's power needs on an annual basis.
It also includes a Tesla Powerwall 2.0 battery from Green Mountain Power. The utility sells these batteries to homeowners in its service area at a highly subsidized installed cost of $1500, and then draws from them during periods of peak demand. The batteries also provide homeowners with backup power—an estimated 8-12 hours during an outage, according to Green Mountain's website.
Of course a home can only take full advantage of renewable power if it's super-efficient. The designers and builders need to reduce the energy load—and the electric bill—to where the monthly savings exceed the cost that the panels add to the mortgage payment.
Although actual savings depend on the homeowners, this home's numbers should make it easy to reach that goal. It earned HERS score in the mid-40's, which means it should use less than half the energy of a comparable home built to the minimum energy code standards.
What Does This Have To Do With Offsite?
Good question. The answer, according to Unity Founder Tedd Benson, is that the pre-engineering, manufacturing efficiencies and quality control possible with offsite makes getting those scores easier and less expensive.
Unity is one of two manufacturers we know of that manufacture closed, pre-insulated wall and roof panels. (The other is Blueprint Robotics in Baltimore, but if readers know of more companies doing this we want to hear about them.)
The drawing shows a typical Unity wall section.
Cavities are filled with blown cellulose insulation at the plant, and can achieve R-values from R21 to R53, depending on wall thickness.
If the local code requires an insulative sheathing Unity installs a wood fiber-based, vapor-open board from Steico. Otherwise, they use Huber's Zip System sheathing.
An interior OSB layer serves as an air barrier and vapor retarder. According to Operations Director Andrew Dey, however, the walls still have enough vapor permeability to dry to the inside or outside, which means they will perform during heating or cooling season.
An interior service layer for wiring, framed with 2x3 studs. The service layer can be insulated on site, after the wiring inspection.
Making a Tight Fit
Energy efficiency has as much to do with air sealing as with insulation, and a blower door test on the Guilford home measured an air leakage rate of 1 air change per hour (ach) at 50 Pascals. Anything below 5ach at 50 is considered tight.
Offsite also make airtightness a lot easier to achieve. The interior OSB obviously plays a role but so do the tight tolerances made possible in the factory.
When building panels inside, out of the weather, you can ensure dimensional stability by controlling the wood's moisture content. Although Unity uses stable I-joists for a lot of its wall studs, they use solid lumber to frame openings, corners and other elements, and they're quite picky about the wood they use. The spruce 2x's stacked up in the plant were the best looking ones we've seen in years.
This pays during the build. Field crews place double-bulb EPDM on every joint wherever two panels meet. Panels are sized to ensure optimal gasket compression, so dimensional stability is obviously important.
Rim joists are difficult to seal correctly, so Unity eliminates them by placing first and second story panels on top of one another and attaching the second story to the inside of the wall. The second story rim sits on top of the 2x3 service layer. To ensure that they're strong enough to support the second-floor loads, the service layer's studs are fastened through to the main wall studs. "It's a highly engineered wall system," says Dey.
Unity shows what's possible using a highly optimized design and manufacturing process. But what about price?
The Xyla sells for $134 to $195 per square foot, depending on size and options. "You will find places where these numbers are high relative to local costs for conventional, code-level building, and other places where these costs are competitive with conventional building," says Benson. However he insists that the costs are in line with other high-performance builders.
He also projects further price improvements, as the company completes more and more of the work in the factory. For instance it will eventually pre-wire the service layer and install drywall. (Electrical connections between panels will be made with the type of connectors used by modular manufacturers.) And, as mentioned in our last blog, they are getting ready to manufacture fully completed kitchen and bath modules that can be craned into place. "We will get more and more competitive, especially as code requirements go up," he says.
The point is that while Unity works at offsite's cutting edge, it doesn't intend to remain a niche builder. The Keene plant can manufacture 100 homes per year with just one work shift, and Benson has been talking with investors about possible plants in other parts of the country.
As offsite's costs come down relative to stick building, high performance building could soon become the mainstream.
A Unity Net Zero home in Guilford, Vermont
Typical Unity wall section
A. Interior Finish, shown 5/8” thick
B. Service Layer - thickness varies
C. 7/16” OSB - Airtight - Structural Grid Line
D. Structural Insulated Framing @ 24” o/c. Dense Pack Cellulose E. Exterior Sheathing
F. Drainage Layer / Rainscreen
G. Exterior Cladding, Shown 3/4” Thick
To ensure airtightness, field crews place gaskets between all panels during construction. The second floor sits on the interior service layer, which is engineered to transfer structural loads to the main wall panels.
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