Twin Cities Habitat for Humanity in Minneapolis has built its first net-zero energy project, a single-family house in the city’s north end that was designed by architecture students at the University of Minnesota.

According to Matt Haugen, communications manager for the Habitat chapter, the house is part of Green Homes North, a redevelopment initiative in a part of the city especially hard hit by the real estate collapse several years ago. There, the city is working with five developers, including Habitat, to build 100 energy-efficient homes.

Since the launch of the project, Habitat has built five or six homes in the area to LEED Silver standards. Habitat did not seek LEED certification for the houses because the process was too arduous, Haugen said, but the experience proved to be good practice for taking construction to the next level. Volunteers started working on the net-zero house in May 2013 and wrapped up their work in November.

The house was designed by architecture students at the University of Minnesota working under the direction of instructors Lucas Alm and Daniel Handeen. Together, they developed plans for the house over the course of a semester.

The Minneapolis project is one of a number of net-zero or Passivhaus homes that Habitat chapters around the country have built in the last several years. Although super efficient designs still represent a tiny fraction of Habitat’s efforts to provide affordable housing, they are popping up all over the place.

Panelized construction speeds work on site

The two-story, 1,618-square-foot house is built on an urban infill lot of 0.11 acre, according to a blog describing the project. Students used Passive House Planning Package (PHPP) energy modeling software and built full-scale mockups for some details, such as window openings in the 16-inch thick walls. There are three bedrooms, 1 1/2 baths, and an additional 816 square feet of unfinished space in the basement. There’s also an unattached garage behind the house.

Above-grade walls are built with panelized 2×6 framing sections assembled at a Habitat facility elsewhere in Minneapolis and trucked to the site. Wall cavities are filled with fiberglass batt insulation, and walls are covered with 8 inches of extruded polystyrene insulation, donated by Dow, for a total R-value of about 58, according to designers.

Below-grade walls are insulated to R-38, with R-20 at the slab edge and R-30 beneath the slab, all with XPS. Rim joists are sealed with 6 inches of polyurethane spray foam insulation, and all wood-to-wood connections are sealed with caulk. The roof is framed with raised-heel trusses and insulated with 24 inches of blown-in cellulose, for an R-value of about 75.

Other features:

• Heating and cooling are provided by two Mitsubishi ductless minisplit heat pumps, with one head per floor.
• Domestic hot water comes from three roof-mounted flat-plate solar thermal collectors. Designers said the drain-back system eliminates heat loss when the sun isn’t shining. The system will provide an estimated 80% of the family’s hot water needs.
• Solar electricity is provided by a 5.7-kilowatt system that includes thirty 190-watt panels manufactured by TenK Solar. The PV panels are mounted on the garage roof.
• Triple-paned windows are manufactured by Richlin. The units have foam-filled PVC frames and spaces between the panes of glass are filled with krypton. Windows have a U-value of 0.13 and a solar heat gain coefficient of 0.22.
• Water-conserving fixtures include toilets that use 1.28 gallons per flush, 1.5 gallon per minute (gpm) bathroom faucets, and 1.5 gpm showerheads.
• Whole-house ventilation is provided by a Zehnder heat-recovery ventilator (HRV). Transfer grilles and fans circulate air between rooms even when doors are closed, designers said. Fresh air is delivered to the bedrooms and the living room; stale air is exhausted from the kitchen and bathrooms.
• Exterior paneling is a mix of cementitious panels and boards mounted in a rainscreen assembly

Total project costs were $213,000, which included the cost of solar equipment and the land, plus the value of in-kind donations but not the estimated 6,000 hours of volunteer labor that went into construction, Handeen said.

Airtightness was tested at 1.28 air changes per hour at a pressure difference of 50 pascals, and the house has a HERS rating of 4.

The idea is to make energy efficiency more affordable

The two instructors who led the design efforts, Dan Handeen and Lucas Alm, cut their teeth on the University of Minnesota’s entry in the 2009 Solar Decathlon, a competition for college students sponsored by the U.S. Department of Energy.

“We both were really struck by the massive investment that went into the Solar Decathlon competition, both time-wise and monetarily,” Handeen said by phone. The experience was a motivator to find local partners as well as less expensive means of achieving the same ends.

A design for the house started percolating in a seminar of 26 students and wrapped up in in a graduate-level studio of 13 students (two of the students were involved in both parts of the process). Work took place over a single semester.

“We did really encourage the students to work in teams and play on each other’s strengths,” Handeen said. “Not only did they get a chance to work on a real world project but they get a chance to work in a real world team fashion. Unlike many of the projects in architecture school where you’re just a lone designer doing it from start to finish, they had to learn how to work together.

Added Alm: “It was a neat way to involve a lot of students at a lot of levels.”

Alm, a practicing architect as well as instructor, said he’s tried pursuing projects of this kind with clients in the past, but often runs into a roadblock because of costs.

“I think the Passivhaus community is confronting this quite a bit now, too,” he said. “It’s like at a certain point this affordability piece starts to make things a little more unobtainable. That’s why it’s great for Dan and I to have these opportunities to test things out and see if they work.”

In the end, says Handeen, one key to keeping costs down is by limiting the size of the house.

“The other thing is how we educate the building and buying public about the value of what they can’t see, such as HVAC and envelope,” he said by email. “People are generally very distracted by finishes and fixtures, rather than seeing how else they could invest the same money that would provide them better cash flow down the line. We really have yet to figure out first-cost optimization in this climate. At current energy prices and current mortgage rates it’s difficult to make it pencil out.”

The future at Minneapolis Habitat? Hard to say

The process was rewarding for the students who designed the house, and the family that now lives in it was “over the moon” to be selected. But a net-zero project is a big step up.

“This was kind of a ‘let’s do it once and see what we learned’ project,” Haugen said. “We’ll go forward with that. I think right after the process we were thinking, ‘Oh, man, that was a big climb,’ and I think it will be a while before we take on something this much out of comfort zone again.

“But we know this is the direction we’re going in terms of we need to make sure that all of our homes are as efficient as possible. The idea behind Habitat is that you create a home that is sustainable for a family both from an environmental standpoint but more importantly from an economic standpoint.”