Habitat’s New Net-Zero House in Minneapolis
University architecture students have designed a house with 8 inches of extruded polystyrene insulation on the exterior side of the wall sheathing
Twin Cities Habitat for Humanity in Minneapolis has built its first net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. 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 LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. 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 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. 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 2x6 framing sections assembled at a Habitat facility elsewhere in Minneapolis and trucked to the site. Wall cavities are filled with fiberglass batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. , and walls are covered with 8 inches of extruded polystyrene insulation, donated by Dow, for a total R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. 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 XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.. 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.
- 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 PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. 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 kryptonA colorless, odorless inert gas, often used with argon in fluorescent lighting and sometimes used as gas fill in high-performance glazing.. Windows have a U-value of 0.13 and a solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. of 0.22.
- Water-conserving fixtures include toilets that use 1.28 gallons per flush, 1.5 gallon per minute (gpmGallons per minute. Measure of liquid (usually water) flow.) bathroom faucets, and 1.5 gpm showerheads.
- Whole-house ventilation is provided by a Zehnder heat-recovery ventilator (HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ). 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 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. 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 HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. 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(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. 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."
- Twin Cities Habitat for Humanity