A Superinsulated House in Rural Minnesota
A tight building envelope and plenty of insulation mean low energy bills — even for a home with electric resistance heat
Electric resistance heating systems have a bad reputation. While the required equipment is cheap (and sometimes cheap-looking), homes with electric heat are known for their high fuel bills.
Yet some residential designers are beginning to rethink the old prejudice against electric resistance heating systems. After all, if a house has a very tight, very well insulated envelope, the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. can be quite low, and so can the utility bills — even when using an expensive fuel like electricity. Moreover, all-electric homes don’t need a chimney, avoid minimum utility charges for natural gas, and don’t have any worries about fuel storage, fuel fumes, or backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney.. Electric resistance heaters have much fewer maintenance issues than appliances that burn gas or oil.
Finally, if the homeowners ever want to install solar panels on their roof, the electricity usage in an all-electric home can eventually be balanced by a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. array.
Integrated design works well
An excellent example of an energy-efficient all-electric house is one designed by Rachel Wagner (of Wagner Zaun Architecture) for Gail Olson and Erik Peterson in Esko, Minnesota. Gail Olson is the fourth generation of her family to live on the 65-acre farm where the new farmhouse was built. The home was completed in 2009.
Using an integrated design approach, Wagner pulled together a team that included the homeowners, builder Steve Johnson, and energy consultant Michael LeBeau (of Conservation Technologies). Wagner recalls, “I’m proud of how well the integrated design process went, from the site assessment, to interviewing the clients and understanding their needs, wants, and goals, to weaving it all together. It yielded a result that is pleasing and functional and really durable.”
The owners are delighted with their house. Olson said, “I feel incredibly lucky to have a designer and builder who work on low-energy houses in this climate.”
Location: Esko, Minn.
Size: 1,950 s.f. plus 1,200 s.f. basement
Basement walls: R-38 ICFs from TF Systems (concrete core with 4-in. EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. on each side)
Sub-slab insulation: 8 in. EPS (R-37)
Above-grade walls: Double 2x4 walls, 15 in. thick
Wall insulation: Dense-packed cellulose (R-54)
Attic insulation: 22 in. cellulose (R-80)
Windows: Duxton fiberglass-framed windows
Window glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill.: Triple glazing; south fixed windows are U-0.17, SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. = 0.50; east, north, and west fixed windows are U-0.16, SHGC = 0.31
Roofing: Standing-seam steel
Design heat load: 15,100 Btu/h
Space heating: 6 kW electric-resistance boiler and wood stove
Heat distribution: In-floor hydronic tubing in basement slab; hydronic wall radiators elsewhere
Domestic hot water: 105-gallon Marathon electric-resistance water heater
Mechanical ventilation: Venmar HE-1.8 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.
Air leakage rate: 0.4 ach50
Cost: $436,210, including design cost and site work
Annual energy use: 12,858 kWh of electricity plus 1.5 cord of firewood
Homeowners: Gail Olson and Erik Peterson
Designer: Rachel Wagner, Wagner Zaun Architecture
Energy consultant: Michael LeBeau, Duluth, Minn.
Builder: Steve Johnson, Two Harbors, Minn.
The two-story, three-bedroom home follows classic passive solar design principles. Double-stud R-54 walls, R-80 attic insulation, and fiberglass-framed windows with orientation-specific triple glazing all ensure that space heating needs are extremely low.
It’s easy to frame double-stud walls
According to Johnson, the double-wall framing was straightforward. “The window rough openings were lined with boxes made from 1/2-inch OSB,” Johnson said. “The top plates were tied together by one layer of 3/4-inch plywood.”
Johnson is a fan of double-stud walls. “This was the first double-wall house that I had built,” he said. “I think one of the things that impressed me was how simple it was. What we need is ‘better building, less technology.’ It was really just a matter of building another wall inside of the outside one.”
The walls were insulated with dense-packed cellulose, and that part of the job went smoothly. “The insulation contractor had already insulated double-stud walls before, and they are very good at it,” said Johnson. “They knew how to pack it in there. The fire code requires that double-stud walls be divided into compartments with drywall or plywood, every ten feet of linear wall, so we sectioned off the walls, and that helped the insulation crew.”
Like many builders in Minnesota and Canada, Johnson uses interior polyethylene as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.. “We used Tu-Tuf, which is a high-quality poly, as the air barrier and the vapor barrier,” he said. “Adhesives and tapes stick really well to the Tu-Tuf. All the poly seams are lapped at a stud and taped with 3M tape. We used Tremco acoustical sealant between the bottom plate and the subfloor.”
Insulating the rim joists with cellulose
The integrated design process allowed Johnson to provide input on air sealing details. “The rim joist area was tricky, especially because we used floor trusses,” said Johnson. “This was an example of why it was nice to work with Rachel — she brought me in early in the design process, and I made some suggestions. We wanted to end up with a continuous air barrier and good R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. at the rim joists. We wanted the rim joists to have the same R-value as the walls. We designed a system on paper, and we told the truss company that we needed vertical members in the floor trusses in a strategic spot — the right distance away from the end of the trusses, under the plane of the inside wall of the double-stud wall above — so that we could install solid blocking between the trusses. That gave the insulators somewhere to fasten the fabric. After the cellulose insulation was installed, we added a layer of rigid foam as blocking. We caulked the rigid foam on all four sides, which was only possible because the blocking was there.”
Wagner enjoyed the challenge of coming up with a rim-joist detail that avoided the use of spray foam. “Except for the minimally expanding foam used at the windows and doors, it’s a house without spray foam,” said Wagner.
The blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. results — 0.4 ach50 — were gratifying to everyone on the team.
How should we heat the house?
Natural gas is unavailable at the site, and the homeowners and the design team spent some time considering a variety of heating systems. “My sense is that the air-source heat pumps aren’t efficient in our climate,” Olson told me. “Some people are installing ground-source heat pumps, but we felt that the payout was not worth the investment for us. Oil isn’t a very common option in this area. I didn’t want to have a propane tank in addition to electrical service.”
They eventually settled on using electric resistance heat. Instead of installing electric resistance baseboards, however, they went with an electric boiler. “One thing I’ve learned from Mike LeBeau is that a hydronic distribution system offers a lot of flexibility in the future,” said Wagner. “If the owners ever want to switch to propane or add a solar thermal system, they can. Those options wouldn’t be available if we went with electric baseboard units.”
Although many PassivhausA 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. designers warn against the installation of a wood stove in a tight house, Wagner didn’t hesitate to recommend one here — even though the air leakage rate is only 0.4 ach50. A Hearthstone Tribute wood stove was installed. “I have had no difficulties putting a wood stove in such a tight house,” said Wagner. “I’ve done it more than half a dozen times, all in houses testing at less than 1 ach50. In the houses that also have a range hood, we caution the homeowner to pay attention before turning on the fan. Two homeowners report having to sometimes crack a window when the wood stove is used at the same time as an exhaust appliance like the range hood or the clothes dryer. We’ve provided dedicated combustion air routes in a couple of the houses, but not all.”
The homeowners use their wood stove frequently during the winter. “We usually have a fire for 2 or 3 hours in the morning if it’s 10 degrees and sunny, say, or 20 degrees and cloudy,” said Olson. “We build another fire in the evening for 2 or 3 hours. We burn mainly aspen from the farm (we have a lot of aspen) along with some birch. If temperatures are below zero and cloudy, I usually keep a fire burning all day. I do need to crack a window in the basement if I am using our electric dryer and have a fire burning in the stove. Otherwise we get a backdraft from the stove. It’s convenient for me to do this because there is a basement window close to our dryer.”
Energy bills and construction costs
The local electric utility has a complicated rate schedule, but the bottom line is that Gail and Erik pay between 8.6 and 10.2 cents per kWh, including the monthly service fee.
On average, the owners are spending $1,227 per year on electricity, including the cost of space heat. They burn about 1 1/2 cord of firewood each year.
The construction cost of the Esko Farmhouse was $426,210. That figure includes design and construction administration fees ($27,300), the cost of an energy consultant ($1,500), and site costs ($31,410 for the well, septic system, and driveway). The cost for the Duxton triple-glazed windows was $22,000.
When asked what she might have done differently, Wagner said, “I would have been a little more aggressive with the amount of south glazing. I adhered to the usual formula, making the south glazing equal to 8% to 9% of the floor area. But the house has an unusual layout, with the living room in the northwest corner. Because of that I think I could have pushed a little harder on the south-facing glazing, and increased it to maybe 10% or 11% of the floor area. The homeowners are using their heating system more than I expected. Their January heating bills are higher than I expected, and January and February are usually our sunniest winter months. So part of me is wondering, could the house be getting more southern gain? Have I been overworried about overheating?”
Wagner isn’t sure that the heating system was the best choice. “The electric boiler is not what I would usually prefer,” she said. “The owners are using more electricity and less wood than I anticipated. I thought they would use more wood, but the wood stove is just used for supplemental heat. If I had known that, I think I would have urged them to install a propane boiler.”
I also asked Gail Olson if she would have done anything differently. “I could have made the house smaller,” she said. “If we had wanted to make it more efficient, the foundation could have been a slab on grade instead of a basement. It would use less energy if it were smaller.”
A feeling of solidity, and no drafts
According to Steve Johnson, the project was a big success. “The homeowners deserve a lot of credit,” said Johnson. “They set out to build a really good energy-efficient home. They wanted to see how far they could take this. They went to a local design conference in Duluth, where they met Rachel — which was a good move. The design is not boring, but it has no bump-outs or cantilevers or beams that penetrate the envelope. The design was conducive to achieving these goals.”
Olson is very happy with her house. “I like the even heat,” she said. “It is spacious. We have views and a connection to the outside from almost everywhere in the house. The passive solar design is fabulous. The house heats up well in the winter and is shaded in the summertime. I love the deep window sills — I have two 6-foot tomato plants right now. I like the feeling of solidity that comes from these 15-inch thick walls. I grew up in a settler’s log cabin, and all my life I’ve always lived in drafty house. I love having reliable heat with no drafts.”
Last week’s blog: “Occupant Behavior Makes a Difference.”
- Mark Teskey
- Rachel Wagner
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