A Leaky Old House Becomes a Net-Zero Showcase
Renovators wrap a 1970s ranch house with a new SIP shell
Jane Bindley had a dream: to turn her 1978 ranch in central New Hampshire into a net-zero-energy house. How hard could that be?
As it turned out, pretty hard. But with help from a dedicated team of experts and a generous budget, Bindley achieved her dream.
Can a north-facing house be net-zero?
Bindley chose her team wisely. She hired a New Hampshire company, Garland Mill Timberframes, to renovate her home. Ben Southworth from Garland Mill is an experienced design/build contractor. When it came time to choose an energy consultant, Southworth advised Bindley to select Marc Rosenbaum, one of the most experienced designers of net-zero-energy homes in the country.
Southworth doubted that Bindley’s nondescript ranch was worth saving. “I told her, ‘It will cost more money to take it apart than to bulldoze it,’” said Southworth. “But she answered, ‘It’s structurally sound, and I can’t imagine putting the house in a landfill.’ ”
The house sits on the shore of Squam Lake, with a spectacular view of the lake to the north. Most of the home’s windows face the view. “We were killed from a solar perspective,” said Southworth. “The house is up against a big hill on the south side, and the hill has tall trees. We put as many 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 as we could on the south roof. Since we were aiming for 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., the PV array defined what our heat load had to be.” The house ended up with a 7.5-kW PV system.
Location: Holderness, N.H.
Construction completed: 2008
Area: 3,400 square feet (including finished basement)
Heating degree days: 7,500
Heat loss at design temperatureReasonably expected minimum (or maximum) temperature for a particular area; used to size heating and cooling equipment. Often, design temperatures are further defined as the X% temperature, meaning that it is the temperature that is exceeded X% of the time (for example, the 1% design temperature is that temperature that is exceeded, on average, 1% of the time, or 87.6 hours of the year).: 23,400 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h
Annual heat load: 8,500 kWh
Annual domestic hot water budget: 2,660 kWh
Basement floor R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. : R-25
Basement wall R-value: R-40
Wall R-value: R-52
Roof R-value: R-73
Blower door test results: 330 cfm50 (shell area 6,243 square feet)
Windows: Triple-glazed double low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. Thermotech windows with fiberglass frames
Window area: 568 square feet
New roofing: standing-seam steel
Space heating system: Water Furnace ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. with three 220-ft. deep vertical closed ground loops; heat distributed through in-floor hydronic tubing (95°F water); supplemented by a woodstove.
Domestic hot water: 2 solar thermal collectors connected to 200 gallons of storage; electric resistance backup.
Mechanical ventilation: Renewaire ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.
PV array: 7.5-kW array (Sunpower PV modules)
Annual PV production: 6,800 kWh
Design/build services: Garland Mill Timberframes
Energy consultant: Marc Rosenbaum
Rosenbaum rose to the challenge. “The house has an incredible building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., which was an attempt to compensate for the drawbacks of the site and the north-facing windows,” said Rosenbaum. “The south roof gets a lot of winter shading. There was a low-slope roof with a 5/12 pitch, and we couldn’t raise the roof or the solar array because of zoning restrictions. So we needed a kick-ass envelope. The envelope specs came from doing the math.”
Creating a very tight, well insulated envelope
The home’s vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding and roof shingles were stripped and the interior of the house was gutted. Most of the materials removed from the house were recycled or reused.
Adopting the “chainsaw retrofit” approach, Southworth and Rosenbaum decided to cut off the home’s roof overhangs. Once the seams between the existing 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. panels were sealed with peel-and-stick tape, the roof and all of the exterior walls were covered with new 6 1/2-inch-thick structural insulated panels (SIPs). The urethane-insulated SIPs are rated at R-35.
Wrapping the home with SIPs was unusual; many builders would have simply wrapped the house with 6 inches of rigid foam. But Southworth is an experienced timber-framer who prefers to use techniques with which he is familiar — and he's used SIPs for years.
Once the SIPs were attached to the framing and sealed at the seams with spray foam, the house already had a pretty decent thermal envelope. But Southworth and Rosenbaum weren’t done. The next step was to fill the 2x6 stud walls with closed-cell spray polyurethane foam, bringing the R-value of the walls up to R-52. The rafters were sprayed with foam until the roof totaled R-73, while the basement walls were sprayed to achieve R-42. The basement floor was insulated with R-25 of rigid foam.
All of the existing windows were replaced with triple-glazed fiberglass windows from Thermotech. Although Rosenbaum tried to talk Bindley into reducing the area of north-facing glass, she didn’t want to give up her dramatic lake view — so most of the home’s 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. still faces north.
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.-tight
Taping the exterior sheathing and spraying the interior with polyurethane foam created a very tight building envelope. Rosenbaum used a theatrical fog machine to track down a few stubborn leakage paths.
A pre-retrofit blower door test put the home’s air leakage rate at 4,000 cfm at 50 Pascals. After retrofit work was complete, a second 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. showed that the house was now Passivhaus-tight, with a leakage rate of only 330 cfm at 50 Pascals.
What kind of heat pump is most cost-effective?
The house has two heat sources: a wood stove and a water-to-water ground-source heat pump. The in-floor hydronic distribution system uses 95°F water circulating through PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating. tubing. Since the water temperature is significantly lower than the temperature for some radiant floors — especially staple-up systems, which sometimes require 130°F or 140°F water — the heat pump's efficiency is much higher than it would be if it needed to raise the water to a higher temperature.
Ductless minisplit air-source heat pumps are much less expensive than ground-source heat pumps (GSHPs). Although a ductless minisplit requires a little more electricity to operate than a GSHP, a net-zero-energy house can provide the necessary electricity by specifying a somewhat larger PV array than would be needed for a GSHP. The cost of the extra PV modules is generally much less than the incremental cost of a GSHP compared to a ductless minisplit unit.
When asked why he specified a GSHP rather than a ductless minisplit, Rosenbaum identified two main reasons:
- The limited area available for the PV array favored a heating system that used as little electricity as possible.
- Japanese ductless minisplits rated for below-zero outdoor temperatures were not available in the U.S. when the Bindley project was under construction.
“Japanese minisplit units are getting better all the time, and they cost much less than a ground-source heat pump,” said Rosenbaum. “I predict that the ground-source heat pump industry will be eviscerated by the Japanese minisplits.”
Since domestic hot water needs to be at a higher temperature than the water circulating through the radiant floor, the domestic hot water system is entirely separate from the space heating system. The two roof-mounted solar thermal collectors are connected to two hot water storage tanks; backup heat is provided by an electric resistance element.
True net-zero performance
Utility bills confirm that in 2009, Bindley’s renovated house produced 1,732 kWh more electricity than it used. The extra electricity more than balanced the small amount of firewood (0.2 cord) that Bindley burned. That means that the Bindley house is one of only a handful of U.S. homes able to document 12 months of net-zero-energy performance.
Several factors contributed to the home’s performance, including:
- The home’s exceptional thermal envelope.
- The fact that Bindley covers her windows with movable R-7 foam-filled insulation panels at night.
- The fact that the home is only occupied for about half the year.
Although Bindley sends more energy to the grid than she consumes, she still pays electric bills of $21.41 a month; that’s the minimum fee collected by the local utility, regardless of usage.
Getting to net-zero isn't cheap
Now that the U.S. Department of Energy is informing builders to prepare for a transition to zero-energy design and construction standards, it’s worth contemplating the steepness of the road ahead.
Bindley’s house is an exciting example of elegant engineering, but it cost an arm and a leg. Her insulation package cost $110,000; her PV array cost $60,000; her windows cost $37,000; her Warmboard subflooring cost $20,000. I was unable to obtain cost figures for other major components, including the ground-source heat pump (which may have cost upwards of $30,000) and the solar thermal system (which probably cost at least $10,000).
“Depending on how you crunch the numbers, the house cost between $350 and $400 a square foot,” said Southworth. In other words, Bindley’s deep-energy retrofit cost at least $1,190,000.
“The material choices were expensive,” Southworth explained. “Every decision we made was the most expensive option.” According to Rosenbaum, “The energy package could have been done for less money if we had used less spray foam.”
Bindley loves her new home. “I once lived in a 1970s house that was very drafty,” said Bindley. “This house is a pure delight in the wintertime, because it is invitingly warm and the air quality is so good. The air is moist and there are no drafts.”
One striking lesson from the Bindley job: if you’re aiming to build a net-zero-energy home in a cold climate, your envelope is going to end up looking like a Passivhaus envelope. Although some writers have contrasted the Passivhaus design approach with the net-zero-energy approach, in fact the two design approaches show signs of convergent evolution.
To get to the stringent net-zero-energy goal, you need Passivhaus levels of airtightness and Passivhaus levels of insulation — at least if you expect to fit all of the necessary PV modules on your roof.
Another lesson: if you’re aiming for net-zero energy, renovation may cost more than new construction.
A dream job
For energy nerds, the Bindley job is likely to represent the very archetype, the exemplar, the Platonic ideal of the perfect energy-retrofit job. Jane Bindley didn’t want to build a mansion; she just wanted to turn her 1978 ranch into a zero-energy home, and she had the budget to make it happen.
“Jane Bindley is an amazing person,” said Rosenbaum. “She would show up with coolers full of food and drinks for the workers — with more food than even the construction guys could eat.”
Southworth also remembers the job-site meals. “She would come every Thursday and make lunch for us, using ingredients from Whole Foods,” said Southworth. “She was wonderful.” Southworth also noted that Bindley, a physical therapist, “was always dragging guys off and giving them a massage.”
MORE INFORMATION AND PHOTOS: A True Net-Zero Gut Rehab, New England-Style
Last week’s blog: “Best Construction Details for Deep-Energy Retrofits”
- Ben Southworth
- Jane Bindley
- Heather Burkham
- Fletcher Manley
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