Image Credit: Ben Southworth The view is to the north. Since most of its windows face north — the direction of the lake views — the Squam Lake house breaks a basic rule of passive solar design. The photo shows the north-facing sliding-glass doors before retrofit work began.
Image Credit: Ben Southworth A chainsaw retrofit. After the roof overhangs at the eaves and rakes were lopped off (following the “chainsaw retrofit” technique), the seams between the panels of plywood roof and wall sheathing were sealed with peel-and-stick tape.
Image Credit: Ben Southworth Spray foam seals the roof. After the exterior roof and walls of the house were encased in a new shell of 6 1/2-inch-thick structural insulated panels (SIPs), the rafter cavities and stud bays were filled with closed-cell spray polyurethane foam.
Image Credit: Ben Southworth SIP attachment detail. The wall SIPs sit on a steel L-bracket. The SIPs were attached to the existing studs with 9-in. screws.
Image Credit: Ben Southworth Now THAT'S an electric bill. Jane Bindley has accumulated an electricity credit with her local utility, so her electric bill shows that in each of the last 12 months, her electricity usage is zero kilowatt-hours.
Image Credit: Jane Bindley Lots of glass, but no drafts. To retain her view of Squam Lake, Jane Bindley decided to retain extensive north-facing glazing. The doors' triple glazing prevents comfort complaints.
Image Credit: Heather Burkham An open plan. The completed kitchen includes a concrete countertop.
Image Credit: Fletcher Manley Multiple heat sources. Although space heating is usually provided by a ground-source heat pump, the house also has a wood stove.
Image Credit: Fletcher Manley Natural materials. Interior walls were finished with American Clay plaster.
Image Credit: Fletcher Manley
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 PV panels as we could on the south roof. Since we were aiming for net-zero energy, 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: 2008Area: 3,400 square feet (including finished basement)Heating degree days: 7,500Heat loss at design temperature: 23,400 BTU/hAnnual heat load: 8,500 kWhAnnual domestic hot water budget: 2,660 kWhBasement floor R-value: R-25Basement wall R-value: R-40Wall R-value: R-52Roof R-value: R-73Blower door test results: 330 cfm50 (shell area 6,243 square feet)Windows: Triple-glazed double low-e Thermotech windows with fiberglass framesWindow area: 568 square feetNew roofing: standing-seam steelSpace heating system: Water Furnace ground-source heat pump 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 ERVPV array: 7.5-kW array (Sunpower PV modules)Annual PV production: 6,800 kWhDesign/build services: Garland Mill TimberframesEnergy consultant: Marc Rosenbaum
Rosenbaum rose to the challenge. “The house has an incredible building envelope, 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 vinyl 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 sheathing 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 2×6 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 glazing still faces north.
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 test 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 PEX 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.”
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Last week’s blog: “Best Construction Details for Deep-Energy Retrofits”