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Net-Zero Home Has Power to Spare

Posted by Green Builder Staff

Dec 5, 2017 3:20:15 PM

This net-zero-plus design can spare some solar for the electric car.

Focusing on total cost of ownership helped builder David Jones earn his first-ever Housing Innovation Award from the U.S. Department of Energy for a home he constructed in Litchfield, Conn. The 1,380-square-foot home—the Perkins Road House—was Jones’ first to be certified to the DOE Zero Energy Ready Home (ZERH) program. And it was the first home that could be classified as “affordable” for the west Connecticut custom home builder whose company, Revival Homes, has built a solid reputation over 30 years of constructing high-efficiency luxury custom and antique reproduction homes.

 Revival Perkins solar panels on roof

Revival Homes’ builder David Jones wanted to change the perception of green homes being only for the affluent by building the Perkins Road House, one that was affordable for the average resident.

“There is a perception that ‘green’ or high-performance homes are something that only the wealthy can afford,” Jones says. “We wanted to design a house that would shatter that perception, both when considering the initial cost of construction and even more importantly when considering the total cost of ownership, which we defined as initial construction costs plus energy costs plus maintenance costs.”

Revival Homes chose to build the house to the criteria of the DOE Zero Energy Ready Home program (see box on ZERH requirements).

Jones joined forces with architect Kate Briggs Johnson of Responsive Designs and the DOE Building America research team led by Steven Winter Associates to come up with an affordable high-efficiency design for the Connecticut climate that met the DOE program requirements. Homeowners were enthusiastic about the approach, as the builders’ primary design goal was to minimize the total cost of ownership.

Jones and his team were so successful with the home’s compact design and highly efficient shell that, with the addition of a 7.6-kW solar photovoltaic system, the home achieved a Home Energy Rating System (HERS) score of -12. In other words, the home will produce more power than it uses in a year. The homeowners should have $0 electric utility bills and should also have enough surplus to power an electric car.

Even without PV, the home would achieve a HERS score of 41, well below the 80 to 100 typical of new code-built construction. Jones achieved all of this at a cost of about $135 per square foot, including the PV.

Beyond The Box

 Revival Perkins precast concrete wall

The home’s first-floor walls are made of precast structural concrete wall panels, consisting of R-12.5 of rigid XPS foam—adhered to a 2.5-inch layer of concrete.

The resulting design was a two-story, Cape Cod-style three-bedroom, two-bath home with a simple floorplan and a small footprint. The home’s concrete slab floor was poured and finished before any interior walls were added. The house is only 26 feet at its widest point, allowing the second-floor joists to be clear-spanning, open-web trusses that don’t require interior supports. These two features allow interior walls to be moved easily in the future if uses for the rooms change over time.

Spare conduits were also installed to accommodate electrical and plumbing for a solar thermal water heating system if one is desired in the future. Jones notes this adaptability is important for a home he expects to last several hundred years.

To achieve the very efficient building enclosure at a low cost, Jones asked his design team to “think outside the box.”

For the first-floor walls, they chose precast structural concrete wall panels, which are usually used for basement walls. The panels are factory-made, with an R-12.5 layer of rigid XPS foam insulation adhered to the 2.5-inch-thick layer of concrete and EPS foam wrapping the concrete studs, which are set at 20 inches on center and faced with a steel surface to nail drywall to. The wall cavities are filled with R-23 mineral wool batts, then covered with 0.625-inch fire-code drywall and plaster. The unfaced mineral wool batt is fire-, moisture- and pest-resistant and dense enough to be cut with a saw for a precise fit with RESNET Grade 1 installation quality. All of these components together provide thermal mass walls with an insulation value of R-35.5.

The second floor has walls on the gable ends only. These walls are made with structural insulated panels (SIPs), which consist of two sheets of OSB sandwiching a layer of expanded polystyrene (EPS) rigid foam. The SIPs used on these walls were 10.5 inches thick, providing an R-39 insulation value. A 0.625-inch fire-code drywall was installed over the SIPs and covered with 0.125-inch natural plaster to serve as the inside wall surface while providing additional thermal mass. On the exterior, a dimpled house wrap product covers the SIPs, providing a rainscreen and weather-resistant barrier behind the vinyl shingle siding.

 Revival Perkins concrete slab.jpg

Perkins Road’s concrete slab floor, poured and finished before any interior walls were added, and clear-spanning joists on the second floor allow easy relocation of interior walls in the future.

The roof is also made of 12.25-inch-thick SIPs that provide R-45 of insulation. The relatively narrow roof needed no ridge beam or rafters; the SIPs themselves provide all of the structural support, simplifying air sealing and reducing labor and material costs. The entire roof is covered with self-adhered membrane, which provides air sealing as well as a leak barrier should the 130-mph-rated shingles ever blow off in a storm.

Jones used large SIP panels (8 feet by 22 feet), which enabled quicker assembly with fewer seams to seal. All SIP joints were sealed with a two-part spray foam injected under pressure to fill the entire joint. Seams were also sealed on the interior side of the house with a very tenacious air sealing tape. These air sealing measures plus the solid panel construction reduced air leakage in the home to 0.76 air changes per hour at 50 Pascals pressure difference, well below the 3 ACH 50 required by the 2015 International Energy Conservation Code.

One of the most unusual features of the home is its foundation, or lack thereof. The concrete first-floor wall panels sit at grade on crushed stone. Before pouring the floor slab, the builder graded the site level, dug 32-inch-deep trenches for the building’s perimeter, filled them with crushed stone and continued with 10 inches of crushed stone extending across the entire house site. The concrete walls were then set in place over the trenches.

Then, three layers of 1-inch rigid XPS were set down within the perimeter of the walls with seams staggered and taped. The builder then stacked 8 inches (R-40) of EPS rigid foam inside the base of the wall cavities to insulate the edges of the 5-inch concrete floor slab. Before pouring the slab, he laid down a thick polyethylene vapor barrier, then loops of PEX tubing that could be used for in-floor radiant heat at some future point. But at this point, they are being used to pre-warm well water, as much as 10 to 15 degrees according to Jones, before it reaches the hot water tank. The concrete slab was sealed and stained to provide the finished flooring for the first floor of the home.

Natural Circulation

 Revival Perkins mini split.jpg

A ductless mini-split pump with one inside air handler provides all of the home’s heating and cooling. 

A mini-split heat pump provides heating and cooling. Although the heat pump has only one air handler, located on the first floor, Jones monitored room temperatures over the winter and found temperatures varied less than 4 degrees from the warmest room to the coldest. The home’s passive solar orientation with large south-facing windows contributes to beneficial passive solar heat gain, while the home’s significant thermal mass and extensive insulation provide a buffer against temperature swings during the day and even over several days. The heating system was turned off for several weeks during the winter and Jones found the house never dropped below 50 degrees Fahrenheit.

The home’s heat pump hot water heater is located next to the bathrooms and laundry, which helps reduce wait times at the tap. The kitchen sink is equipped with an on-demand electric hot water heater; this provides nearly instant hot water at the kitchen sink, where hot water is often used in short intervals, without the complications and energy requirements of a recirculation system.

Integral Durability

At the homeowners’ request, Jones added aging-in-place features like a first-floor laundry, master bedroom and full bath with a curbless shower. The first floor is only 6 inches above grade, so it is wheelchair accessible with a low ramp.

Jones felt he was also successful in meeting the clients’ primary goals: to minimize the total cost of ownership (energy cost plus maintenance cost plus construction cost) and to encourage others to follow a similar path. “When total cost of ownership is considered, this house is incredibly inexpensive,” he says.

Simple finishes, factory-built components, minimal mechanical equipment, and a small footprint helped Jones achieve the low construction costs of $135 per square foot (including the solar PV system, but excluding the site, well and septic).

“It has zero energy costs and it has extremely low anticipated maintenance costs (over the next 30 years) compared to typical homes and especially most high-performance homes,” Jones says. “The ductless mini-split system is far simpler than many heating and cooling systems; it is less likely to fail. But if it were to fail, it is easier and less costly to replace than other systems.”

The use of concrete offers even more energy efficiency benefits, Jones adds. The concrete walls can’t burn, rot or support mold, and they never need painting. They will withstand bumps from lawn mowers and storm damage far better than stick-built homes covered in traditional siding. All exterior trim is PVC, which won’t rot. The concrete floor never needs replacement. The PEX plumbing tubes won’t corrode and are unlikely to freeze in the winter. And, home temperature is consistent: Jones notes that the house goes for weeks without the heat on in winter and does not drop below 50 degrees Fahrenheit. 

Perkins Road House Key Features

  • DOE Zero Energy Ready Home Path: Performance
  • Walls: First floor R-35.5: precast concrete wall panels with continuous R-12.5 rigid foam plus steel stud interior wall 24-inch o.c. with R-23 mineral wool cavity insulation. Second floor R-10.5 structural insulated panels (SIPs), draining house wrap, vinyl shingle siding
  • Roof: Full self-adhered membrane, architectural shingles
  • Attic: R-45 12.25-inch SIPs
  • Foundation: Concrete slab has 8-inch R-40 rigid foam at perimeter and R-15 rigid foam under the floor, pre-cast concrete wall panels sit at grade on 3-foot-wide trenches filled with crushed stone and drained to daylight
  • Windows: Triple-pane, vinyl-framed, argon-filled, low-E windows; south facing U=0.18, SHGC=0.39, others U=0.17, SHGC=0.2
  • Air Sealing: 0.76 ACH 50
  • Ventilation: Continuous exhaust fans, auto-open skylights
  • HVAC: 18,000-BTU ductless SEER 21, HSPF 12 air source mini-split heat pump with single inside air handler
  • Hot Water: Heat pump hot water heater, electric on-demand water heater at kitchen sink
  • Lighting: 100 percent LED. Exterior lights on motion and daylight sensors
  • Appliances: Heat pump clothes dryer, Energy Star refrigerator, dishwasher, bath fans
  • Solar: 7.6-kW PV
  • Water Conservation: EPA WaterSense-certified fixtures
  • Energy Management System: Programmable thermostat. Programmable skylights
  • Other: Electric car charging station, passive solar water preheating through concrete slab. No-VOC interior paints and finishes. Passive heating and cooling with thermal mass concrete walls and slab and 0.75-inch drywall. All construction debris recycled. Disaster-resistant concrete and SIP construction

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