Source: Energy.gov, Office of Energy Efficiency & Renewable Energy. Photos courtesy of Thomas Fullam
From the US DOE
“It can’t be done.” Those words were enough to motivate Tom Fullam of Vassalboro, Maine, to build his first high-performance house, which earned him a 2011 silver Energy Value Housing Award from the National Association of Home Builders Research Center. It was soon followed by a second, even higher performing house, the first home in Maine to meet the requirements of the U.S. Department of Energy’s DOE Zero Energy Ready Home certification.
Fullam, a building science educator and construction consultant, had been running load calculations to determine sizes for heating systems and started wondering “what would it take to build a house so efficient you wouldn’t need a boiler?” This is a big question in a state where 70% of homes have oil boilers – a higher share than any other state in the union.
On a recent consulting project, Fullam showed the homeowner how the energy-efficiency measures he suggested would only add $4,000 to the cost of building his new home, compared to one built to local code. Except that the home no longer needed a boiler and a chimney, which costs about $13,000 installed, so the super-efficient home was actually projected to cost $9,000 less than a home built to code. “When my client saw this, he said ‘I’m building it your way,’” said Fullam.
Fullam plugged in some aggressive insulation and glazing values and got a 1,500 ft2 home in his central Maine climate, with a projected heating load of under 10,000 BTUs. Since this is in the International Energy Conservation Code’s (IECC) zone 6, none of his local builder friends believed it was even possible. So Fullam decided to build one.
One minisplit heat pump provides all of the heating and cooling the 1,200-ft2 home needs. A heat recovery ventilator (HRV) provides needed ventilation for the super-airtight home. The HRV and its ducts are installed above a second ceiling in the utility closet, keeping the HRV in conditioned space and leaving the primary ceiling air barrier intact. The HRV brings in fresh air, which is warmed by the heat exchanger, then further warmed as it enters the living space near the heat pump. Stale air is drawn from return vents in the bedrooms and baths, which help pull the conditioned air through the home.
The first home, started in 2008, was a 1,250-ft2 single-story home with an enclosed porch and unheated basement. It achieved a HERS of 38 before photovoltaic (PV) panels were installed on the roof; this is compared to standard construction of HERS 100. His home had double-wall construction with 8.5 inches of dense packed cellulose and 3.5 inches of fiberglass batt in the wall cavity, for an insulation value of R-40, and blown cellulose covered the ceiling deck to a settled depth of 18 inches for an R value of R-60. These far exceeded local practice of R-19 for walls and R-30 for ceilings. Triple-pane windows provided an insulation value of R-6, where R-2 to R-4 is common. A solar water heating system provided for radiant floor heat and domestic hot water. Fullam also added 2.9 kW of PVs on the roof, bringing the HERS down to about 15.
Fullam wanted to do better by showing he could make the costs of such buildings comparable to code-minimum construction. “Maine has the oldest housing stock, oldest population, and highest heating oil dependency in the nation,” he said. “My goal is to reach the $170-180,000, turnkey market for older people on a fixed income. They want a house that is two bedrooms and two baths, with a garage. I want it to be high-efficiency, high-quality construction, fully handicapped accessible, and low cost.”
WALL CONSTRUCTION SEQUENCE
1. Install outer wall.
2. Install ceiling joists and foam block.
3. Install vertical batts in outer wall.
4. Tack horizontal batts to outer wall studs.
5. Tape wall vapor barrier to ceiling joists & foam.
6. Attach top plate of inner wall to ceiling joists with vapor barrier laid over top plate and draped to floor.
7. Lay bottom plate over bottom end of vapor barrier. Pull barrier taut and tape to floor slab. Screw bottom plate to slab.
8. Construct inner wall of studs and 2nd top and bottom plates. Install between 1st top and bottom plates.
9. Tape top and bottom plate-barrier joints.
10. Attach ceiling vapor barrier with tape (no staples) to joint tape.
11. Install foam floor pad and tape to floorwall joint tape.
He built a second home with a HERS score of 35 without PVs, or 11 with them, based on projected PV production. It turned out the home’s 3.9-kW photovoltaic system produced a surplus of 1,100 kWh the first year, meaning the home a true net-zero energy building.
Blower door tests for whole house air leakage were conducted on both homes. A typical older home in Maine might have air leakage of 12 to 30 or more air changes per hour at ACH 50 (50 Pascals of pressure). ENERGY STAR for Homes (Version 3) and DOE Zero Energy Ready Home require air leakage rates of less than or equal to 4.0 and 2.0 ACH 50, respectively, in IECC climate zones 5 through 7. Fullam’s first home tested at 1.25 ACH 50, and the second home tested at 0.49 ACH 50. This is below even the super-airtight 0.6 ACH 50 requirement of the Passive Haus program. Fullam attributes the incredibly low air leakage rates to the floor-to-ceiling air barrier layer he incorporated in the double-wall shell used in both homes. Following guidance published by Building America research partner Building Science Corporation for cold climates, he installed a polyethylene fiber vapor barrier on the outside face of the inner wall using tape so there were no air holes from staples. This vapor barrier, connected to the ceiling and floor vapor barriers, completes a continuous air barrier for the entire building envelope (see figure and sidebar for construction sequence). All wiring went through the inner wall and plumbing went through the slab to avoid holes in the air /vapor barrier. (Note vapor barriers are not recommended in the mixed or hot climates.)
Fullam chose mineral wool to insulate the walls of the second home because of its higher R value and resistance to fire, moisture, and pests. The attic floor was covered with R-70 (26 inches settled to 20 in.) of blown cellulose. He used advanced framing techniques like two-stud corners and insulated headers. He pre-drilled the inner wall studs with a hole at 24 in. from the bottom plate to run wiring.
After wiring was installed in exterior walls and before any interior partition walls were built, the entire ceiling and perimeter walls were sheet rocked. This provided a complete fire break and a second unbroken air barrier. “There is no waste when you do this,” said Fullam. “The sheet rockers can come in with the largest sheets they can handle (4’x14’) and there is little to cut around so it goes up fast.”
Wiring for ceiling fixtures ran up through exterior walls to the attic. Fullam put the attic access on the outside wall on a gable end of the house then ran an 18-inch-wide gangplank on top of the insulation from the attic access door to the other end of the house. Any wiring routed through the attic went to junction boxes labeled by room name and fixed to a board running along the trusses within easy reach of the gangplank. “Electricians love it,” said Fullam.
The home’s only heat source, a ductless mini-split heat pump, is well suited to the home’s low 6,600-BTU winter heating load. Solar thermal water heating panels provide 77% of the home’s hot water needs. To reduce hot water consumption, Fullam installed low-flow plumbing fixtures and designed a compact plumbing layout where all hot water fixtures are within 10 feet of the tank and hot water is distributed through a central manifold with direct PEX tubing from the tank to each faucet.
The exterior wall’s center layer of mineral wool batt extends between the inner and outer wall framing to stop thermal bridging at the windows. The vapor barrier (a translucent mesh fabric) covers the outer face of the inner wall and is wrapped around the framing and taped to the inside face of the inner wall with a rugged air sealing tape. A plywood and drywall box is constructed to line the window opening after the triple-pane windows are installed. This drywall and plywood box is less expensive than solid wood, has less cracks in the corners from expansion and contraction, and fits into a preformed pocket in the window frame to form an airtight seal around the window.
Fullam estimated the second home cost about $4,800 more to build than a home built to the 2006 IECC, including purchase and installation of the PV and solar water heating systems. The HERS rater projected annual energy cost savings of $2,587, for a total projected annual utility bill of $320 when the PV production was included. However, as noted earlier, the PV has actually been performing better than projected and Fullam saw PV production revenue of $734 for the first year based on utility credits for the surplus power.
Fullam is recommending DOE Zero Energy Ready Home to all of the participants in his home building classes as well as to clients in his construction consulting business because he can show what these energy savings can mean for homeowners, especially those on fixed incomes. Both the building’s history and the third-party verification that is a mandatory part of DOE Zero Energy Ready Home proves “that it can be done.”
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