DOE's Home Case Study - It's All in the (Envelope) Details

June 30, 2014
July/August 2014
This online-only article is a supplement to the July/August 2014 print edition of Home Energy Magazine.
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“It can’t be done.” Those words were enough to motivate building science educator and consultant Tom Fullam of Vassalboro, Maine, to build his first high-performance house. The home achieved a HERS index score of 38 and earned him a 2011 Silver Energy Value Housing Award from the National Association of Home Builders Research Center. That soon led to a second, even higher-performing house, the subject of this article, which has earned distinction as the first home in Maine to achieve DOE’s Zero Energy Ready Home certification.

The first house, started in 2008, was a 1,250 ft2, three-bedroom, two-bath home with an enclosed sun porch that achieved a HERS index score of 38 before PV panels were installed on the roof (a home built to International Energy Conservation Code [IECC] 2006 would typically score a HERS 100).

The 3.9kW PV solar panels on the detached garage more than meet the electricity needs of this ultraefficient home in central Maine.

fe_Gilbride_photo2One mini-split heat pump, centrally located between the kitchen, living room, and dining room, is well suited to the home’s low 6,600 Btu winter heating load and provides cooling in the summer.

fe_Gilbride_photo3The home’s exterior walls consist of an inner and an outer wood-framed wall, both insulated with mineral wool batt, with a third layer of batt between the two walls. This central layer of 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. It 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.

Table 1. Project Overview

Table 1. Project Overview

PrintFigure 1. 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, 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 ductless heat pump (see green dot). Stale air is drawn from five return vents located in the bedrooms, storeroom, and bathrooms (see red dots), which helps pull the conditioned air through the home.

exterior wall detail_021114
Figure 2. The builder was able to achieve an incredibly low air infiltration rate of 0.49 ACH50 with a meticulously applied air and vapor barrier that was continuous from ceiling to walls to floor.

Fullam has made some changes to the first house, which he currently uses as an office and classroom, to improve its performance even further. He added R-15 of insulation under the floor to increase the insulation level to R-40 between the floor and the unheated basement. He also moved the evacuated-tube solar water-heating panels off the roof, where they were continually getting covered with snow despite the 10/12-roof pitch, and mounted them on a stand-alone rack in the backyard. Then he installed 2.9 kW worth of PV panels on the roof. Fullam estimated that these changes brought the home’s HERS rating down to about 15. His utility bills, which are now down to $380 for the year, are showing that the PV is meeting 50% of the home’s annual 3,950 kW electricity needs.

Raising the Performance Bar

Despite this remarkable performance, Fullam felt that he could do better. And more importantly from his perspective, he wanted to show that he could bring the cost down to something comparable to code minimum construction. “Maine has the oldest housing stock, oldest population, and highest heating-oil dependency in the nation,” says Fullam. “My goal is to reach the $170,000–180,000, 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.” (See “Key Features,” as well as Table 1 and Figure 1 for detailed information on the project.)

Make It Tight

One test of a home’s performance is a whole house air leakage test, which is required as part of the DOE’s Zero Energy Ready Home evaluation. A typical older home in Maine could have air leakage of anywhere from 12 to more than 30 DOE’s Zero Energy Ready Home requires homes in IECC climate zones 5 through 7 to have air leakage of 2 ACH50 or less.

Fullam’s first home tested at a remarkable 1.25 ACH50. The home was so airtight that when the first blower door test was done, the energy auditor had to buy a C ring adapter for his blower door equipment to get an accurate reading. Fullam asked the same energy auditor to test the airtightness of the second home while he was conducting an inspection of the solar-thermal and PV systems. The auditor kept getting an error message because the house was so airtight, and this test was done before the drywall was taped and mudded. He didn’t want to buy an even smaller adapter ring for his blower door equipment, so Fullam waited for the official HERS rating test to find out his blower door results. The HERS rater brought a D ring adapter with him, and the official test yielded an amazing measurement of 0.49 ACH50, below even the superairtight 0.6 ACH50 requirement of the Passive House program.

Fullam attributes the incredibly low air leakage rates to the floor-to-ceiling air barrier layer he incorporated into the wall construction technique he employed in both houses (see “Wall Construction Details” and Figure 2).

After the building’s double-walled shell was constructed, but before any interior partition walls were built, the entire ceiling was Sheetrocked in one unbroken plane. According to Fullam, “There is no waste when you do this. The Sheetrockers can come in with the largest sheets they can handle [4 feet x 4 feet], and there is no cutting around walls and closets so the ceiling goes up fast.” Fullam uses 5/8-inch Sheetrock, which provides a complete fire barrier and a second air barrier between the living space and the attic.

All waste plumbing went through the slab and was roughed in before the slab was poured.

After the wiring was installed in the exterior walls, the walls were Sheetrocked in one continuous pass all the way around the house, before the interior partition walls were constructed, to provide another continuous air barrier along the home’s thermal boundary. “This is now recognized as a preferred practice by the National Association of Home Builders Research Center,” says Fullam.

Most wiring for interior walls was routed through the exterior walls and stuck out of the walls where interior walls would be installed. 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. Although most builders put the access in a hallway ceiling, it’s a challenge to air seal and insulate the hatch adequately while maintaining the ability to open it. Fullam ran an 18-inch-wide gangplank above the insulation from the attic access door to the other end of the house. Any wiring that wasn’t routed directly from the electric box to an interior wall was routed up to the attic and run along a nailboard that Fullam attached to the trusses next to the gangplank. Junction boxes were installed along the nailboard, and wiring for each room was routed to its respective junction box, which was labeled with the name of the room. “Electricians love it,” says Fullam.

Most of the ceiling lights are LED flat fixtures mounted to the ceiling. Although they look like recessed can lights, they attach to regular or shallow octagonal boxes, not cans. Electric boxes for these lights and any other holes for wiring through the ceilings were sealed with spray foam from the attic side. The ceiling deck was then covered with 26 inches of blown cellulose, which settled to a 20-inch-thick layer, providing R-70 of insulation value.

In the bathrooms, Fullam used a vinylceramic composite tile that is warmer to the touch than tile but more durable than vinyl. The product was adhered directly to the slab with a no-VOC adhesive.

Key Features

The following are the performance features that led this Maine home to becoming a DOE Zero Energy Ready Home.

Walls: Double-wall construction 2 x 4, 24 inches on center, two-stud corners, insulated box headers, three layers (total of 10½ inches, R-45) mineral wool unfaced batts, vapor barrier on outside of inner wall, drain wrap, untreated white cedar shingle siding

Roof: Standing-seam white metal roof, SRI 81.6, 6/12 pitch

Attic: 20 inches (R-70) cellulose installed on ceiling deck over vapor barrier

Foundation: 18-inch x 16-inch thickened-edge slab foundation with 4 inches (R-20) rigid foam under full slab at slab edges

Windows: Triple-pane, argon-filled, PVC-framed, low-e, U = .21, SHGC = .40

Air sealing: 78 CFM50, 0.49 ACH50 Ventilation: HRV, 2.04 CFM/W

HVAC: One mini-split air source heat pump, 26 SEER, 10 HSPF Hot water: Solar-thermal, two flat-plate collectors with electric backup

Lighting: 100% interior lighting LED

Appliances: Energy Star refrigerator, dishwasher, washer

Solar: 3.9 kW PV

Water conservation features: All fixtures EPA WaterSense

Energy management system: Smart Meter

Fullam chose a white standing-seam roof with a solar-reflectance index of 81.6. While white roofs are common in the deep South, Fullam explains, “Acadia National Park [off the nearby central Maine coast] has the highest smog problem of any national park in the country, so every cool roof helps.” The attic has gable end vents near the peaks. With a darker roof, the attic air would heat up more in warm weather, sending more air out of the gable vents but pulling more moisture-laden air into the soffit vents. This humid air can condense on the underside of the roof as the roof cools at night, causing mold problems. Fullam’s white roofs cause the attic to heat up less. This pulls less humid air in, so the attic stays drier. And, Fullam notes, with R-70 of attic floor insulation there is little chance of gaining beneficial solar heat through the roof in the winter. Under the standing-seam metal, the roof was decked with plywood, which was covered with a waterproof roofing membrane.

Fullam found an inexpensive triple-pane window manufactured in New Brunswick, Canada. The extruded uPVC framing incorporates the exterior trim, brick mould, and nailing flange, as well as hollow warm-edge spacers and a trim pocket. This pocket provides an easily sealed receiver for the wood or Sheetrock trim that finishes the inside of the window box. Fullam used Sheetrock attached to plywood rather than solid wood for considerable cost savings on the 7-1/2-inch-deep window boxes. On the doors, he attached Sheetrock corner bead to the back of the interior door molding to create a similar pocket. On the first house, he had used Sheetrock for the sides and top and solid wood for the sills, but gaps appeared in corners due to inconsistent shrinkage between the Sheetrock and the wood. With the second house, he went to Sheetrock on all four sides, which provided consistent expansion and contraction, then topped the sill with a decorative piece of slate. The windows consist of three panes of glass. Two panes have low-emissivity coatings to minimize heat loss, and the space between the panes is filled with inert argon gas to help prevent air leakage through any tiny holes in the seal. On the first house, Fullam used windows with a solar heat gain coefficient (SHGC) of .20 and a U-factor of .18, which translates to an insulation value of R-6. On the second home he chose windows with a SHGC of .40 and a U-factor of .20, which translates to an R-value of R-5. “We did it to save birds. The lower-SHGC (R-6) windows have a coating that creates a slight mirror image on the exterior surface. Birds can’t tell it’s a window and they fly into it.”

Where the vapor barrier had to be cut for window openings, Fullam folded it over the interior wall framing around the window opening and taped it. The middle layer of insulation came right to the plywood-backed Sheetrock box, so thermal bridging was reduced even around the windows.

Ventilate Right

Because the home is so airtight, a whole-house balanced ventilation system that would intentionally bring fresh air into the home as well as exhausting stale air was essential for good indoor air quality. A heat recovery ventilator (HRV) was installed in a utility closet next to the kitchen. The HRV pulls stale air from five points in the home: the far corners of the two bedrooms, a storage room, and the two bathrooms. To avoid puncturing the ceiling air barrier, the 6- and 4-inch HRV ducts are routed above a second dropped ceiling installed in the hallway. The stale air is pulled through a heat exchanger and exhausted outside. Fresh air is pulled from a port on the south side of the building (the warmest location for this cold-climate home) and supplied to a central location next to the home’s only heat source, a mini-split ductless heat pump, which is mounted between the living room, kitchen, and dining room. This incoming air is warmed as it passes through the heat exchanger and then further warmed by mixing with the air coming from the heat pump. The HRV’s five return registers help to pull this warmed air throughout the home.

The ductless heat pump is well suited to the home’s low 6,600 Btu winter heating load and has no trouble maintaining indoor temperatures of 70°F in the main living areas and 68°F in the bedrooms. Fullam selected the most energy-efficient HRV he could find (it uses 30 watts to recover 83% of the heat), and he set it to meet the ASHRAE 62.2 standard for continuous ventilation.

The outdoor unit for the heat pump is mounted 36 inches above the ground near the back door. The horizontally mounted solar-thermal panels protect the heat pump and back door from the weather.

The home met all of EPA’s Indoor airPLUS specifications, which is a requirement of the DOE Zero Energy Ready Home program. The site was graded away from the house on all sides. All paints, adhesives, and cabinets are low or no VOC. There is no carpeting.

Using Sun Power

The solar-thermal water-heating panels provide 77% of the home’s hot-water needs. Low-flow plumbing fixtures and a compact design with all hot-water fixtures located within 10 feet of the hot-water tank and central manifold distribution (direct PEX tube piping from the tank to each faucet) help reduce hot-water consumption. Fullam installed 6-foot tubs in the houses. “People think this is a luxury item, but you actually use less hot water with a 6-foot tub than with a 5-foot tub because you don’t have to bend your knees to soak,” he says.

The home is equipped with PV panels. Fullam installed the 3.9 kW system on the roof of the detached garage, which was constructed before the home was started, “so I used the PV credits to build the house,” says Fullam.

Although the home has a HERS rating of 11, it is performing more like a zero energy home. “The PV system will produce 700 to 1,100 kW more than I use annually,” Fullam says. “My December 2013 usage for our all-electric home was 647 kW for the month. It was 772 kW for January 2014, and I have 1,400 kW credits still to use. The HERS rater needed to stick to the formula to compare apples to apples, but it greatly underestimated what the home has actually produced.”

Fullam notes that storms caused a four-day power outage in December. He used a 2,000W backup generator to run a 1,500W space heater for about 50% of the time, and it kept the whole house warm. Fullam notes that the first house was without power during the four-day ice storm and never dropped below 55°F.

Wall Construction Details

To achieve a continuous air barrier from floor to ceiling, the builder took the following steps.

  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 and 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 second top and bottom plates. Install between first top and bottom plates.
  9. Tape top and bottom plate barrier joints.
  10. Attach ceiling vapor barrier with tape (not staples) to joint tape.
  11. Install foam floor pad and tape to floor with wall joint tape.
  12. Install ceiling drywall and exterior wall drywall.
  13. Install and drywall partition walls.

Spreading the Message

Fullam has no immediate plans to build another house himself, but he is involved in the construction of many homes through his classes and consulting work. Fullam teaches to small class sizes, using his first home as both a classroom and a model. “Five people in my recent class all plan to build homes this way in the next year. From previous classes, one home is almost done; another couple have been in theirs for a year,” says Fullam.

“On one house, the husband was a salesperson for a big local lumber company. They sell house plans and whole building packages,” says Fullam. “We took one of their 28 x 48 floor plans and converted it using my energy efficiency recommendations. It cost only $4,000 more than a house built to code, except that it no longer needed a boiler and a chimney. Since a new boiler costs about $13,000 installed, the house was actually projected to be $9,000 cheaper than a home built to code. When the salesman saw that, he said, ‘I’m building it your way.’ He ended up returning $15,000 of his construction loan back to the bank because he didn’t need it.”

Fullam’s reach has extended far beyond his classroom. The lumber company invited him to speak at its annual convention to over 200 contractors. He has been interviewed on WABI TV. Articles about his work have appeared in several local papers and on several web sites, such as Green Homes of Maine. He is also involved in the national Green Homes and Solar Tours.

“My message to other builders is, ‘It doesn’t cost more. You are just redistributing where you spend the money. The carpenters and insulators get more, the heating contractor gets less,’” says Fullam.

learn more

Get more information about the DOE Zero Energy Ready Home program (formerly known as the DOE Challenge Home program).


Fullam was attracted to DOE’s Zero Energy Ready Home program because he wanted a program that included third-party verification. “When I did the first house, people still didn’t believe the results I got. It was really important for me to have that third-party verification to prove it could be done.” Fullam also needed the verification process to be cost-effective for low-cost housing. With DOE’s Zero Energy Ready Home, the only cost for certification is the $750–800 fee to the HERS rater to do the documentation. “Other nationally recognized certification programs are far too expensive for homes in this price range, which limits their acceptance,” says Fullam.

Theresa Gilbride is a researcher with the Pacific Northwest National Laboratory, a multiprogram laboratory managed by Battelle for DOE. Theresa works in residential buildings research, especially in support of DOE’s Building America program.

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