DOE ZERH Program Keeps Energy Efficiency Simple

Three-time ZERH winner TC Legend keeps efficiently lighting things up with its latest energy-saving home.

Builder TC Legend Homes of Bellingham, Wash., aims for quality and sustainability in every home it constructs. Company founder Ted Clifton Jr. has found the U.S. Department of Energy (DOE)’s Zero Energy Ready Home (ZERH) program an ideal way to achieve that goal.

Project Info

Project name: House in the Hollow, Bellingham, Washington

Builder: TC Legend Homes, Bellingham, Wash.,

Completed: July 2021 

“The DOE [ZERH] program is a great benchmark not only for ourselves, but for all those looking to build or purchase a home that is eco-friendly,” Clifton says. “Since the program incorporates ENERGY STAR and Indoor airPLUS, holds high standards for energy efficiency, and is nationwide, it is an easy way to compare homes across the board and that reduces confusion for purchasers.”

TC Legend Hollow HERS Index FULLIt’s also great that the DOE program has a very low-cost barrier, Clifton adds. “There are many other certification programs that charge large fees, making it difficult for small builders to participate,” he says. “Or, the cost of the fees gets passed along in higher home prices, increasing the barrier to purchase for many people.”

Since 2015, TC Legend Homes has built all of its homes to the DOE program criteria. The builder typically constructs one spec home and five or six custom homes for buyers each year. It currently has nine homes under construction. In 2022, the builder was recognized by DOE for its efforts with a Housing Innovation Grand Award in the category “Custom for Buyer under 2,500 square feet.”

That custom home, known as “The House in the Hollow,” became the builder’s third grand award from the ZERH program. The 2022 grand award winner is a 1,935-square-foot, two-story home in Bellingham, Wash. The home achieves a Home Energy Rating System (HERS) index of minus 23 when the solar photovoltaic panels are included. TC Legend’s homes routinely achieve HERS scores of 10 to 20 below net zero. These “positive energy” homes realize enough energy savings and photovoltaic (PV) power production to run the home, as well as an electric car or two, with zero power bills.

TC Legend - South Elevation

“The House in the Hollow” led to the builder’s third grand award from the DOE’s Zero Energy Ready Home (ZERH) program for its energy-saving efforts. The home achieves a Home Energy Rating System (HERS) Index as long as minus 23.

Business as Usual—Only Better

Like all TC Legend homes, the all-electric Hollow model is constructed with structural insulated panels (SIPs) with insulated concrete form (ICF) foundation walls, slab-on grade floors, triple-pane windows, HRV fresh air systems, air-to-water heat pumps, and solar panels.

To reduce heating and cooling loads, TC Legend Homes relies on its tried-and-true base design: A four-inch slab is poured over four inches of R-20 high-density foam insulation. The stem walls around the slab are made of insulated concrete form (ICF) blocks consisting of a six-inch reinforced concrete core wrapped with 2.625 inches of rigid foam on the interior and exterior for an R-23.6 total stem wall.

The ICF stem walls reduce heat loss through the edges of the slab, and also reduce the labor costs involved in forming the stem walls since the easily assembled foam blocks create a stay-in-place form for the concrete.

TC Legend often stains and seals the concrete slab to serve as the finished floor of the home because the exposed concrete readily absorbs passive solar heat and also easily transfers warmth from the radiant floor loops.

The home’s above-grade walls consist of 6.5-inch R-29 structural insulated panels (SIPs) that use a graphite-enhanced expanded polystyrene core wrapped with oriented strand board (OSB). The walls are finished with house wrap and fiber cement siding.

All seams are sealed with SIP tape and any exposed ICF foam is covered with metal flashing. The bi-level roof is constructed of graphite-enhanced SIPs, using 10.25-inch R-49 SIPs for the north-facing roof and 12.25-inch R-59 SIPs for the longer south-facing roof. The SIP roof panels are topped with underlayment and lifetime-warrantied asphalt shingles.

TC Legend - Master bath

Windows are triple-pane glass with three low-emissivity coatings and argon gas fill that reduce heat transfer.

In place of solid wood splines, foam inter-panel connection splines were used in the north roof and I-splines were used in the south roof to support the longer span of the south-facing roof. SIP construction offers seismic category D resistance; the SIPs are rated to last 100 years.

“SIPs also create less framing construction waste than traditional stick framing and are generally stronger than stick framing,” Clifton says. “On top of that, SIPs are super quick and easy to install, making the labor much cheaper than traditional stick framing and therefore helping keep our houses more affordable.”

The TC Legend team has the installation down “like a well-oiled machine,” and can install the SIPs on an entire two-story house in just eight days, Clifton adds. “ICFs are also quick and easy to install,” he says. “They create less material waste and require less concrete, which helps to reduce the home’s embodied carbon totals.”

ABCs of SIPs

SIPs make for a very airtight structure with almost no thermal bridging. The home is slab on grade, so there is no air leakage from the floor. All SIP panel joints are double-bead mastic sealed. Windows are foamed-in and sill plates are sealed with SIP mastic.

After the mechanical trades have completed their work and all windows and doors are sealed, the builder uses a proprietary aerosolized acrylic product that is distributed throughout the home while it is pressurized, with a blower door fan to seal any remaining cracks in the building envelope. The final blower door test on this home was 0.54 ACH50.

To provide mechanical ventilation for their super-tight homes, TC Legend installs heat recovery ventilators (HRV) as the standard ventilation system in all of their homes. An HRV brings in outdoor air that is tempered with outgoing stale air in a heat exchanger to warm incoming air in winter and cool incoming air in the summer.

They added a MERV 17 HEPA filter unit to the fresh air intake of the HRV that includes an inline fan to pull in air that passes through the HEPA filter, which filters out 99.97 percent of particles down to 0.3 microns. The fan-powered filter unit is operated by the same variable-speed switch that controls the home’s powerful range hood fan, so when the range hood fan is on, just the right amount of makeup air is brought in through the HEPA filter.

The incoming air is tempered through the heat exchanger on the HRV to minimize blasts of cold or hot air. The main house ventilation is always on. It can be boosted to a pre-programmed setting of 10, 30 or 60 minutes by timers in the bathrooms. There is a sensor in the main living area that will boost the system as needed to keep CO2 below 1,200 parts per million. There are also humidity sensors in the bathrooms.

TC Legend uses an air-to-water heat pump for efficient space and domestic water heating at a coefficient of performance (COP) of 3.92 (i.e., the heat pump is 3.92 times as efficient at making heat or hot water as an electric-resistance furnace or water heater).

According to Clifton, “This is much more efficient than a heat pump water heater and, because the compressor and fan are outside, it is not scavenging heat from the house or adding noise inside.” Space heating is delivered via radiant floor loops downstairs in the exposed concrete floor slab and via a fan coil unit upstairs, as well as via a heating unit in the supply side of the HRV. The heat pump also provides cooling with a COP of 6.75.


Super Windows

The home is designed to take advantage of passive solar heating and shading with a floor plan that is longer in the east-west direction and puts most of the windows on the south side of the house.

The windows and doors are triple-pane glass with three low-emissivity coatings and argon gas fill that reduce heat transfer. The windows include fixed-glass windows with a U value of 0.15 and a solar heat gain coefficient (SHGC) of 0.46, and casement windows with a U value of 0.18 and a SHGC of 0.34.

The home has only two west-facing windows and five east-facing windows, and these windows have a lower SHGC, thus allowing in less solar heat gain than the north- and south-facing glass. Roof overhangs protect south-facing clerestory and first-story windows from summer sun, while existing deciduous trees help shade east- and west-facing glass.

Recognizing the need to maximize any daylight available in the cloudy Pacific Northwest, TC Legend often incorporates shaded south-facing clerestory windows into its designs. Clifton noted the visible light transmittance of their windows is 0.59 on fixed windows and 0.43 on casement windows.

“This is almost double the light transfer of some other triple-pane windows,” Clifton says. The home’s interiors are painted white to maximize daylight and when the sun goes down, 100 percent LED light fixtures illuminate the interior.

Clifton notes that TC Legend “really lives by the statement, ‘If you’re not actively trying to improve the world around you, then what are you doing?’” He adds that the company “believes in leaving the world a better place than we found it.”

One goal of the company is to start building larger developments of 25 or more homes that are affordable and just as eco-friendly as their current homes. “Owning a (Zero Energy Ready) house should be available to everyone, not just the wealthy,” Clifton says.


Key Features

Air sealing: 0.54 ACH50, all SIP joints sealed with double beads of mastic and tape. Windows have a double gasket. Whole-house aerosolized acrylic sealant.

Appliances: ENERGY STAR washing machine, dishwasher, refrigerator, exhaust fan.

Attic: No attic; vaulted ceilings.

Energy management system: CO2
sensors; programmable HVAC system.

Foundation: Slab on grade: 4-inch R-20 EPS under slab; ICF stem walls 11.25-inch R-23.6.

Hot water: Combination air-to-water heat pump for space and water heating, 50-gallon, 3.92 COP; electric tankless backup water heater.

HVAC: Air-to-water heat pump with 50 percent central air handler, 50 percent radiant, 6.75 COP for cooling, 3.92 COP for heating. Passive heating design with exposed concrete floors.

Lighting: 100 percent LED, passive and shaded vertical glazing.

Roof: SIP gable and shed roof: 10.25-inch R-49 (north) and 12.25-inch R-59 (south) graphite EPS SIPs, asphalt underlayment and asphalt composite roofing.

Solar: 10.56-kilowatt (kW), 33 rooftop panels, microinverters.

Ventilation: HRV to HVAC; timer, CO2 and humidity sensors for boost setting.

Walls: SIPs, R-29: 6.5-inch graphite EPS SIPs; house wrap; fiber cement lap siding.

Water conservation: EPA WaterSense rated fixtures; central manifold with PEX piping.

Windows: Triple-pane, argon-filled, low-e3, vinyl, U=0.15-0.18, SHGC=0.46-0.34.

Other: Low-VOC and recycled-content products. Aging-in-place design. EV charger installed.