Passivhaus in Chapel Hill: Page 2 of 3

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Traditional bungalow architecture
The traditional bungalow architecture of this home in Chapel Hill, North Carolina, belies its incredible energy efficiency and adherence to the strict principles of Passivhaus design.
15-inch-thick walls
The 15-inch-thick walls have an insulation value of R-42.
The home’s open floor plan
The home’s open floor plan allows for easy distribution of heating and cooling without ductwork.
Floating framed walls
The floating framed walls inside the concrete shell help minimize thermal bridging and heat transfer, and provide wall space for running plumbing and electrical.
4-inch-thick concrete floor
Stained and polished 4-inch-thick concrete floors provide thermal mass to moderate temperature swings and store heat from the sun.
The Seniors enjoy the sunny deck
The Seniors enjoy the sunny deck on the south side of the house.
Well-Insulated roof
Insulating roofs to Passivhaus standards can be challenging because of the high R-values required and the amount of space this takes. In this home, a 10-inch strip of 1.5-inch-thick foam board, which has been carefully incorporated into the trim pattern, beefs up R-values under the eaves.
Insulated roof (detail)
In this home, a 10-inch strip of 1.5-inch-thick foam board, which has been carefully incorporated into the trim pattern, beefs up R-values under the eaves.
Triple-pane, argon-filled windows
Triple-pane, argon-filled windows take up about 100 square feet of the south-facing wall.
An air-to-air heat exchanger
An air-to-air heat exchanger exhausts stale air and brings in fresh air, but keeps about 75% of the heat while doing so.
Air-source heat pump
The air-source heat pump delivers heat to the living room.
LED track lighting system
The high-efficiency LED track lighting system provides significant energy savings compared to conventional incandescent bulbs.
Efficient Washer
Super-efficient appliances save money, energy, and water.
Traditional bungalow architecture
15-inch-thick walls
The home’s open floor plan
Floating framed walls
4-inch-thick concrete floor
The Seniors enjoy the sunny deck
Well-Insulated roof
Insulated roof (detail)
Triple-pane, argon-filled windows
An air-to-air heat exchanger
Air-source heat pump
LED track lighting system
Efficient Washer

Three-quarter-inch CDX plywood was used to sheath the roof, and a self-adhering membrane applied over the sheathing provides a water- and air-tight seal around every nail or screw used to attach the roofing material to the sheathing. A layer of 2-inch-thick open-cell icynene insulation applied to the underside of the roof sheathing helps complete the air seal. Two-by-fours span the underside of the I-joists, to which drywall was attached, and the remaining 131/2-inch space was filled with blown-in cellulose for an R-value of 62.

Pairing Comfort & Efficiency

The home’s passive solar design, plus superior insulation and airtightness, reduce the reliance on mechanical heating systems. Heat from passive solar gain and a Fujitsu ductless heat pump is distributed via airflow through a heat recovery ventilator (HRV). The HRV passes stale indoor air through a heat exchanger, which transfers about 75% of the heat to fresh, incoming air. Although an airtight home is highly energy efficient, an HRV is necessary to ensure good indoor air quality. The 200-cubic-foot-per-minute HRV eliminates the need for bathroom vents, since the HRV also eliminates excess moisture from the home with the rest of the indoor air.

To maximize the home’s efficiency and minimize its energy use, energy-efficient appliances—like an Electrolux electric induction range, a Bosch Axxis front-loading clothes washer, and LED track lighting—are used. Chris and Leigh Ann also prewired their home for a future PV system.

Meeting the Standard

There have been a plethora of green building standards over the past two decades, and rightfully so—buildings are a major energy user and contributor to global climate change. In the United States, 76% of all electricity is used for heating, cooling, appliances, and lighting. Among standards set by the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) certification, Living Building Challenge, and the 2,000-Watt Society, Passivhaus standards stand apart in their simplicity and adaptability to individual climates to achieve the same energy-use goals regardless of location. This adaptability is similar to how today’s building codes function—and Passivhaus standards are probably the easiest of the green standards to transfer to our existing code-enforcement mechanism. Unlike LEED certification (and other holistic strategies, like the Living Building Challenge), Passivhaus focuses only on energy use. Renewable energy or sustainable material use is not brought into the equation, as it is for LEED and other certification programs. This single-minded purpose generally results in a 90% reduction in typical heating and cooling use and a 70% reduction in overall energy use compared to homes built according to today’s conventional standards. Even compared to the standard LEED-certified building, the overall reduction is still about 30%.

It’s hard to say that we should have just one green building standard or another, because of so many new and innovative building materials, the acceptance of grid-tied PV systems, and a broader understanding of how a building functions as part of the living landscape. Fortunately, the PHIUS understands that trying to become established in opposition to the existing green building standards is counterproductive—they are trying to make it simpler to integrate Passivhaus certification with the existing Home Energy Rating System (HERS) Index.

HERS rates a home or building’s energy efficiency—a typical resale home scores 130 on the index; a conventional new home usually scores 100. A negative score implies that a home produces more energy than it consumes—a concept that may be met with some skepticism by green building professionals, since this number does not account for a home’s embodied energy—energy used to extract materials, produce products, and transport them to the building site. While the Seniors’ home does not have a HERS Index score, its blower door test resulted in 0.51 ACH at 50 pascals of pressure, or about one air exchange every two hours. For comparison, an Energy Star home based on the EPA guidelines will have a typical value of 3.5 ACH at 50 pascals—that’s one-seventh as air-tight as the Seniors’ home. Since HERS raters are now common in most parts of the country, this may help make the Passivhaus certification accessible to all potential builders.

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