A Passive House with Classic Foursquare Flair
“It was new, it was German, and it had a funny name,” says David Peabody, Peabody Architects, Alexandria, Virginia, “but we saw that it’s the future.” He’s referring, of course, to Passivhaus, a system by which homes are meticulously designed and constructed according to principles developed by the Passivhaus Institute in Germany.
In 2011, Peabody set a new course for his firm by completing, in collaboration with O’Neill Development, the first certified Passive House in Bethesda, Maryland. “And it’s a good thing,” says the architect and Certified Passive House consultant, “because that home has led to all of our other projects since.”
Built on spec, the Bethesda Passive House achieves three goals, Peabody explains. The first is demonstrating that a traditional American four-square can be constructed using Passive House principles. Because there are several other four-squares in the neighborhood, the house would fit in stylistically. Second, the home’s construction incorporated traditional construction materials and methods. Third, building using Passive House principles didn’t add lots of extra costs. “We tracked all of the extra costs associated with making the home a Passive House, and those costs came to approximately 8 percent of the construction cost and 5 percent of the total project cost,” he says.
A growing family bought the five-bedroom house, enjoying it for two years. They moved because they needed more space (several sets of twins were involved). The homeowner, who led a consultancy focused on energy and the environment, told the Wall Street Journal that the family would miss the home’s low utility bills, high air quality, and quiet. They’d also miss the “green cred” the house had with the kids.
“From the first homeowners, we learned first-hand that the extreme claims made about the comfort and energy efficiency of Passive Houses are all true,” Peabody says. “Their monthly energy bills averaged within $5 a month of our energy model predictions, and they never found a difference of more than 1 degree between the basement and the third-floor attic temperatures.”
The new owners, a couple with two teenagers, are equally pleased with the home. “We didn’t have our hearts or minds set on living in a Passive House,” says the homeowner. “But once we realized how energy efficient the house is, we thought it sounded great.”
Moreover, he adds, “It’s a lovely home with lots of light, nice touches, and finishes. It’s also a very practical house. Many newly built homes in this area are oversized. David put a lot of thought into right-sizing the home, and the Passive House concepts make the home comfortable and quiet, with continuous fresh air. It feels natural. We’re happy to know we’re not operating a steamboat of a house.”
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A Box for Passive House Principles
The American four-square is essentially a box, Peabody says, which makes this traditional architectural style—from the standpoint of surface/volume ratio—perfect for a house utilizing Passive House principles. “We pretty much followed what we learned through our research and training, and used energy modeling software to figure out how much insulation we needed,” Peabody says. “We also put together a team of the smartest subs we know and took a real team approach. We hit all the targets pretty easily.”
The entire team was involved, from the documents phases through completion. The concrete and insulation subcontractors helped develop practical details for sub-grade thermal bridge-free construction. The SIP installer and window manufacturer helped develop details for thermal bridge-free window installation. The job superintendent consulted throughout the process. As a result, each team member knew how their work was integral to the success of the entire project.
The walls of the 4,120-square-foot home are eight-inch InsulSpan SIPs covered with two layers of expanded polystyrene for an R-value of 36, with Tyvek and vertical furring strips creating a rain screen. HardiePlank fiber cement clads the exterior of the house.
The roof is 12-inch SIPs, with standard asphalt shingles installed over felt, for an R-value of 45. The team used four inches of EPS on the exterior of the foundation and 5.5 inches of fiberglass on the interior to insulate to R-44. The concrete slab has four inches of EPS with a vapor barrier underneath and is R-20.
The Thermotech triple-glazed windows have insulated sashes and frames. At the entrance is a Mastergrain fiberglass door with EPS insulation in a Thermotech frame. Deep roof overhangs protect the second-floor windows from overheating in the summer. Motorized awnings decrease solar gain in the summer on the west side of the home. On the south side of the house, porch overhangs shield the interior from the summer sun.
“We made the mechanical system as invisible as possible,” Peabody says, “so when you walk into the home, you don’t notice anything different than you’d find in a regular home.” The house has two Zehnder ComfoAir 350 energy-recovery ventilators. One serves the attic and second floor; the other the basement and first floor.
“The Passive House relies on an ERV [energy recovery ventilator system], which performs like a giant fan that brings in air from the outside and exhausts air from the inside,” Peabody says. “The ventilation system makes this house a healthy house, because it continuously vents old air from the kitchen, bathrooms, and laundry, and replaces it with fresh outside air.”
A hot-water line from the gas boiler runs to a heat exchanger. The boiler also supplies hot water to an insulated tank with an integral solar heat exchanger, which is ready to accommodate a future solar hot water system. A 36,000 BTU Mitsubishi ductless mini-split air-source heat pump helps with cooling and dehumidification during the summer months.
As the project was built as a prototype, the design team installed a monitoring system that collects data about interior air quality, temperature and humidity, and energy used by each piece of equipment. The current homeowners pay about $57 a month on average for cooling and heating. The average energy bill is $280 a month in the Bethesda area.
Living in a Passive House
Before purchasing, the current homeowners learned a lot about the house while listening during inspections. “The inspectors repeatedly mentioned how thoroughly the house was built, and how clean and organized the systems are,” the homeowner says. “So far, we haven’t run into any issues. The people who maintain our systems are great. They come out and do maintenance twice a year, and that’s it.”
“Living in a Passive House is great,” he adds, “and the Passive House approach is a great idea if you’re considering building a house. This home is solidly constructed and will be standing for hundreds of years. It’s warm in the winter, cool in the summer. The overhangs and awnings control passive solar throughout the year. The design team incorporated lots of clever details. All of this adds to our quality of life in this home. It all makes a lot of sense.”
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Camille LeFevre
Camille LeFevre is an architecture and design writer based in the Twin Cities.