Passive Perfection

This New York-area Forever House shows how Passive development is done successfully.

Content courtesy of the U.S. Department of Energy. Edited by Green Builder staff.

When Architect Christina Griffin bought a tiny 1905 cottage in the village of Hastings-on-Hudson, New York, she retained the physical footprint of the old home but gave the property an all-new carbon footprint.

She replaced the 998-square-foot gas- and oil-burning house with a modern 1,713-square-foot, two-story carbon-neutral home that meets the U.S. Department of Energy’s Zero Energy Ready Home (ZERH) certification requirements.

Judges for DOE’s Housing Innovation Awards were so impressed with the zero-carbon “Forever House” that they gave it a grand award for decarbonization. The home also achieved a “Passive House + Source Zero” designation from Passive House Institute US (PHIUS) and was recognized by PHIUS with a Passive Project Design Competition award.

A roof-mounted solar array generates backup power that can be used during outages. A smart control device then directs energy to specific loactions, such as a computer, refrigerator and lights.

This project is the fifth for which Griffin has sought a Passive House designation, and the first home designed by her firm, CGA Studio Architects, to receive a ZERH certification. Griffin first heard about the DOE program through PHIUS, as the two programs have a collaborative relationship. “PHIUS makes it pretty clear on its website that Passive House is a path to DOE ZERH,” Griffin says.

Griffin’s home energy rater, Anthony Lisanti of Integral Building & Design, Inc., also recommended that she consider pursuing a DOE Zero Energy Ready certification.

Griffin has long been interested in sustainability. She built the first LEED for Homes platinum-rated project in Westchester County, N.Y., in 2010. (That home also earned a design award from the American Institute of Architects.)

A speech by early Passivhaus pioneers Dr. Wolfgang Feist and Katrin Klingenberg in New York City in 2011 inspired Griffin to become a Certified Passive House Consultant with PHIUS in 2012 and with Passive House International (PHI) in 2015.

A Passion for Passive

Griffin is an active member of New York Passive House, a network of professionals, builders, and contractors working together to problem solve and promote Passive House projects.

A math whiz since high school, Griffin taught herself the modeling programs used by Passive House. With practice, she learned to run a WUFI (moisture risk) 3D model on a home in two hours. Griffin says the WUFI model provides a clear picture of what impacts different energy-efficiency options can have on a home’s performance.

While Griffin is passionate about Passive House design, she found that many of her clients were reluctant to embrace the approach because they were intimidated by unfamiliar brand names and construction practices.

To help customers become more comfortable with the concepts, Griffin decided to construct a Passive House for herself and her husband that she could use as a model home. The resulting home, completed in June 2022, has already hosted more than a dozen tours for fellow architects, builders, school groups, and members of the public.

The tours sold out within two hours. “Taking a tour takes the mystery out of Zero Energy and Passive House projects,” Griffin says. “It’s really a revelation to people when they can see and feel the energy-efficiency features we include in these homes.” 

One thing Griffin includes in the tour is a 90-second video comparing the thermal envelope of a Passive House to putting on a down jacket with continuous insulation between outer and inner airtight layers. “I’ve put a down jacket on this house,” she says.

Although Griffin has experimented with several building materials over the years, she settled on stud walls and blown cellulose, both for their low-embodied carbon content and because they are common in the construction industry. 

The home’s double walls are constructed of two 2-by-4, 16-inch on-center walls set 8 inches apart, with exterior CDX plywood sheathing on the exterior. A smart vapor control membrane covers the inside face of the interior wall to hold in the 18 inches of dense-packed cellulose, which provides a whopping R-55 insulation value to the walls.

The builder tacked 2-by-2 furring strips over this interior vapor retarder to provide a service cavity behind the drywall so electrical boxes could be set and wiring run without having to put holes in this air barrier layer. 

The exterior CDX plywood sheathing layer was topped with a 4-layer smart vapor-control waterproof membrane with a three-dimensional mesh, which serves as an integrated vented rain screen behind the tongue-and-groove random-width, horizontal knotty cedar siding with a semi-solid stain finish.

The attic is insulated with 18 inches (R-64) of dense-packed cellulose held in place from below by an airtight smart vapor-control membrane. Like the walls, 2-by-2 furring strips create a space between this membrane and the ceiling drywall to provide an air-sealed space for electrical boxes and wiring.

The attic itself is ventilated with continuous ridge and soffit vents. The CDX plywood decking is covered with a taped and sealed weather-resistant membrane and is topped with fiberglass roof shingles.

Carbon Footprint Fighters

Because Griffin is “totally committed to having the lowest carbon footprint we can,” she was very conscientious about the materials she chose, such as the cellulose insulation, which is made from recycled newspapers. That is one reason she chose to reuse the existing concrete foundation walls, rather than breaking them up and pouring new high-embodied carbon concrete. 

Because the basement walls and slab were already in place, Griffin left the basement uninsulated and installed insulation below the subfloor with dense-pack cellulose insulation in the floor cavities, and 1.5 inches of moisture-resistant rigid mineral wool insulation attached to the bottom of the floor joists over an airtight smart vapor-control membrane.

Griffin was able to retain the original 2-by-8 joists and diagonal plank subfloor for 72 percent of the flooring. Where new flooring was installed, she used 2-by-10 floor joists; therefore, 72 percent of the floor was insulated to R-34 and 28 percent was insulated to R-41.

To complete this highly insulated building envelope, Griffin selected triple-pane windows with an insulation value of U=0.15 and a solar heat gain coefficient (SHGC) of 0.34. The PHIUS-qualifying windows were purchased from a nearby vendor in New Jersey.

The house design included large south-facing areas of glass for beneficial solar gain in the winter. To minimize unwanted heat gain in the summer, these windows are shaded by trellises, roof overhangs and a covered porch.

The air sealing strategy for the home consisted primarily of the airtight smart vapor control membrane that wraps the entire interior of the home, including beneath the ceiling insulation, on the interior of the wall insulation, and below the floor insulation, with service cavities between this membrane and the drywall in the walls and ceiling to maintain the air barrier.

In addition, a smart vapor-control weather-resistant barrier and rainscreen was installed over the sheathing on the exterior walls. A similar membrane was installed over the roof sheathing. Also, a liquid-applied air barrier was applied at the windowsills and basement sills. All of these techniques combined to achieve blower door test results of 0.82 air changes per hour at 50 Pascals pressure differential.

ERV in Action

An energy recovery ventilator was installed to bring in fresh air that is tempered and filtered before distribution throughout the home via a separate duct system from the heating and cooling system. The ERV exhausts air from the bathrooms and kitchen.

The ERV has controls to notify homeowners when the filters need to be changed. 

The 1,700-square-foot home is heated and cooled by a single ducted mini-split air-source heat pump with a heating seasonal performance factor of 12.6 HSPF and a seasonal energy efficiency ratio of 18 SEER. The system has one outdoor compressor and one indoor air handler that is ducted to supply all rooms of the home.

The heat pump has a rated output capacity of 21,600 Btu for heating and 18,000 Btu for cooling. Passive solar heat gain in the winter is provided by large south-facing windows in the music room, living/dining area, and all bedrooms.

The roof-mounted solar photovoltaic (PV) array has 28 panels that can provide 10,920 kWh per year of energy. As power is generated, it is sent to the home’s 9-kWh battery to power the home. Surplus power is sent to the grid and the homeowner receives a credit for it on their utility bill. A data monitoring system tracks energy usage and solar power generation.

During a power outage, a smart control device directs power from the batteries to specific locations, such as the computer, refrigerator and lights. Griffin and her husband can control this from an app on their cell phones. The home’s solar power generation is expected to exceed the needs of the house, but the system was oversized to take into account the possible purchase of electric cars.

Training, Retraining—and More

Griffin built this home to not only “talk the talk” but to “walk the walk.” She has also served as a Passive House consultant for several years. In 2013, she co-founded the Center for Sustainable Development, a nonprofit that conducts seminars and workshops on Passive House and green materials for homeowners, architects, engineers, and builders.

Griffin acknowledged that finding or training knowledgeable contractors is a challenge. “Training is really difficult,” she says. “Even if the head framer knows what’s going on, the other guys often don’t. There is a lot of retraining.” 

She arranged to have the manufacturer of the vapor-smart membranes come to the site to teach the workers how to install the membranes and tapes. When she can’t find a Passive House contractor, Griffin sometimes serves as her own general contractor. For the Forever House, she was fortunate to get an experienced contractor and site supervisor to help finish the project.

Future Forever House-similar projects are on the horizon. CGA Design Studio includes three architects and an interior designer. “We do about 15 to 20 homes per year, including single-family new homes, townhomes, mixed-use, renovations, and retrofits,” Griffin says. “We have quite a few 8- to 16-unit multifamily projects in older downtowns along the Hudson River and we are dedicated to designing these buildings to achieve Passive House and DOE ZERH certification.”