Clay Brook Passive House: How New Hampshire’s First Phius-Certified Home Achieved a Record HERS Score of 17

Nestled in the White Mountain National Forest near Gorham, New Hampshire, the Clay Brook Passive House is more than an elegant two-story residence. It is the first Phius-certified single-family home in the state and holds the lowest HERS score (without renewables) ever recorded in the United States at just 17. These are not minor achievements that the owners mention in passing. They have even printed shirts that read, “Ask me how I built the first single-family Passive House in New Hampshire with the lowest HERS Score in the country.” The home proves that ambitious performance targets are achievable even in extreme climates, a lesson that resonates with builders pursuing high-performance residential construction everywhere. For context on how family run home builders how family ties create a competitive advantage can drive project success, the collaborative spirit behind Clay Brook offers a compelling parallel.

The White Mountain Site: Building in One of America’s Toughest Climates

The Clay Brook Passive House sits just eight miles from the summit of Mount Washington, a location renowned for having some of the most erratic and severe weather in the continental United States. The summit has hosted a continuously occupied weather station since 1932, and the highest wind speed ever recorded on Earth (231 miles per hour) was measured there on April 12, 1934. No other weather station is closer to the home. Because HERS raters must use the nearest weather station when determining cooling and heating design temperatures, Michael Browne of Advanced Building Analysis relied on Mount Washington summit data for this project. That extreme dataset is one reason the home achieved its record-breaking score.

The home is located in Climate Zone 6-A, a designation that demands rigorous insulation and airtightness standards. With a gross floor area of 2,217 square feet and a conditioned floor area of 1,885 square feet, the two-story home includes two bedrooms, two offices, two bathrooms, and an unconditioned garage with electric vehicle hookups. The contrast between this project and other novel construction methods is striking. While this home relies on wood framing and dense-pack cellulose, innovative approaches like robotic brutalism inside the first multistory 3d concrete home in the us tackle performance from a completely different material direction, yet both aim for the same goal of durable, efficient shelter.

An Architect Shaped by Hurricanes and the Appalachian Trail

Hans Breaux, principal architect and founder of Portland-based Project CO+OP, is the designer behind the Clay Brook Passive House. A licensed architect in Maine, Massachusetts, and New Hampshire, Breaux brings a unique background to his work. He was in architecture school in Louisiana when hurricanes Katrina and Rita devastated the Gulf Coast. Seeing that level of destruction gave him a visceral understanding of why the built environment must be resilient. Family members who suffered from mold sensitivities in the aftermath drove home the point that resilience extends long after a crisis passes. This realization led Breaux to believe that buildings must be adaptive and responsive to their environment to keep people safe. “That created a sense of, ‘Okay, there’s kind of a problem. There are things to solve,'” he recalls.

After graduating in 2010, Breaux thru-hiked the Appalachian Trail from Georgia to Maine over roughly six months. He settled in Belfast, Maine, where he worked at the Belfast Community Coop and absorbed the principles of cooperative organizing. Those values later became the foundation of his firm. As his career progressed, he worked on cutting-edge projects including the Maine Coast Waldorf High School, the first Passive House-certified high school in the country. The recession-era housing slump in Maine, which saw newly authorized permits fall by nearly 70 percent from their 2005 peak, shaped his understanding of the construction industry’s cycles and the importance of durable, high-quality building. For more on how project teams sustain quality through tough conditions, a look at ibs 2014 a look inside the new new home book offers valuable perspective on residential construction standards.

Performance Data That Sets a National Benchmark

Clay Brook achieved a HERS Index Score of 17 without photovoltaic panels, making it the lowest-scoring home on record in that category. The performance metrics are remarkable for a residence in Climate Zone 6-A. Every number in the table below tells part of the story.

The heating demand of 11.79 kBTU/ft2/yr and peak heating load of 5.5 kBTU/ft2/yr are exceptional given the extreme winter temperatures near Mount Washington. The cooling demand of just 0.64 kBTU/ft2/yr reflects the natural shading from the heavily forested site and the high-performance envelope. An airtightness of 0.019 CFM50 per square foot demonstrates the precision of the construction team’s work. The home was certified under Phius CORE 2021 on June 4, 2024, and the owners installed a photovoltaic system in summer 2025, putting the home on track for Phius ZERO certification. For a comparison of how similar performance targets were met in a different region, see how leed platinum certification and net zero performance lessons from a new hampshire home demonstrate that high performance is attainable across multiple certification frameworks.

Building Assemblies Designed for 250 Years

The owners wanted their home designed so that it would remain comfortable for 250 years. Every assembly decision was evaluated through that lens, balancing natural materials with long-term durability. The foundation begins with a four-inch concrete slab floating on a 16-inch layer of Glavel foamed glass aggregate, a Vermont-made material with an R-value of 1.7 per inch. The aggregate provides excellent drainage and thermal insulation while using recycled glass processed in an electric kiln powered by renewable energy. The effective R-value of the foundation averages R-27. Breaux describes Glavel as “incredibly light; it is like moving bags of potato chips.” A small amount of foam was used under the footings, and concrete was chosen for the polished ground-floor slab despite its higher embodied carbon, because durability justified the trade-off.

The wall assembly uses a double-stud system with a continuous layer of dense-pack cellulose insulation and plywood sheathing, achieving an effective R-value of R-46.8. Inboard of the sheathing, Pro Clima Intello serves as the smart vapor barrier, chosen because plywood is more vapor-open than OSB. An uninsulated service cavity protects the vapor control and air control layers from puncture. Because the assembly lacks exterior insulation outboard of the sheathing, Phius raised concerns about potential condensation on the interior face of the sheathing in extreme cold. The team performed a dynamic hygrothermal model to prove the wall was safe, and the builder installed moisture monitors inside the wall assembly for real-world verification. Two roof assemblies deliver similar performance: parallel chord trusses filled with dense-pack cellulose achieve R-82, while raised-heel gable trusses with loose-fill cellulose reach R-83.

Triple-pane uPVC windows by Logic Windows & Doors, sourced through Pinnacle Windows Solutions, were oriented to capture western and southern light in winter and to frame direct views of Mount Washington on clear days. The mechanical system includes a hybrid air-source heat pump domestic hot water heater, Mitsubishi air-source heat pumps for heating and cooling, and a Zehnder ERV, which Breaux calls the “gold standard” for maintaining comfort. Interestingly, while the house has two heat pump units (one per floor), only the ground-floor unit has ever been turned on, and even that is rarely needed. “The house is incredibly comfortable,” Breaux says. For more on how thoughtful design accommodates family living at any scale, the story of designing small family home stumpf residence lessons offers helpful principles on space planning and occupant comfort.

The Power of Trust and Cooperative Building

Breaux credits much of the project’s success to the relationship between Project CO+OP and Maine Passive House, led by CEO Katrina Bell and founder Jesper Kruse. The team had worked together on previous projects, building a foundation of mutual trust. “There is a lot of trust between me and the builder, and that was a big part of the project,” Breaux says. “If they wanted to do a detail in a certain way, they would literally sketch it on a napkin and send it to me. I would draw a couple lines and send it back to them.” That sense of camaraderie extended to the owners, who Breaux still considers friends. The cooperative principles that shaped Breaux’s early career in Belfast proved directly applicable to high-performance construction: democratic decision-making, collaboration between firms, and a commitment to benefiting the local community.

The owners themselves were deeply involved in the design process. One owner, who works in IT, grew increasingly interested in energy modeling as the project progressed. Beyond operational energy, the owners focused on embodied carbon and prioritized natural materials whenever possible. The driveway uses pervious rather than impervious asphalt. Only a few trees were cleared from the heavily forested site, preserving natural summer shading and minimizing landscape disturbance. The collaborative dynamic between a forward-thinking design team and engaged owners demonstrates how residential projects can push performance boundaries without sacrificing livability. A similar design philosophy appears in canadian modern inside the cantilevered bay of fundy home on a granite ledge, where site-responsive design and material honesty produce equally striking results.

A Model Where Prediction Matches Reality

“All models are wrong. Some models are useful,” building scientists often say. Climate change and the difficulty of modeling human behavior inside a home mean that real-world energy use frequently deviates from predictions, especially during the first year of occupancy. At the Clay Brook Passive House, something unusual happened: the energy model predicted a total annual use of 8,027.51 kWh, and the actual first-year consumption was 8,039 kWh. The difference is barely more than a rounding error. This accuracy did not happen by accident. It resulted from Breaux’s years of Passive House experience, Maine Passive House’s expertise in high-performance construction, and the team’s established working relationship, reinforced through participation in local organizations like passivhausMAINE.

The project demonstrates that high-performance building requires science, refined construction techniques, and materials that maintain integrity for decades. But as the Clay Brook Passive House proves, these demands are far easier to meet when a strong sense of community and shared passion drives the work. The home is not just a technical achievement. It is a model for how trust, cooperation, and a commitment to excellence can produce homes that serve their occupants and the environment for generations. For those looking to improve their own home’s performance, even small upgrades can make a meaningful difference. Advances such as a new spin on toilets how american standards vormax single jet flush technology is changing bathroom plumbing show that innovation in residential construction extends to every room of the house, reinforcing the idea that better building is a whole-home endeavor.