Passive House Lessons: The Three Rs

Construction of the Spring Valley Passive House is a classic case of researching, rebuilding, and recycling.

“I’m a mechanical engineer and mechanical engineers love to engineer their way out of problems with mechanical engineering solutions,” says homeowner and builder Daniel Colombini. But Colombini admits that better mechanicals alone were not going to be enough to turn the drafty old 1930s home he had bought in Ossining, New York, into a comfortable place for his family.

Research into energy-efficient homes sparked an interest in Passive Houses and convinced him that, for a truly comfortable, long-lasting home, the best option was to start over with a highly insulated building envelope.

The home is equipped with 13.2 kW of photovoltaic panels and 32 kWhs of battery storage—enough electricity to power the home and an electric vehicle or two.

Colombini’s research led him to the U.S. Department of Energy (DOE)’s Zero Energy Ready Home (ZERH) program. The end result was Spring Valley Passive House, a new 3,585-square-foot two-story home that achieved DOE ZERH certification and received a DOE Housing Innovation Grand Award.

His home was also the first project to achieve both LEED Platinum and Passive House certification in Westchester County, and one of the first in New York state.

The home achieved a HERS score of minus 9, providing enough electricity to power the home and an electric car, thanks to the 13.2 kW of roof-mounted photovoltaic panels and two 16 kWh batteries.

“We learned a lot about construction,” says Colombini. “To do ZERH-level construction is not a big budget spend; these kinds of improvements should be done by everyone.”

Colombini says total project costs were less than 10 percent over standard construction for a similar-sized home. Much of that added cost came from the Passive House qualifying windows and doors, which were imported from Poland. To achieve DOE ZERH certification alone would have added less than 5 percent over standard construction, Colombini estimates.

Sustainably built

To meet the Passive House criteria, Colombini went well beyond even the high insulation requirements of the 2021 IECC. The effort starts with a 2-by-4, 16-inch on-center exterior wall that is filled with R-12 mineral wool batts and wrapped with OSB that is sealed and taped at all seams.

To the exterior of the sheathing, Larsen trusses were attached through to the studs. The Larsen trusses are factory-made I-joists or site-built trusses consisting of plywood and 2-by-4s salvaged from the demolition of the old home. The 12-inch-deep trusses were wrapped with a breathable air- and water-control membrane, then stuffed with R-42 of dense-packed cellulose. The membrane was held in place with vertical 1-by-4 furring strips that were topped with hemlock siding.

Lumber salvaged from the original house was used wherever possible on the new construction, and 75 percent of demolition and construction debris was diverted from the landfill. Much of what couldn’t be used on site was donated, given away, or sold for re-use on other projects.

The foundation included about 75 linear feet of existing concrete block wall that was retained from the original building and insulated for reuse as part of the basement walls. The basement and crawlspace walls were insulated with R-40 of XPS; 4 inches (R-20) of XPS was installed under the newly poured slabs.

The roof is a simple gable rafter roof constructed of 18-inch Larsen truss rafters at 16-inches on center. The rafters were topped with 5/8-inch OSB sheathing and a vapor-permeable water-resistant 3-ply membrane underlayment under the asphalt shingles.

On the south-facing roof where the solar panels were to be installed, this underlayment was topped with 2-inch furring strips installed over the rafters. That was topped with another layer of sheathing to provide an attachment surface for the solar panels while avoiding holes in the primary weather and air barrier.

The home is heated and cooled by a variable refrigerant flow (VRF) heat pump system with a heating efficiency of 9.0 HSPF, a cooling efficiency of 16 SEER, and a capacity of 3 tons (36,000 Btu/hour). The heat pump serves three indoor evaporator units (one per floor), each equipped with a MERV 13 filter.

A separately ducted energy recovery ventilator (ERV) provides fresh tempered air throughout the home. The outside air intake is filtered with a MERV 13 filter. The ERV operates at three speeds, depending on air quality needs in the home.

“Providing a ventilation system that is separate from the heating and cooling system allows for simpler ventilation operation that runs constantly, regardless of heating and cooling demand,” says Colombini. “This works well for a Passive House, which often does not require heating or cooling, but always needs ventilation.”

The HVAC was designed to be as simple as possible, using readily available high efficiency equipment. “Our experience shows that overly complex designs may indicate great efficiency on paper,” Colombini notes. “But in practice, systems must be easy to operate and maintain by building occupants and service vendors.”
Project challenges

Even though the project team sought simplicity on the design side, there were some implementation issues. Supplies were sometimes difficult to get due to the pandemic. Reuse of lumber helped to balance some material costw spikes and shortages, as did some creative switching of materials such as using siding made of hemlock instead of cedar.

Another challenge was finding skilled labor that understood what needed to be done to meet the objectives of the project. “Finding contractors who were able to do this was nearly impossible,” Colombini remarks. “Everyone learned a lot on the job.” 

Several training sessions took place on site to teach the subcontractors various envelope assembly details. For example, extreme care was taken to provide a sealed envelope. This required constant supervision and quality control, Colombini says.

Still, Colombini is optimistic. “Many of these strategies could be easily implemented by any design and construction team that is educated on their implementation,” he says. 

Colombini says there is one huge point that the development team must keep in mind. “What are the fastest, most cost-effective ways to meet the design criteria that we’re trying to hit?” he notes. “That’s the question we have to keep asking.” 
That perspective has Colombini excited to take what he learned on this house and try another one. “I’d like to do one in Vermont,” says Colombini. “Going to a colder climate zone, with half the budget, is my challenge.”

Daniel Colombini’s Spring Valley Passive House is a blend of repurposed materials, including its wooden structure and concrete foundation.

Much of the house is constructed from repurposed materials. For example, 75 linear feet of existing concrete block wall was retained from the original building and insulated for reuse as part of the basement walls.

Site-built and I-joist Larsen trusses create a 12-inch wall cavity exterior of the sheathing. Together with the 2-by-4 framing on the interior, they form a wall 16 inches deep that holds R-57 of insulation.