Housing the people: Modular passive homes emerge
On an August morning in the Fairmount Heights neighborhood in Maryland, two prefabricated units sat in a parking lot ready to be transported down the street. They were to be lifted onto a carefully constructed foundation on a site in the residential community.
The units had arrived on trucks to this community just outside Washington, D.C., from the factory in Delaware where they had been prefabricated. One unit was the first floor, which was lifted into place in 90 minutes. The other was the second floor unit, which had to have the roof “unfolded”and the eave overhangs attached. These last two steps took place on site so the second story could be transported from Delaware to Maryland more easily.
From the outside, the units looked like simple wood-framed volumes wrapped in the characteristic green of the Huber ZipSystem sheathing, with taped joints, installed windows and the roof fully shingled. On site, prefabricated knee walls were installed and gable panels lifted into place. In less than eight hours, the 1,600-square-foot house was ready for the electrician, plumbing and mechanical contractors to come and make their connections.
At the end of June, Beracah Homes in Greenwood, Delaware, began construction of its first modular passive house. Passive building consists of a set of design principles used to attain a quantifiable and high level of energy efficiency within a specific comfort level to heat and cool the building.
David Peabody Architects had been contacted by the Housing Initiative Partnership (HIP) a nonprofit developer based in Prince Georges County, Maryland. HIP is dedicated to revitalizing neighborhoods and creating home ownership programs for low and moderate income families. It has been providing single family homes for sale and multifamily homes for rent to families since 1988. Since the early 2000s, HIP has included sustainability as a part of its commitment to providing high quality projects, but it had never built a modular or a passive house before.
Peabody believes that there’s a lot of potential for building modular passive house duplexes and fourplexes that could compete in a subsidized housing sector.
Jocelyn Harris of HIP procured a grant from the State of Maryland to build their first passive house. Maryann Dillon, the executive director for HIP, had previously toured the Habitat for Humanity / Parsons School of Design Empower passive house entry in the 2011 DC Solar Decathlon. There she had been introduced to David Peabody and O’Neill Development, Inc.
After receiving the grant to build a passive house, HIP selected Peabody to execute the project, and he proposed using modular construction. Peabody is a certified passive house consultant, and his office had previously designed and built two custom passive houses in Bethesda and Rockville, Maryland. The Bethesda project was a spec project in partnership with O'Neill Development, and the one in Rockville was a private commission.
This project presented the new challenge of being a modular house designed to be affordable to a lower income buyer, a construction process of which Peabody Architects had no first-hand experience. After researching modular construction companies, Peabody decided to partner with Beracah Homes. Beracah had been building Energy Star homes and took this challenge.
Fabrication and Construction
The fabrication of the house started in the factory at Greenwood and the construction of the foundation, related site and utility work took place in Fairmount Heights. The framing incorporated new, nonstandard features into the construction such as double wall studs filled with Densepack fiberglass insulation.
Passive Buildings must incorporate the following:
- Continuous insulation throughout its entire envelope;
- No thermal bridging;
- Extremely airtight envelope (.6ACH@50Pascale);
- High-performance windows (typically triple glazed) and doors;
- A balanced heat and moisture recovery ventilation system to provide constant filtered fresh air when the house is closed and in the heating or cooling mode.
For certification, proprietary energy modeling software is used to determine a building's energy use. Passive houses use, on average, 15% of the energy for heating and cooling as a house of the same size built to code. Some details of the assembly for the modular passive house are listed below. For more technical information about the project please visit David Peabody Architects.
The Modular Passive House Combination
David Peabody believes that modular construction can bring the price of passive houses down within reach of a wider population, creating a larger market for these super-efficient houses. This house is the prototype putting that idea to the test.
The team faced many challenges developing the prototype. An overarching construction goal was to construct as much of the house in the factory as possible to shorten the schedule and control the energy performance.
Peabody, with his associate Izumi Kitajima, who’s also certified as a passive house consultant, studied Beracah's production methods and developed building envelope details that would work with those methods.
For the next project, the highly iterative drawing process of this project will not be necessary and the goal will be that the design, permit and fabrication drawings will all be the same drawing set!
Similarly, the construction of the next building will incorporate efficiencies that the prototype did not have. The prototype was delivered to the site with a completely finished interior; plumbing fixtures and bathroom mirrors were already installed but the exterior had only been completed to the sheathing so if the house did not pass the blower test, any leaks could be much more easily sealed than if the house had the exterior finish already installed.
Beracah had paid such meticulous attention to detail that the first test performed by Conway Energy had a result of .65ACH50Pa (air changes per hour at 50 pascals of pressure). With some minor reparations and additional sealing, the house easily exceeded the passive house standard of 0.60ACH the equivalent of a single 2 3/4”diameter hole in the envelope. The second test result was even better. Passive houses are 10x more airtight than an Energy Star home and 20x more airtight than a standard code-minimum home.
The house was cooled using a window unit, and the temperature difference was only 8 degrees. The greater difference in temperature yields higher contrast images from the thermal imaging camera. The leaks found in the first test were due to some minor mistakes, which were easily rectified; a few joints were taped again that had been damaged by foot traffic and some penetrations that had to be made on site were only sealed with foam and needed to be taped.
There were other challenges with the prototype. Detailing of a modular home has to take into account the shipping of the completed units and this can change the detailing.
When asked about this challenge, David Peabody said, “it requires having both a floor and a ceiling on every unit, so between every floor you have not only floor joists but ceiling joists. Modular construction is done from the inside out rather than the outside in as in traditional stick-built construction. That required us to rethink where the air barrier would be placed (exterior sheathing surface). Similarly the roof had to be hinged and shipped down the road in a way that it could be erected in the field.
That required figuring out how to make sure the hinges didn't penetrate the air barrier and how to break the thermal bridging at the hinges and how best to insulate that attic space. There were just a lot of places like that where we had to scratch our heads to figure out how to insulate and seal and still keep this standard factory construction.”
Peabody believes that there’s a lot of potential for building modular passive house duplexes and fourplexes that could compete in a subsidized housing sector, where the ratio of exterior envelope to net square footage of the building is lower. Peabody Architects has taken what it has learned from this experience building passive houses and is now applying them to deep energy residential retrofits. It is pursuing passive house certification of a landmark building in Washington, now under way.
HIP is trying to justify the 17% greater hard costs of the project over a comparable project using traditional building methods. These costs are primarily in the envelope, double studs, triple glazed windows and a solar hot water heater, which pushes the rate of return to over 14 years. The expectation is that the next project would cost less than the prototype, but the costs are unlikely to come down to make it beneficial for the client.
The ability to shorten the schedule and control the quality of construction makes the modular model very appealing despite the limited standard interior finishes on offer. They will also consider using an energy recovery ventilator in future projects since this is so beneficial to the indoor air quality. The three largest expenses for the clients are mortgage payments, transportation costs, and utility bills, so HIP will continue to study ways to make the homes less expensive to heat and cool.
About the Author
Pamela Sams, AIA, is a technical director at the Washington, D.C. office of Gensler.