Passive House, Actively Green
- By Jesse Pyles, Doug Fox
- April 1st, 2013
Unity College in central Maine is a small liberal arts college with a big voice in the national sustainability conversation. We take seriously our leadership role in higher education and across sectors, preparing our students for leadership roles of their own in a changing world. From our unique sustainability science focus throughout the curriculum, to our first-in-the-nation commitment to divest our endowment from fossil fuels, we aim to model viable approaches to sustainability education that improve learning, engage the community, and decrease environmental impact.
This approach to sustainability informs our planning and campus building efforts as well. In recent years we’ve undertaken several infrastructure projects that advance green building and renewable energy well beyond our campus borders, working with pioneers in the field to demonstrate the viability of these groundbreaking approaches. In 2008 we partnered with Bensonwood Homes and the Massachusetts Institute of Technology (MIT) to build the first LEED Platinum, net-zero energy residence for a college president: the Unity House. We’re the first college in Maine to finance a solar energy project through a unique power purchase agreement (PPA) with ReVision Energy. Now, Unity College is home to another first, the TerraHaus.
When it was completed in August of 2011, the TerraHaus opened as the first college or university residence hall built to the Passive House standard, one of the strictest building energy performance standards in the world. After nearly two years housing Unity College students, TerraHaus has not only lived up to its energy-efficient billing, but last year received the official designation of Certified Passive House by The Passive House Institute US of Urbana, IL. At its most basic, Passive House certification demands that a building uses 90 percent less energy for space heating than a similar building in the same climate.
TerraHaus was designed and built by GO Logic Homes of Belfast, ME, with landscape design services by Ann Kearsley Design of Portland, ME, and significant input from students, faculty, and staff. The integrated design approach to TerraHaus ensured that Unity’s sustainability values and educational goals were primary considerations in the development and construction of the house.
The 2,100-sq.-ft. residence is modeled to use the equivalent of 50 to 75 gallons of oil per year for space heating, less than 10 percent of the heating load for a home this size in this climate. It achieves this remarkable level of performance through efficiency from: 1) solar orientation, 2) superior air sealing, and 3) super-insulation. The TerraHaus relies primarily on these “passive” space heating methods with only minimal backup heating from active mechanical systems as needed — in this case, a cold-climate heat pump, a heat recovery ventilation system, and small electric baseboard units. Let’s consider passive heating strategies more closely.
The placement of a house can have a significant effect on the solar energy that is available for heating the structure. TerraHaus takes the angle of the sun by season and the features of the site into account to maximize solar gain in the winter and reduce the amount of sunlight entering the house in the summer. Designers chose German windows that allow more solar radiation and less heat loss than were commercially available domestically. The concrete slab and kitchen island of TerraHaus absorb that solar heat during the day. (Thermal mass releases heat during the day too, but on a net basis it absorbs more than it releases.) At night, this heat is released at a rate related to the drop in temperature, counteracting the loss of solar gain. Passive solar designers use various formulas to optimize the ratio of glazing to thermal mass.
Superior Air Sealing
Air infiltration is one of the main energy concerns with conventional housing construction. By reducing or eliminating leaks — from gaps in door and window frames, the meeting point of different building materials, perforations into the attic from wiring and plumbing (to name just a few points of concern) — the home’s demand for space heating is significantly reduced. The Passive House standard for air sealing is rigorous, requiring mechanical ventilation to ensure adequate air exchange for occupant comfort. TerraHaus relies on a heat recovery ventilation system (HRV) to reduce heat loss during fresh air exchange.
Some air-sealing measures in TerraHaus include meticulous taping and caulking between SIPs (see below), and the use of thick polyethylene sheeting that extends from beneath the foundation up the length of the exterior wall. GO Logic estimates that the extra air-sealing measures used to achieve the Passive House standard can add $2,200 to the cost of a home, but can lead to about a 27 percent reduction in fuel use, making the payback period quite short.
Conventional framing has some energy conservation drawbacks that are reduced with Structural Insulated Panels (SIPs). In traditional construction, the framing lumber itself forms thermal bridges between the conditioned interior space and the outside of the building, leading to heat loss in the winter. Additionally, traditional framing can be difficult to air seal. The SIPs for the TerraHaus are “sandwiches” of oriented-strand board sheathing with a graphite-coated expanded polystyrene foam core (chosen for its low global warming and ozone potential). They form a continuous barrier without thermal bridging from studs and run the entire height of the wall. The SIPs were custom cut in the factory and the joints between panels were carefully caulked and taped on site. Cellulose was added to the interior stud wall cavities for a total wall R value of 50.
A Model for All
Unity College and GO Logic developed TerraHaus as a model for higher education, the building community, and the region. The GO Logic team has presented widely on the TerraHaus, as have Unity faculty, staff, and students. Professor Doug Fox’s Passive House course introduced students to building energy concepts and engaged them in home-weatherization outreach in the local community. Since its completion, the College has hosted dozens of tours and special events at TerraHaus, encouraging visitors to rethink green building in Maine.
TerraHaus has also gained broader recognition from the architectural community, winning numerous awards. Eco-Structure magazine has named it their residential Evergreen Award winner and Builder magazine gave it their Builder’s Choice award last year. TerraHaus also won the EcoHome 2012 Design Award program’s Grand Award, and the highly competitive New England Design Award from the American Institute of Architects. These juried prizes are a testament to the balance that TerraHaus achieves between architectural design and sustainability performance.
The Passive House standard is a viable, achievable benchmark for building performance in cold climates. TerraHaus further demonstrates its applicability for student housing and meaningful learning on college and university campuses.
Jesse Pyles is sustainability director and senior fellow for the Environmental Leadership Program and Doug Fox is professor of Sustainable Agriculture and director of the Center for Sustainability and Global Change at Unity College in Unity, ME. Mr. Pyles can be contacted at email@example.com. Mr. Fox can be contacted at firstname.lastname@example.org.