This fall, Unity College will boast two cutting edge buildings on campus, Unity House and TerraHaus. How do they compare, and how do they both help Unity promote green building?
The first Unity College building to garner national attention was Unity House, built in 2008. This net zero building was the first college president’s house to earn LEED’s Platinum rating. It was designed and built by Bensonwood Homes. Tedd Benson believes in building for the long term and his Open-Built Concept of “disentangling” systems—plumbing, electrical, framing, siding, etc.—makes for easy upgrades and adaptation over time. Unity House will house its second president and family this July, but the building is laid out in a way that it could someday be converted to multiple classrooms or offices.
Unlike most Bensonwood homes, Unity House’s style has a post-modern ironic feel to it. Rather than confining AdvanTech OSB to its usual hidden use as subflooring or sheathing, Bensonwood’s Hilary Harris, working with environmental educator and presidential spouse Cindy Thomashow, used it as interior trim. The siding includes corrugated metal more often seen on an industrial building than on a $400,000 campus residence. Like several other artistic statements found in the design, though, the trim and siding convey an environmental message, generally about waste re-use or recycling. Advantech is a high quality product made from low quality trees, and the corrugated siding is from recycled metal. Do they work? This innovative building pushes the envelope and invites viewers to answer that question for themselves.
The key sustainable attribute of Unity House, however, is not its recycled content but its net zero performance. Passive solar windows for space heating and a solar thermal system for hot water captures on site much of the energy needed by the home. Its heat pump and all appliances are powered electrically, and its active solar system, a 5.4 kV photovoltaic array, generates more electricity than the building uses.
The PV system we use in Unity House is grid-tied. Solar PV systems generate electricity only when the sun is shining. For continuous access to electricity PV systems store energy by generating more than is needed by the building while the sun is shining, and draw from that storage when the sun is not. Off grid systems use a bank of batteries for storage, and these batteries have a number of environmental concerns as well as limited storage capacity. Grid tied systems are connected to the conventional energy transmission system, the grid, which acts as a huge battery for home solar systems. While the sun is shining, the PV system may generate 4-5 times what the house requires and the excess is sent to the grid where others can use it—commonly referred to as “running the meter backwards.” When the solar system is not generating electricity, the house pulls energy from the grid the way any other home does. Through an accounting system called net metering, the electricity supplier credits the electricity that we generate and subtracts it from our electric use before calculating our bill. At present, the utility companies in Maine do not pay small generators of electricity so we don’t benefit financially from the net annual excess we generate (monthly excess is credited to the next month until the last month of the year). Our excess generation during hot, sunny, summer days, however, helps reduce Maine’s peak usage due to air conditioning because prime air conditioning usage and prime solar collection often coincides.
A Different Design and Message for TerraHaus
The central message of Unity House is that net zero is achievable. To reach net zero, however, superinsulation, superior air sealing and passive solar gain are just as important as the showy PV panels. Having given numerous tours of Unity House, I realized that this part of the message was getting lost. Our visitors tend to focus on the solar panels and the recycled content rather than on Unity House’s low energy requirement. While Unity House does not achieve the energy conservation levels of TerraHaus, it is close. (At less than 5,000 Btu/square foot, TerraHaus will use only 10% of the energy used by code-compliant homes in our region.)
Someday we may add solar photovoltaics to make TerraHaus net zero, but for now TerraHaus unambiguously sets a new standard for energy performance in a residence that will look and feel like it grew out of its rural New England site. Designing to Passive House certification standards for insulation, air sealing and passive heating reinforces the message that before people think about renewable energy they should think about how to reduce their energy consumption. Through careful design, this low energy consumption can be achieved without sacrificing quality of life. We hope that this beautiful residence reinforces that message.
Doug Fox, Director, Center for Sustainability and Global Change, Unity College