Green Building / News

Home Reaches Net Zero

August 29, 2011
KEYWORDS net zero / renovation
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green houseThe Rosses are typical in many ways. A young family, with careers, a child, and plans for more, Sara and Gareth Ross decided to put down roots in 2006. They went to Amherst, Mass., where they wanted to build a dream home in an environmentally responsible manner.

They contacted Coldham & Hartman Architects, an Amherst firm that makes “ecologically intelligent design” a focal point of its efforts. There, Tom Hartman convinced Sara and Gareth that renovating an existing home supported their goals. The Rosses rented a home in Amherst proper while they looked for the right renovation opportunity.

Total Fixer Upper

In 2008 they brokered a private sale. The circa-1890 house represented both challenges and opportunities. However, because of the home’s age and poor maintenance record, the Rosses revisited Hartman’s recommendation about renovation: They considered razing it. In the end, however, the environmental burden that would have placed on landfills led them to choose a comprehensive renovation.

New England winters are harsh, and heating costs are a concern; with energy prices at risk of rising long term, this gave the Ross family another incentive. The energy-intensive renovation developed into a project that would include the house as a system.

Total Home System

Their Coldham & Hartman project manager, Andrew Webster, was excited by the challenge of radically reducing an existing home’s energy consumption. “Here was a chance to demonstrate the strategies that could be used in all kinds of existing housing stock, to show that older homes aren’t lost causes,” he said.

In addition to the HVAC system, the project included the home’s static elements — the walls, ceilings, roof, foundation, and thermal barriers. The architects decided to strip the home of its asbestos siding, lead-painted clapboard, and brick veneer, so they could design an optimally efficient building envelope. They proposed foam-in, foam-out insulation to increase the R-value and provide a better air barrier. Interior cavities were filled with an open-cell foam, and the exterior was clad with a foil-faced foam.

Combined with an equally efficient roof assembly, triple-glazed windows, a low window-wall ratio, the resultant airtight envelope provided whole-assembly R-values of 20 for basement walls, 34 for exterior walls, and 57 for the roof. This reduced the estimated peak winter heat load from over 100,000 Btuh to fewer than 30,000. The maximum estimated cooling load for this 3,200-square-foot home was reduced from 6.8 tons to less than 1.5 — over 75 percent reductions in heating-cooling needs.

A Deep Energy Retrofit rebate from Western Massachusetts Electric, the local utility, gives qualifying homes demonstrating a minimum 50 percent reduction in energy a substantial rebate. Based on energy modeling and inspections alone, the rebate allowed the Rosses to recover $32,000 of their investments very quickly.

Net Zero, Baby

As planning proceeded, another potential goal began to look possible — having the finished home achieve net zero status. A net zero building produces as much or more energy than it consumes, with the excess capable of being delivered back to the grid. When significant federal and state economic incentives were added to the budget mix, harvesting solar power looked like an increasingly smart choice, so the Rosses decided to add a rooftop
solar PV system capable of producing 12.4 kW — enough to support all of their electricity needs.

Adam Kohler, a mechanical engineer with Kohler & Lewis, Keene, N.H., recommended a VRF zoning system from Mitsubishi Electric. He was familiar with the technology and the selection software and believed it was the best solution for this home. As a result of the efficiency improvements to the home’s shell, only one exterior heat pump condenser would be needed.

Plus, because the system delivers refrigerant directly to multiple interior fan units, the architects wouldn’t need to account for the space requirements and complexities of a centrally ducted system. It “gave us the flexibility we needed to conserve space without sacrificing comfort or access to the system,” Kohler said.

The products were also accessible in ways that were critical for portions of the required attic installation. Plus, the system would allow the home’s less-frequently used guest suite and office to be zoned separately.

The Rosses moved into their home in June 2010.

While the Rosses managed their energy use carefully, during a warmer-than-average summer, the combination of the advanced thermal envelope, rooftop PV, and the heat pump system kept the family comfortable while producing
50 kWh per day more energy than the home consumed. In the first six months of operation, the family earned $416 from excess electricity returned to the grid. “The electric car comes next,” said Sara.

www.kohlerandlewis.com
www.mehvac.com

Publication date: 08/29/2011

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