November 2012

Volume 21, Number 11

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More Wall, Less Passive Solar Makes Net-Zero Homes Affordable

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EcoTerra.jpg

EcoTerra, a net-zero-energy house in Quebec, uses a ground-source heat pump, retractable awnings, and a building-integrated photovoltaic system from which waste heat is recovered for domestic hot water and space heating.

Photot: Canada Mortgage and Housing Corporation

By Erin Weaver

There are many ways to achieve net-zero energy. “Affordably” is not the adverb that typically comes to mind first. But a report from the Canadian government outlines key strategies for building net-zero-energy (NZE) houses at a reasonable price.

“Identifying Affordable Net Zero Energy Housing Solutions” highlights current best practices, identifies technologies that “currently do not exist but which would be very useful,” and proposes “technology suites” for future NZE construction. Manitoba engineer Gary Proskiw, who prepared the report for Natural Resources Canada, defines the lowest-cost technology suite as the one yielding “the most affordable NZE house possible while emphasizing reliability and durability.” He adds that “maintenance concerns are minimized since off-the-shelf products are used.”

After gathering input from designers of a dozen NZE homes in Canada, Proskiw advocates:

• light- or medium-weight framing

• maximum south-facing glazing equal to only 6% of the floor area

• electric baseboard heat

• graywater heat recovery

• high-efficiency heat recovery ventilation

• walls insulated to R-60 (for northern climates)

• attics insulated to R-80 (for northern climates)

Proskiw is generally critical of passive solar as a key part of NZE design. He analyzes the economics of window selection, concluding that excessive south-facing glazing and top-of-the-line windows are not likely to be cost-effective.

Taking a 167 m2 (1,800 ft2) house as a model, Proskiw estimates the R-44 walls cost $170/m2 ($15.80/ft2) to build, while a one-square-meter, triple-glazed, low-E, argon-filled window with insulated spacers costs $488. This means each square meter of added glazing costs $318 more than the comparable wall area. Energy modeling yielded a savings of 19 kWh per year for the extra window, estimated at $1.90, giving the window a payback period of 167 years—more than six times the life expectancy of the window. Proskiw concludes that builders should focus on affordable triple-glazed windows with warm-edge spacers to control condensation and should limit glazing to 6% of floor area; the money that would have gone to more expensive windows “would often be better spent on improving the energy performance of some other conservation or renewable energy option.”

Among those options, Proskiw identifies areas in which improvements would be most effective. Domestic water heating systems, for example, tend to be electric because natural gas and propane are potentially hazardous in an airtight house, but electricity is an unnecessarily high-grade energy source for the job. Solar thermal systems “appeal to the philosophical purity of the NZEH concept” but can be problematic in harsh environments, and heat pump water heaters can substantially cool indoor air, not ideal in cold climates. Proskiw advocates the use of graywater heat recovery, though there is a need for systems that can be installed horizontally for use in houses without basements. For exterior walls, Proskiw recommends the use of R-24 batts and optimum value engineering (to minimize framing material), beyond which improving thermal performance drives the cost up rapidly. He mentions manufactured I-profile studs used in Europe as an affordable alternative, but these may not meet codes in North America due to lack of familiarity.

In general, Proskiw recommends that homes be designed with simple shapes to minimize air leakage and that space heating and water heating equipment also be kept simple for low maintenance. He also emphasizes the importance of building commissioning, a practice that is becoming established in commercial building but could improve residential performance as well.

Editor’s note: Proskiw’s report was released in 2010, but it only came to light recently thanks to coverage on GreenBuildingAdvisor.com.

Comments (3)

1 Affordable NZEH posted by Michael Beattie on 11/05/2012 at 03:05 pm

Good article. Ridiculous photo.

2 Trading Passive for Active? posted by Robert Riversong on 04/08/2013 at 01:52 pm

This report is Canadian and may be covering colder climate zones than most of the northern US, but as a 30-year designer/builder of super-insulated passive solar homes in northern New England, I question trading away the passive solar design element for an active make-up energy complex - in terms of affordability, durability, longevity, maintainability, and the fundamental preference for passive over active systems of any kind.

The additional south glazing appears to be not cost-effective only compared to an envelope that is perhaps excessive. In my R-40/60+ homes, 7%-12% south glazing is always cost effective (if reasonably priced windows are used). In addition to the "free" solar heat, the glazing also offers better daylighting, summer ventilation, views and a more pronounced sense of connection to the outdoors.

The easiest and most cost-effective way to reduce a home's energy load is to reduce its size and volume and geometric complexity. The 3,000 SF single-family home noted below is a perfect example of excess compensated by expensive active energy systems. Just a generation or two ago, a 1200 SF 3-bedroom house was the American dream. Are our dreams now simply unsustainable?

3 Case Study: Net Zero House in posted by Lissa Spitz on 04/12/2013 at 12:40 pm

There are a lot of things to like about this case study, but to me they don't make up for the fundamental error of size. If one of the goals of the project was to "create a healthy home for a family of four to live in with as little environmental impact as possible" they missed the mark at 3000 square feet.

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October 26, 2012