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