A sustainable building school in Ontario has completed work on a 2,400-sq. ft. net-zero energy house it’s calling “Canada’s Greenest Home.”

Now on the market for $649,000, the house incorporates a variety of features that enhance indoor air quality and energy and water efficiency, including a composting toilet, a rainwater collection system, a 5-kW photovoltaic system, and nontoxic interior finishes. (GBA last reported on this house in a February 2012 news story, “Teaching Deep Green by Building It.”)

“We were initially quite hesitant to brand this project as ‘Canada’s Greenest,’ ” Chris Magwood, project manager and executive director of the Endeavour Centre, wrote in a blog detailing the project. “The claim was not made to be boastful or to dismiss the work of other designers and builders who have made remarkably green homes… But we were very interested in pushing as many boundaries as possible with this project, to challenge ourselves as designers and builders to make the very best house possible, going beyond what has been done previously.”

Builders are seeking certification under two green-rating programs, LEED for Homes and the Living Building Challenge. But Magwood said any contractor could “reproduce the results of this home with materials and products that are off-the-shelf.”

Prefabricated straw-bale walls and a tight envelope

The home’s exterior walls include two dozen “Bio-SIPs” manufactured by NatureBuilt Wall Systems, whose cofounder financed the project. The 8 ft. by 3 ft. panels with a straw bale core are 16 inches thick and weigh about 250 lb. per lineal foot of wall. The manufacturer says the panels are rated at R-35 and have a 2-hour fire rating. On the inside, panels were drywalled and plastered. The exterior is finished with cement lime stucco.

Like SIPs manufactured with foam insulation, these panels are manufactured off-site and assembled on the foundation quickly. The panels are joined together with galvanized connectors.

In addition to the prefabricated panels, the house also includes some double-stud walls filled with cellulose insulation as well as site-built straw-bale walls, Magwood says, a strategy which gave students experience working with a variety of building techniques.

A blower door test showed that the home’s air leakage rate is 0.63 air changes per hour at 50 pascals. Although the house missed the tough Passivhaus standard by a whisker, it is still much tighter than a conventionally built house.

Windows are triple-pane units manufactured by Inline. They have a U-factor of 0.17, the equivalent of R-6.

Below grade, the house is insulated with a product called Poraver, made in Ontario from expanded glass balls. Magwood writes that Poraver is made mostly as a lightweight concrete aggregate and has an estimated R-value of between 1.5 and 2 per inch (test results are still pending). Builders placed 8 inches of the material under the basement floor. The foundation walls are made from Durisol insulating concrete forms.

High indoor air quality and locally sourced materials

In addition to specifying an energy recovery ventilator (ERV) to supply fresh air to the nearly airtight house, designers also used nontoxic interior materials and finishes to keep air quality high.

The interior walls are coated with clay finish plasters, mixed on site from clay, sand, calcium carbonate, pigment, flour paste, and water. The plaster was applied by trowel in one coat roughly 1/8 inch thick.

The flooring is maple (certified by the Forest Stewardship Council) finished with a factory-applied UV-cured urethane that is free of volatile organic compounds, Magwood writes. The ceilings are finished with whitewash made from hydrated lime, powdered casein (a milk protein), and water.

The builders also made efforts to buy building materials within a 250 km. (155 mile) radius of the building site in Peterborough, Ontario.

“There are many green building products available in other markets (Europe, in particular, leads Canada in this way), but we wanted to avoid importing solutions and meet our targets using only materials from within a 250 km. radius,” Magwood wrote in his construction blog. “For all the major components of the building, we were able to achieve this goal. This keeps transportation energy costs and impacts minimal.”

But he admits it was a struggle because so much manufacturing has been outsourced to cheaper labor markets abroad. “Some categories of products are no longer manufactured in Canada,” he wrote, “or even in North America.”

Not your average plumbing system

At the heart of the building’s high-efficiency plumbing system is an M10 Clivus Multrum compositing toilet, which turns human waste into compost in a large storage bin.

“The tank includes a sprayer and controls that mist the compost pile regularly with a small amount of water,” Magood explains. “This provides the ideal conditions for effective composting: not wet, not dry, but consistently moist. Having provided sufficient nutrients, aeration, and moisture, the rich colonies of bacteria, protozoa, rotifers, actinomycetes, fungi, mold, yeast, and earthworms can best go to work converting solid waste to useful compost. Dry compost material needs to be removed from the tank about once a year.”

Other features include:

• A rainwater collection and filtration system designed to let the homeowners be “water independent.”
• Low-flow plumbing fixtures.
• Foam-flush toilets.

Air-source heat pump for both air conditioning and heat

Magwood says the project originally leaned toward a pellet boiler, in part because a pellet manufacturer was located a half-mile from the house. But the Living Building Challenge has a “no combustion” rule, so designers opted for a Zuba air-source heat pump from Mitsubishi. The heat pump operates in temperatures as low as -22°F.

“The decision to go with an air-source heat pump was made largely based on the cost of installation,” Magwood writes. “While a ground-source unit offers better efficiencies (especially at colder outdoor temperatures), the cost of installation is quite a bit higher, and the payback on the additional investment is well over a decade.

“Given our investment in other technologies for this home, we decided in this case that the lower cost of installation and the very good efficiencies for the unit made it the right decision for Canada’s Greenest Home.”

Two 4 ft. by 8 ft. solar thermal collectors should supply an estimated 50% to 75% of the residents’ domestic hot water, although the accuracy of that estimate, along with net-zero energy aspirations, will depend heavily on the choices made by whoever buys the house.

Magwood says the new owners should be able to earn between $3,500 and $4,000 a year by selling excess electricity to the local utility at a contract price of $0.56 per kWh, so in effect there will be no heating bill.

Extra features — such as solar thermal collectors, PV modules, rainwater collection equipment, and the composting toilet — added roughly $75,000 to the project, Magwood says, but the $270 per sq. ft. in construction costs isn’t out of line with typical custom home costs in the area.

Not a simple project, but within reach

The Endeavour project blog gives a detailed look at the many design and material decisions builders had to make. There are ample opportunities to stumble, but Magwood says problems can be overcome.

“As I’ve learned in the 15 years since building my own home, the process of creating a good, environmentally sensitive, energy-efficient home is not simple,” he wrote in an entry at the start of the project. “The basic concepts are pretty easy to wrap one’s head around, but the execution involves so many decisions and choices, all of which impact each other and the final result. As soon as you throw some unconventional materials and systems into the equation, it can get even more tangled.

“However, this should not prevent prospective owner-builders from designing and building their own homes. There is nothing more satisfying in life than to be responsible for the walls around and the roof over one’s family, and it is a manageable task.”