Until this past summer, I had more or less assumed that a net-zero house, one that didn’t use any fossil fuel to function, could really only be achieved in some ridiculously expensive research and development setting. That may have been true a decade ago, but it isn’t true now. A combination of technical advances and cost reductions has now put a net-zero house within the reach of nearly everyone. Even better, net-zero can be achieved in most buildings in stages, and are investments that are likely to outperform the market in today’s investment climate. The result is a win-win-win situation.
First, what exactly is “net-zero”? There isn’t a hard-and-fast definition, but, in general, net-zero buildings create as much energy as they consume. They typically combine highly efficient construction and appliances with some form of renewable energy generation, usually on-site. But, this can be done in different ways, and sometimes with different goals in mind, and the result is a wide variety of net-zero terms, as delineated in this list from a designer in Waitsfield, VT (his house is in the list below)—
“Net-zero carbon, net-zero cost, net-zero source, net-zero site, near net-zero, net-zero ready…there are many terms used to describe a certain category of buildings that are referred to as “net-zero energy buildings” (or NZEBs).”
In the last six months I have seen or heard about no less than six examples of net-zero buildings, and the variety of approaches in these buildings will give you some sense of the term, I think. (Some of these details are from memory, so forgive me out there if I get something wrong).
Building #1— Kim Quirk is the owner of Enfield Energy Emporium in Enfield, CT, an architectural firm, and she bought and renovated this house and has turned it into a net-zero office space and living quarters. I saw her presentation about this at Solarfest this past summer, and if I recall, the house was originally built in the mid-19th century, and was mostly gutted when she bought it. She had the basement foamed, and did a deep-energy retrofit that included increasing the thickness of the exterior walls and filling them with cellulose insulation. She added a 5kw PV system in the yard, which is net-metered. And here’s the unusual part—for heating, she dug a huge hole under her driveway, about 10 x 12 feet by 10 feet deep, lined the sides with a liner and foam, filled it with sand, water, and tubing, and then buried it. (My rough calculations—about 60 tons of insulated mass). This thermal mass is a huge “Thermos” that can store an entire summer’s worth of heat gathered by a largish array of evacuated-tube thermal collectors. So all summer long they run and pump hot water through this thermal mass (pics here), which brings the temperature up to something like 180 degrees. In the winter another set of tubing pulls the heat out, where it’s radiated into the house in a system of low-temperature (90 degree F) baseboard heat. An interesting approach. One of her goals was zero-combustion in addition to net-zero, and from her talk this summer it sounded as if the building was on its way to achieving her design goals.
Building #2— Architect Bill Maclay’s Dartt House, in Waitsfield, VT. I saw Bill give a presentation about this building last week at Renewable Energy Vermont’s Expo in Burlington. This is another older structure, renovated in much the same way as Kim Quirk’s house. It is actually two or three net-zero projects together—a building that serves as an office, and an adjoining building that he been turned into two apartments. Unlike Kim Quirk’s solar-heated thermal mass method, these buildings use air-to-air heat pumps for both heat and cooling, all powered by a combination of larger PV arrays—one 17kw array that serves as the roof of a carport (last pic on this page), smaller arrays to the rear of the house, and another large net-metered array that is off-site.
Building #3— Oddly enough—our house. Technically a “near-net-zero building” as it is now, as we still use propane for hot water. But we’re on our way to net-zero, via yet a third approach—using sustainably-gathered biomass for heat. In our case, cordwood. Our house is off-grid, with a 3kw PV system and a 1kw wind turbine. With the addition of a bit more PV and solar hot water, we should get all the way to net-zero. Even as is, the building uses only a fraction of the fossil fuel that most Americans use. The house also has a fair amount of passive-solar design features—it is oriented to the south, and most windows and living areas are on that side of the building, and closets and utility areas are on the north. The site is shielded to the north by hills and trees, and open to the south. The building has performed admirably—on sunny days in the winter I can leave home for work with the house at 63 degrees, and come home to a house that is well above 70, all with no heat on, even if outside temps are in the 20’s. We typically use about 2 1/2 cords of wood per winter for heat, which we burn in a single wood stove on the main floor of the open-floor-plan design.
Building #4— Well, “buildings”, plural. A company called Vermod is making net-zero single-wide modular homes to address the need for efficient low-cost housing in the state. With 12-inch-thick walls and triple-pane windows, and a 6kw PV system on the roof, they are energy-self-sufficient. Heating is provided through an air-to-air heat pump that scavenges waste heat from the exit side of the building’s cross-flow air exchanger. Though a third more expensive than standard mobile homes, they are expected to save their owners well over $2,000 a year in utility costs. The “payoff” should come after only about 12 years, in line with what others have estimated for other net-zero projects. More on that in a minute.
Building #5— Middlebury College’s 2013 “Insite” house, its entry to this year’s Solar Decathlon, in which it took 8th place. Built from the ground up by students, it combines a front walkway shaded by solar panels with a modern design made from local and sustainably-sourced materials. This is the third entry in this list that is heated with air-to-air heat pumps—a relatively new technology for home heating that is rapidly entering the market. I toured this house this summer while it was on display at the college, and it seemed very livable, and had many innovative design features.
Building #6— And, last on today’s list, Dr. Mel Tyree’s house in Ellenburg, NY. I also met Mel and his wife at Solarfest, where he gave a presentation about the house (Home Power Magazine article about the house here). His house uses yet another approach to achieve net-zero, a net-metered 10kw PV array with battery backup, a 10kw wind turbine, and an open-loop, ground-source heat pump for heat (sometimes mistakenly called “geothermal”) that uses two wells, on to draw from and another “injection well” to return the water to the earth. This is a very large PV and wind system, but the amazing upside to this is that it not only lets the Tyree’s easily achieve net-zero, but it also powers their two Tesla EV’s. So, net-zero house plus net-zero transportation. In very nice cars. Mel has kept extensive records of all manner of data associated with the house, and has calculated an 11- to 13-year payoff for the additional $65,000 in construction costs. Not a bad investment.
So, my main points— 1) Net-zero buildings are quite possible to build today, either by renovating older homes or in new construction, 2) they can become net-zero in stages, by adding efficiency improvements or renewable generation over time, and 3) the investments typically seem to have payoffs in the 12- to 15-year range. And needless to say, with buildings consuming nearly 50% of all energy used in the U.S., net-zero structures will be a necessary part of the more sustainable world we’re trying to move toward. Yay, planet.