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A Net-Zero-Energy Home in Rural Tennessee

An 8-kilowatt photovoltaic array should provide more than enough energy for this efficient new home

Posted on Mar 27 2012 by Allison A. Bailes III, PhD

On my thousand-mile quality assurance road trip last week, I visited a house that was designed to produce more energy than it uses, making it a net zeroProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. Calculating net-zero energy can be difficult, particularly in grid-tied renewable energy systems, because of transmission losses in power lines and other considerations. energy home. You can take any house all the way to net zero just by giving it enough on-site power production (photovoltaics, wind, hydropower...), but that's not the most effective way to achieve the goal of net zero energy use. First, you want to make the house really efficient, and that's what these folks did.

As tight as a Passive House

If you're a home energy pro, the photo of the duct leakage test fan hooked up to the Blower Door frame may look a little odd to you (unless you work on Passive House or similar super-efficient projects). That's my setup when I was testing the house for air leakage. Yes, I really was running a Blower Door test with a Duct BlasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems. All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage. fan. Even better, I was running the fan with the smallest ring (Ring 3). Because the house was so tight, I needed to use equipment that would give me better resolution than the larger Blower Door fan.

This house was really, really tight. The air leakage was 200 cfm50, which translated to about 0.5 ACH50. For reference, the Georgia energy code requires all new homes to test at 7 ACH50 or less. The Passive House standard for air leakage is 0.6 ACH50. Any house that tight needs mechanical ventilation, of course, and they installed a combined ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. and HEPA filter system made by Broan.

Wondering how they got the house that tight? They took a number of measures, all of them designed to eliminate the common sources of air leakage:

  • Simple design - only 4 corners and a single gable roof
  • Slab foundation
  • Insulating Concrete Form (ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure.) walls
  • Spray foam insulation on the roofline

Super efficient and powered with renewable energy

They did a great job with the rest of the energy efficiency details, too. It's becoming the norm these days, but I'll mention anyway that they used double pane, low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. windows. The ceiling fans, lights, and appliances all were super efficient as well. They also installed a ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. with all ducts in the conditioned attic. That means there was no duct leakage to the outside. Of course, they installed a solar hot water system, too; you can see the solar thermal collector on the roof, to the right of the PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. modules, in one of the photos.

Once they made the house as efficient as possible, the amount of on-site power production they needed to get to net zero was smaller. The photovoltaic system on the south-facing roof ended up at about 8 kilowatts, which actually should put them below net zero. That means that the power company will be sending them money each year.

So, how do you build a net zero energy home? The first thing to do is to have someone like Jack Cowan on your team. Jack is a HERS rater in the Memphis, Tennessee area, and he started his rating company, Cowanhouse, over a decade ago. The house described in this article is going to be his nephew's home, and Jack has helped with the design and construction, as well as being the rater on the project. He's done a number of other renewable energy projects, too, including another net zero house whose owners have never paid a utility bill. Jack did all the modeling and specifications for the PV system here.

Meeting great people doing amazing work is one of the reasons I love what I do so much.

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Image Credits:

  1. Energy Vanguard
  2. Allison A. Bailes III

Mar 27, 2012 9:47 AM ET

NY, National Grid, does not
by aj builder, Upstate NY Zone 6a

NY, National Grid, does not give back money if one generates a surplus.

Question, how noisy is the Broan system? Their bath fans are noisy. Panasonic bath fans are less so.

Nice home, but needs quite a large solar array, and expensive ground source heat pump.

Mar 27, 2012 5:16 PM ET

Response to aj
by Allison A. Bailes III, PhD

aj, I don't have any experience with that system, so I can't answer directly, but the fan itself is in the attic, so I imagine it won't be noticeable in the house. I imagine the 8 kW PV system is going to provide a significant amount of extra energy for the owners. As for the GSHP, I myself would have chosen a mini-split, but these folks wanted this type of system, so that's what they're getting.

Mar 27, 2012 8:32 PM ET

duct blaster
by Hunter Dendy

I love seeing the duct blaster hooked up to the door panel. Good stuff!

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