Net-Zero Energy, Soup to Nuts

Ann V. Edminster, M.Arch, LEED AP

Author of Energy Free: Homes for a Small Planet

Ready or not, by executive order, California homes have been mandated to achieve net-zero energy: new homes by 2020, existing homes by 2030. In the simplest terms, this means that a home will have to produce at least as much clean, renewable energy each year as the occupants of the home use in that year. While definition details and implementation specifics have yet to be worked out, California’s Public Utilities Commission and Energy Commission are working furiously to put the infrastructure in place to achieve this extremely ambitious goal.

Net-zero energy is also taking hold nationally and internationally, with help from the US DOE’s Builders Challenge, the US-Canada Net-Zero Energy Home Coalition, the international Passive House movement (along with its domestic adherents), and other initiatives in Europe and beyond.

Completed net-zero energy home projects exist across the housing spectrum, rawww.netzeroenergyhome.canging from single-family custom homes to production subdivisions to affordable multifamily developments. Each type poses a different degree of challenge, but there are some common denominators. For starters, achieving net-zero energy is going to require a fairly strict adherence to the “loading order” – i.e., incorporating energy features in a sequence that is based on getting the greatest energy performance gains at the lowest costs.

The rest of this article offers a few high-level points about each of the steps in the loading order, which should be prioritized in the order shown.  Optimize the basic building geometry and orientation. The enemies of net-zero energy (and durability and comfort, incidentally) include poor orientation and overly complex building geometry. Translated, this means:

  1. Do your best to orient the house with the long axis east-west;
  2. Keep most of the glazing on the south side;
  3. Maximize your south- and west-facing roof space;
  4. Keep the basic form as close to a shoebox (basic rectangle) as you can;
  5. Think elegant and simple, not fussy and convoluted.

Create a thermally robust building enclosure (appropriate to the climate).

A good thermal enclosure comprises four principal elements:

  1. Above-code insulation (greater R-values than Title 24 requires for your climate zone).
  2. A high quality insulation installation. While this may be hard to believe, most insulation is installed with insufficient attention to detail. Batt insulation, in particular, should be installed so that it is in full contact with all six faces of every framing cavity, with only very small voids and areas of compression. This is absolutely not the norm. Take care to seek out insulation contractors who are energy-efficiency advocates and enthusiasts.
  3. Superior air sealing, to prevent heated and cooled air from leaking through the building enclosure – you paid to heat or cool it, don’t waste that investment!
  4. High quality windows. Windows should have a below-code U-factor (thermal conductance), optimized solar heat gain coefficients (SHGC), and solid construction for longevity. Compression-type operation is also a plus, because it minimizes air leakage – casements, awnings, and hoppers.

 

Minimize heating and cooling needs with passive design strategies.

Passive design strategies are particularly important in hot climates, where air-conditioning can rack up electricity use. Include overhangs and other shading devices that are “tuned” to the direction the windows face and how much direct sun they receive during the hottest times of year. Passive ventilation strategies, such as “thermal chimneys” and windows carefully placed high and low on opposite walls, can also be effective features to help minimize reliance on mechanical cooling.

Passive solar heating can also be helpful, but should be designed with caution to avoid overheating in super-insulated homes. Keep mechanical and water system layouts as simple and centralized as possible. Although a lot of emphasis is placed on efficient mechanical equipment –- and I don’t deny it’s important –- the overall design of the mechanical systems is generally overlooked. Among numerous aspects that should be factored into an efficient design are:

  1. Minimizing the length of duct runs (and hot water piping).
  2. Minimizing duct leakage.
  3. Keeping ducts in conditioned (heated/cooled) parts of the building.
  4. Ensuring all parts of the system are correctly installed and tuned.

Choose efficient mechanical and water heating equipment, lighting, and appliances.

Each different type of equipment has a different efficiency metric, and there are acronyms galore. Here are just a few:

  • Energy factor (EF), for water heaters;
  • Annualized fuel utilization efficiency (AFUE), for furnaces;
  • Coefficient of Performance (COP), for heat pumps;
  • Water factor (WF), for clothes washers.

The American Council for an Energy Efficient Economy (ACEEE.org) offers a comprehensive guide to efficient household equipment and appliances. Three other excellent resources for selecting highly efficient household items are:

  • The Consortium for Energy Efficiency (CEE1.org), which lists appliances in order by efficiency;
  • TopTenUSA.org, which lists the ten most efficient models of a wide range of different devices, from televisions to refrigerators;
  • EfficientProducts.org, which provides additional product listings as well as well-researched and informative product selection guidance.

Incorporate strategies that help occupants control electrical use from plug-in devices.

So-called “plug loads” are an increasingly dominant part of the energy use picture for virtually all American households. The two big categories are entertainment devices and home office electronics. Thoughtful designers can help occupants control the energy use from these devices by understanding how they are used in our everyday lives, and accommodating controls for those uses conveniently. For example, locating plugs and switches for entertainment centers and desks where they are readily accessible can encourage energy conservation behaviors.

Design renewable energy systems to complement your highly energy-efficient dwelling.

Renewable energy systems –- the supply side of the net-zero energy equation -– are typically the most expensive ingredient of a net-zero energy home. Most homes also have limited real estate to devote to them. These factors dictate that we make the most out of our efficiency opportunities before we turn to renewable energy to help us get to net zero. We absolutely can’t achieve net-zero without these systems, but we need to maximize all the other tools in our toolkit, too.