Defining a Zero Energy Capable Home

January 01, 2008
Climate Solutions Special Issue
A version of this article appears in the Climate Solutions Special Issue issue of Home Energy Magazine.
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On August 10, 2006, the city council of Austin, Texas, passed a resolution creating a task force that would work to make all new locally built single-family homes zero energy capable by 2015. The task of implementing this directive—including defining what was meant by zero energy capable—fell to the city’s municipal electric utility, Austin Energy. As the manager of the Austin Energy Green Building (AEGB) program, the job of putting together the needed task force fell to me.

Recognizing the scope and potential impact of this effort, I quickly realized that a key factor in determining the success or failure of the task force would be the makeup of the group. Rather than trying to handpick members of the task force, my team and I decided that we should make a list of the organizations that should be involved and ask those organizations to select representatives from their membership (see Table 1). In February 2007 this effort was rolled into the Austin Climate Protection Plan (see “Austin Climate Protection Plan” for details).

In our first work session, we determined that the definition of a Zero Energy Capable Home would be “a home that is energy efficient enough that it will be cost-effective to install on-site renewable energy systems to make it a true zero energy home.” We further defined that level of energy efficiency as 65% more efficient than a home built to current code (2000 International Energy Conservation Code with local amendments) in Austin.

As with any group given a broad mandate such as this one, our first order of business was to select group leaders. Peter Pfeiffer of Barley & Pfeiffer Architects was selected as the task force chair, and I was chosen to be the vice chair. During a facilitated brainstorming session, the group determined that our primary objectives would be to

  • develop a plan whereby all new single-family homes built in the City of Austin Code Jurisdiction will be Zero Energy Capable Homes by 2015;
  • reduce utility costs to homeowners through reducing energy use and increasing use of cost-effective renewable energy sources;
  • reduce the growth of peak demand on Austin Energy, which will improve efficiency of utility operations; and
  • reduce the growth in the emission of greenhouse gases attributable to the operation of homes.

The deliverables needed to achieve these objectives were determined to be

  • recommendations to City Council for local amendments to the 2006 International Energy Conservation Code (IECC) to be adopted as City of Austin Energy Code;
  • goals for energy efficiency improvements for the IECC cycles in 2009, 2012, and 2015; and
  • a process and plan for oversight of the implementation of the project.

Since air conditioning is the biggest user of energy in Austin homes, and is also the area with the most room for improvement, the first set of code changes focuses on taming that beast. Starting on January 1, 2008, the City of Austin Energy Code will require that residential heating, ventilating, and air conditioning (HVAC) systems be tested for duct leakage, air balancing, and static pressure. Batch testing following Energy Star procedures will be allowed for production builders. The new code will require the submittal of Manual J documentation for HVAC system sizing, and the installation of Minimum Efficiency Reporting Value (MERV) 6 or better filters. These measures, combined with the new code requirement for a radiant barrier system in nonreflective roofs, will greatly reduce HVAC energy use. Testing the building envelope to reduce infiltration and requiring that 25% of interior lighting be high efficacy rounds out the major code changes.

To determine the impact of these changes on energy use, we modeled a home built to the new code and compared the results to a model of the same home built to the current code. These results show electric energy savings of 2,515 kilowatt-hours and gas savings of 400 cubic feet, or a 19% improvement for electricity and a 1% improvement for gas. When the energy savings are converted to British thermal units, the total energy savings equals an improvement of 11% in energy use over the same home built to the current code.  The house the task force chose to model is a 2,263 square foot model currently being built in Austin by a national production builder. This model is fairly representative of new homes being built in Austin today, and, as a model used by a production builder, is probably more representative than the composite home we have used in the past to model code changes. We used EnergyGauge modeling software developed by the Florida Solar Energy Center to perform the modeling.

Using information provided by the builders and contractors on the task force, we determined that these code changes would add $1,179 to the cost of a typical home built in Austin. The energy savings at current utility rates are $228, which would result in a five-year simple payback. Even better, when the cost of these changes is rolled into a $200,000 mortgage at 6.5% interest, the utility bill savings exceed the additional mortgage costs, providing an immediate payback to the homeowners.

After completing the local amendments to the 2006 IECC, the task force turned to goals for the energy code cycles in 2009, 2012, and 2015. Our job was to determine what energy efficiency improvements we could reasonably expect to require in each of these code change cycles and to identify the areas we thought these future changes should come in.
A typical process to set goals and identify measures might be to set the goals first and then try to identify code changes that would get us to the goal. The task force decided that these steps could not be separated but had to be done concurrently. Some of the future code changes were almost inevitable given the changes that we had previously instituted. For example, the lighting and radiant barrier changes made in the 2006 amendments had a very positive impact on electric energy use. But the heat loss from the lighting changes all but negated any improvements in space-heating efficiency from the HVAC testing and other measures. Our research into lighting led us to believe that there would be an even greater opportunity to improve lighting efficiency in 2009, and that this would further increase the heating penalty. So we knew that we would have to increase wall thermal performance just to keep gas use from rising.

Using an iterative process that alternated between research and modeling, we determined that we could reasonably expect to see further improvements in energy efficiency in subsequent code changes as follows: 19% in 2009, 18% in 2012, and 17% in 2015. Together, these changes would achieve our goal of a 65% improvement in the efficiency of a code-built home by 2015.

For the 2009 code cycle, we identified these changes: increasing high-efficacy lighting from 25% to 90%; increasing wall assembly performance to a level equivalent to R-15 insulation; increasing window performance; and improving water-heating system efficiency by 5%. These changes will give us more than 95% of the energy savings needed to meet the goal for 2009. Between the time the current code changes go into effect on January 1, 2008, and the time the 2009 IECC is published, Austin Energy Green Building will be working with builders and designers to determine where the additional 5% improvement will come from. Measures that may be included in the changes to the 2012 and 2015 codes have also been identified, including improvements in national efficiency standards for appliances. These improvements in appliance efficiency will have to play a big part in any plan to substantially improve energy efficiency in homes, because plug loads can represent between 20% and 30% of total electric energy use.

This process may seem simple and straightforward enough that it could be done in any city in the United States, but the key to making our effort successful was the collaboration among all of the groups involved. The city of Austin and Austin Energy have been operating energy and water conservation programs since 1982. One of those early programs was the Austin Energy Star Homes program. Energy Star provided an alternative path to energy code compliance for single-family builders. Rather than just inspecting the homes and passing or failing them, this program worked with the designers and builders to find the most cost-effective and energy-efficient ways to meet or exceed code requirements. In 1991 the Energy Star program became the Green Building program, and this collaborative relationship with the builders has continued. This mutual respect and trust between city staff and builders made it possible for us to transform the code change process from one of confrontation to one where collaboration helped everyone involved and all of us came out ahead.

The 2006 IECC with these local amendments was adopted as the City of Austin Energy Code on October 18, 2007 and will take effect on January 1, 2008. The final report plan from the Zero Energy Capable Homes Task Force was adopted by the council on the same day.

This is just the first step of many that will be required to meet Austin’s ambitious goals for energy efficiency and carbon emissions reduction, but it is a positive step. I look forward to similar progress on Austin’s other climate protection initiatives.

Richard Morgan is the manager of the Austin Energy Green Building program. AEGB is responsible for private sector green building efforts and the development and improvement of the energy code.

For more information:

To learn more about Austin Energy Green Building, visit

Austin Climate Protection Plan

With its progressive Climate Protection Plan, Austin is taking the lead among the nation’s cities in their efforts to combat global warming. There are five broad elements to the plan. Each element targets a different sector in the community.

Municipal Plan
  • Power 100% of city facilities with renewable energy by 2012.
  • Make entire city fleet carbon neutral by 2020 through use of electric power and nonpetroleum fuels (with alternative fuel emissions offset through mitigation).
  • Develop departmental climate protection plans, including policies, procedures, targets, and reporting for maximum achievable reduction of greenhouse gas (GHG) emissions and energy consumption in all city departments.
  • Develop a city Of Austin employee climate education campaign and programs and incentives to help employees reduce their personal carbon footprint and engage in community outreach for climate protection.

Utility Plan
  • Achieve 700 megawatts in savings through energy efficiency and conservation by 2020.
  • Meet 30% of all energy needs through renewable resources by 2020, including 100 megawatts of solar power.
  • Achieve carbon neutrality on any new generation units through lowest-emission technologies, carbon sequestration, and offsets.
  • Establish carbon dioxide (CO2) cap and reduction plan for all utility emissions.

Homes and Buildings Plan
  • Make all new single-family homes zero energy capable by 2015.
  • Increase energy efficiency in all other new construction by 75% by 2015.
  • Require disclosure of historic energy use; facilitate and require energy efficiency improvements in existing homes and buildings at point of sale.
  • Enhance incentives and requirements for Green Building program; develop carbon neutral certification.

Community Plan
  • Establish City Climate Action Team to inventory GHG emissions from activities communitywide.
  • Work with stakeholders and technical advisors to develop recommendations for short-term and long-term reduction targets and implementation strategies for the metro area.
  • Establish key areas for study, including transportation; land use planning; emerging technologies; waste management; natural areas and landscapes; and multigenerational education.
  • Report recommendations to city council in one year.

“Go Neutral” Plan
  • Develop online carbon footprint calculator for individuals and small businesses, and provide individually tailored assessments for complex organizations and entities.
  • Develop a menu of local GHG reduction strategies that citizens, businesses, and organizations can fund through the purchase of carbon offset credits.
  • Develop a program to recognize households, businesses, and groups that achieve carbon neutrality.
  • Promote carbon neutrality among visitors by providing mechanisms and incentives for the purchase of offset credits by airport travelers, conventions, trade shows, and festivals.

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