How Low Can We Go? Driving to Zero Net Energy Buildings

by Penni McLean-Conner, NSTAR

Reducing energy waste has become a top priority in many states, hence the rapid development and expansion of energy efficiency (EE) programs nationwide. Many energy thought leaders ponder, “How low can we go? Can we achieve zero net energy buildings?”

Yes, and zero net energy buildings—which combine aggressive EE measures with renewable distributed generation such as solar—are in production in the U.S. and around the world.

Massachusetts is one of several states that have found zero net energy buildings save energy costs and reduce greenhouse gas (GHG) emissions. The state has developed recommendations to make zero net energy buildings the norm by 2030. As utilities advance their EE programs, it is important they develop, understand and test strategies to ensure zero net energy buildings are appropriately developed.

Zero Net Energy: What is It?

There are a variety of definitions with respect to zero net energy buildings. A 2006 Department of Energy National Renewable Energy Laboratory publication, àƒ-à¢â€š¬à…-Zero Energy Buildings: A Critical Look at the Definition, defines four approaches:

A deep retrofit on a house, for example, involves advanced and fairly invasive EE measures that radically improve the energy performance of an existing home, such as an external wall superinsulation build out.

Zero net site energy: Energy produced on-site is at least equal to the energy used.

Zero net source energy: Energy produced on-site is at least equal to the energy used when energy use is accounted for at its source. In this case, there is accounting for energy used to generate and deliver energy to the building.

Zero net energy cost: The money a building owner pays a utility for energy services and use is at least equal to the amount that the utility pays the owner for generating and exporting energy.

Zero net energy emissions: Onsite production of emissions-free, renewable energy is at least equal to the energy used that comes from emissions-producing sources.

Piloting Zero Net Energy Buildings

There are many efforts to advance building practices associated with zero net energy buildings via pilots. These pilots involve deep energy retrofits combined with renewable distributed generation achieving 50 to 90 percent reductions in energy usage.

A deep retrofit on a residential building, for example, involves advanced and fairly invasive EE measures that radically improve the energy performance of an existing home. Dramatic energy reductions are achieved by addressing all energy loads including space conditioning, appliances, plug loads and hot water. These projects are costly because building practices and materials are not yet standardized.

Efficiency measures that address buildingsàƒ-à¢â€š¬à¢â€ž- shells are fundamental in achieving energy reductions associated with zero net energy buildings. These measures typically include an external wall superinsulation build out, attic insulation enhancements, foundation wall and slab insulation, extensive whole-house air sealing and high-performance windows. These shell enhancements require a build out of existing shells that involve building out window frames to support deeper walls and building out roof eaves.

The EE measures are combined with renewable energy sources. These measures include leveraging natural daylight to displace light fixtures, installing solar hot water systems and augmenting with distributed renewable generation.

To date in Massachusetts, five houses have completed deep retrofits. John Livermore remodeled his 1973 house to achieve zero net energy. His efforts should reduce energy use for heating 70 percent. Livermore installed solar generation and hot water to address the remaining energy needs. He expects to produce excess electricity equal to 1,500 kWh annually.

“My motivation for taking action to reduce our familyàƒ-à¢â€š¬à¢â€ž-s carbon footprint was the understanding that carbon emissions need to be reduced by about 90 percent by 2030 in order to stabilize the earth’s climate systems, and the realization that I needed to take personal responsibility for reducing our emissions,” Livermore said. “We set out to demonstrate what can be done to reduce the carbon footprint of a suburban homeowner on a modest budget, with an overall goal to reduce our home’s net energy usage by 90 percent. “We are currently well on our way to achieving this target”

As utilities and other EE program administrators expand EE plans, they should include development of zero net energy buildings. With research and pilots devoted to zero net energy buildings, standardized building practices and materials will become market-ready, catapulting these building practices to a cost-effective measure.


Penni McLean-Conner is the vice president of customer care at NSTAR, the largest investorowned electric and gas utility in Massachusetts. McLean-Conner, a registered professional engineer, serves on several industry boards of directors, including the Massachusetts Technology Collaborative and CS Week. Her latest book, “Energy Eficiency: Principles and Practices,” is available at

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