Net zero carbon: Closer to possible than you might think

Homeowners committed to minimizing their climate impact may be familiar with the concept of net zero energy homes that produce as much energy as they consume over the course of a year from clean, renewable sources. Increasingly, these buildings are not using fossil fuel at all, eliminating onsite combustion and becoming zero carbon emissions as well.  Thanks to newer generations of cost-effective materials that save and generate energy, the ability to make one’s home net zero energy and free of carbon emissions is closer than ever before.

Zero emissions and zero energy are related, but different, concepts, said Rick Ames, Boston Building Resources board president and architect at Next Phase Studios. Net zero energy could include fossil fuel consumption on site, while zero emissions means no fossil fuels and no on-site combustion. “Net zero energy is aspirational, especially for someone with an older home,” Rick noted. “But you can electrify the building and get to zero emissions as well.”

The New Buildings Institute names five aspects of a zero-carbon building: energy efficiency, renewable energy, grid storage and integration, building electrification, and embodied carbon in the materials used for construction or renovation. Increasing energy efficiency involves insulation, air sealing, and efficient appliances and systems powered by electricity. With the electrical load established, a renewable energy source (usually solar panels) can be designed to power it.

Renovations now in progress at the Reuse Center at Boston Building Resources will make the building net zero through heavily insulated walls and roof, an airlock for material pick-up, an all-electric heating and cooling system, and a rooftop solar array to power it all. The City of Boston Department of Neighborhood Development has the objective of making all new-construction buildings net zero by 2030 or sooner as part of a path to net zero carbon by 2050.

Air sealing and insulation

Preventing energy loss with air sealing and insulation is the first place to start when reducing carbon. Architect Travis Anderson of PlaceTailor recommends focusing on “an airtight boundary, good windows, and a good jacket” of insulation.

A deep energy retrofit is a “major surgery” approach that involves new windows, doors, and cladding. However, most homeowners will opt for something less comprehensive—and less expensive—than this $50,000–$80,000 endeavor. Rick recommends contacting Mass Save for an energy assessment to understand the process better and to identify low-hanging fruit for each individual situation.

Spray foam is very effective for air sealing. However, many spray foams use propellants that are highly volatile and damaging to the environment. Rick recommends that, when you work with an insulation contractor, ask them to use a spray foam that emits no VOCs and uses non-damaging propellants. Environmentally friendly insulation options include cellulose and mineral wool.

Newly constructed or tightly sealed buildings use air exchangers to ensure that carbon monoxide and other toxins do not build up inside. A very high level of air sealing needs to be reached before an air exchanger is necessary. Energy-recovering air exchangers, used for years in Europe, are now making their way to the United States, Travis noted.

Efficient electrical appliances and heating systems

Heat pump image by by shixart1985 | Creative Commons license

Many homeowners have already switched to LED lighting and smart thermostats. Changing from a gas range to an electric one is straightforward, with the help of a plumber and an electrician, to eliminate a use of fossil fuels.

But what about home heating? Air-source heat pumps now provide a viable alternative to the fossil-fuel-powered boilers and furnaces heating most Boston-area homes. Heat pumps are highly efficient systems that move heat in or out of a building to provide heat in winter and cooling in summer using vapor compression, as a refrigerator does. Heat pump technology has been used for many years in Asia for home cooling, and has now advanced to the point where it works well in cold climates to generate heat. Heat pumps work effectively even when temperatures dip below freezing, and they cool more efficiently than conventional air conditioning systems.

“BTU for BTU, electricity costs as much as 23% more as a fuel source,” said Rick. But the efficiency gained with a heat pump system can offset the higher fuel cost. “Heat pumps are much more efficient than any boiler can be,” Rick explained. “If you layer on any solar at all, it becomes a no-brainer, because you’re providing your own fuel.” Heat pumps are also available to run a dryer, water heater, or refrigerator.

Converting an older home to heat pumps must happen in concert with air sealing, insulation, and door/window/envelope upgrades, as well as a solid effort at conservation. Even with a more efficient system, a drafty home will cost more to heat and cool.

How do heat pumps work?

Heat pumps harvest energy from phase change—that is, the energy taken in or released when a substance changes to or from solid, liquid, or vapor. For example, when water reaches its boiling point, a large input of energy is needed to push the water into its vapor form, steam. When the steam cools and returns to its liquid state, the phase-change energy is released.

“By putting a refrigerant under pressure, the pump is able to get a phase change reaction that generates much more energy than what was needed to crank that pump up,” explained Rick Ames, architect at Next Phase Studios and chair of the Boston Building Resources board of directors. The Massachusetts Clean Energy Center and the Green Energy Consumers Alliance have more information.

Solar power

With a well insulated and air-sealed home, entirely powered by electricity, a home’s energy needs can be met with a rooftop solar array. Money spent on a solar installation will be recouped in future energy savings. On a nationwide average, the payback period for solar is about eight years, but it can be much less depending on the situation. Mass Save and the Massachusetts Clean Energy Center have more information about rebates and financing options.

With air sealing, insulation, efficient equipment, and a clean, renewable energy source, a homeowner can reduce their energy bills and increase comfort while minimizing their impact on the planet as never before.