While energy consumption in new homes is decreasing thanks to stricter building codes, there is a dearth of performance standards for retrofits to older residences and those that do exist rarely include enforceable or quantifiable carbon reductions. To provide additional guidance for decarbonizing existing homes, my master’s thesis, which was named a best student paper on housing by the Center last year, proposes a framework that can help identify the greatest opportunities for residential carbon reductions.

I found that considering anticipated changes to lower-carbon fuel sources for electricity generation and assigning a time-weighted value for carbon emissions can significantly impact assessments of which retrofits are likely to save the greatest amounts of carbon at the lowest price. Using this approach could help policymakers, design professionals, activists, and homeowners choose from a variety of decarbonization strategies.

In my thesis, I assessed the carbon- and cost-effectiveness of three decarbonization strategies for residential retrofits:

• Electrification: swapping out gas-fueled stoves, water heaters, and space heating systems for higher-performing electric systems like an electric heat pump water heater and air source heat pump.
• Envelope upgrades: improving ceiling insulation, wall insulation, windows, and infiltration barriers.
• Renewable energy: adding roof-mounted photovoltaic (PV) systems and battery storage systems.

I broke down each strategy into distinct retrofit interventions. In comparing these approaches, I not only looked at the operational carbon reductions associated with different interventions, but also assessed three additional factors:

• Embodied carbon: the Greenhouse Gas (GHG) emissions associated with the manufacturing, transportation, installation, maintenance, and demolition of building materials over a specified period of time. When considered together, a building’s embodied carbon and operational carbon make up the building’s total carbon, or life-cycle carbon.
• Electric grid decarbonization: the reduction of GHG emissions associated with utility-scale electricity generation through increased use of low-carbon energy sources.
• The time value of carbon (TVC): the concept that carbon reductions today are worth more than the same level of carbon reductions in the future due to the urgent need to draw down GHG emissions in the near term.

These factors are often excluded from analysis of energy performance upgrades, or, if they are included, are rarely analyzed all together.

I focused on retrofits of single-family homes built before 1980 that have not undergone significant energy upgrades because these homes use much more energy than newer homes. In addition, I examined the impacts in three different locales that not only had different climates but are served by electricity systems that produce different levels of emissions (because they generate power from different mixes of facilities). Specifically, I examined retrofits in Houston, which has a hot-humid climate and gets its power from a system with a mid-range rate of emissions; in Los Angeles, which has a temperate, dry climate and gets its power from a system with a relatively low rate of emissions; and in Chicago, which has a cold climate and gets its power from a system with a relatively high rate of emissions.

Envelope upgrades across the cities tended to rank highest in terms of carbon reduction per dollar spent. This suggests that in existing homes without proper insulation and air sealing, upgrades would significantly reduce both GHG emissions and energy costs. Moreover, envelope upgrades in homes with higher energy consumption, often in colder climates like Chicago, yielded greater life-cycle carbon reductions than similar upgrades in Los Angeles or Houston. In Los Angeles, PV upgrades made financial sense but did not reduce as much carbon as other retrofit interventions. Also, because California’s grid already relies on a high share of renewables, the electrification retrofits in Los Angeles made more sense from a carbon-standpoint than they did in the other two cities.

Since grid-level emission rates would decrease with grid-level improvements, electrification upgrades tended to rank higher when those improvements are taken into account. In Houston for example, as the grid decarbonizes, switching from gas to electric heating would quickly yield particularly large carbon reductions. On the other hand, as grid-level emission rates decrease, home retrofits focused on renewable energy tended to rank lower than before. And accounting for the TVC generally caused retrofits with high initial carbon investments, such as the addition of heat pumps with carbon-intensive refrigerants, to drop in ranking.

These results show that considering life-cycle carbon and the TVC can significantly affect assessments of different approaches to decarbonizing existing homes. Given the growing interest in reducing carbon emissions associated with existing homes (including substantial new federal funding for such efforts), the methods detailed in my thesis can hopefully help researchers, policymakers, designers, and, ultimately, homeowners make more informed decisions about decarbonizing the existing residential building stock.