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The Goldilocks Approach to Residential Retrofits

December 2023
by Brendan Kavanagh

Rachel and I were honored to give the Day 2 keynote at NESEA Building Energy Boston conference this past March. In our presentation, Why We Stopped Doing Deep Energy Retrofits, we shared the story of how and why Byggmeister has shifted away from super-insulating homes to lighter envelope improvements coupled with a greater focus on eliminating onsite fossil fuel consumption.

We can’t meet our climate goals without rapidly reducing carbon emissions from existing buildings, but how best to do this? That’s the million-dollar question. Byggmeister’s current position is that for single-family homes, we should be investing a bit less in envelope improvements and more in electrification.

Since our talk, we’ve sought out and received a lot of great feedback from our colleagues. The following three questions came up frequently so we felt we should write up and share our thinking on them.

  1. How much should we improve the building envelope before we electrify homes?
  2. How big of a problem are refrigerant leaks and what does this mean for the scalability of heat pumps?
  3. How sure are you that moderate energy retrofits are a more cost-effective decarbonization strategy for existing homes than deep energy retrofits?

1. BUILDING ENVELOPEIs there a certain level you need to get your building envelope to before converting to heat pumps?

Our short answer is no, we don’t think that there is a standard metric of airtightness or heating load that all homes need to get to before we electrify them. That isn’t to say that we are in favor of throwing heat pumps in every home without improving the envelope. On the contrary, we preach and practice envelope first then mechanicals. On our projects, we ensure that homeowners complete all subsidized insulation & air sealing measures available through the MassSave program. Then, we work with them to figure out what the next best steps would be to decarbonize. Usually there are at least a few practical and affordable opportunities to go further with the envelope, but sometimes there just aren’t. A common example is when a homeowner’s heating system is failing and due to budget constraints, they might not be able to afford converting to air source heat pumps and implementing all recommended envelope measures. In those situations, we prioritize the heat pump conversion and focus the envelope work on the most impactful areas, because the alternative could lock the home into another fossil fuel system for a couple of decades.

2. REFRIGERANT LEAKSYour presentation showed a lot of carbon emissions associated with refrigerant leakage. Are we at risk of doing more harm than good by converting to heat pumps?

Even when accounting for the impact of refrigerant leakage we are confident that in most situations converting to heat pumps will reduce the carbon footprint of your home, typically by a lot. There are a variety of reasons for this which are well summarized in this National Resources Defense Council article; however one key factor to highlight is that most U.S. households already have or are likely to get some form of air conditioning, which use and leak refrigerant similar to heat pumps. Unfortunately, refrigerant leakage is nothing new or exclusive to heat pumps, so while it’s not a reason to avoid converting it is an important factor to account for in evaluating energy retrofit work. Both our experience and the limited data we could find indicate that most heat pump systems have minimal leakage, a few leak a lot, but all systems do result in at least a small amount of refrigerant leaking into the atmosphere. The data we found on average leakage rates from residential heat pump systems indicates a range between 1-10% annually, so for the sake of our presentation we assumed a leakage rate of 5%. When you calculate total emissions between 2020 and 2050 that works out to a total leakage of 150% of the refrigerant initially charged in the system. We are hopeful that this overstates the impact of refrigerant leakage for a few reasons. First, the EPA mandated refrigerant phase down will likely result in the heat pumps we are installing now being replaced with heat pumps that use less potent refrigerants before 2050. Second, older refrigerants will become more expensive due to the phasedown and therefore installers and homeowners will be motivated to fix leaks instead of just topping off the refrigerant levels. Third, we are already seeing more heat pump systems designed with less total refrigerant and fewer site-made line set connections which substantially reduce the potential for leaks. In short, while refrigerant leaks are cause for concern, we do not think that we should avoid using heat pumps because of them. Rather, we should continue to rapidly deploy heat pumps and at the same time focus on reducing leaks.

3. MORE COST-EFFECTIVE: Your analysis showed that moderate energy retrofits are much more cost-effective at reducing carbon emissions than deep energy retrofits. Given what we know about how deep energy retrofits perform, that seems intuitively implausible. Are you sure you’re right about this?

Those who attended our talk may recall that we compared the emissions of a moderate retrofit we did in 2019 to a hypothetical deep energy retrofit of the same house. Some attendees questioned the modeled energy use of the hypothetical DER which showed an energy use intensity (EUI) of 16 kBtu/hour, suggesting that it should be substantially lower. We decided to redo our analysis assuming an EUI of 6 kBtu/sf — the same EUI as the 2014 DER we discussed earlier in the presentation.

In this new analysis the first year of operation shows the differential cost effectiveness of the moderate retrofit drop from 300% (original analysis) to 250% more cost effective than the DER. Looking out to 2050, the cost effectiveness of the moderate retrofit drops further from 250% in the original analysis to a much slimmer margin of 50% more cost effective. While this is substantially lower, the point stands that in both the short and long term, a deep energy retrofit still seems to be less cost effective at reducing carbon from existing homes when compared to a more moderate energy retrofit.

We fully acknowledge that deep energy retrofits have important benefits that go beyond energy and carbon savings, including but not limited to, comfort and resilience. The challenge is, for many (if not most) homeowners our moderate energy retrofit measures already pushes the limit of what they are comfortable investing in their homes. We also recognize that modernizing the electrical grid to reliably meet the increased demand from all-electric buildings and vehicles is a significant challenge, but we can’t allow this challenge to stand in the way of electrifying every home we can as quickly as we can.

In the late 2000s Byggmeister was a pioneer of the deep energy retrofit approach. For more than a decade, we dreamed of bringing DERs to scale and proposed superinsulation on dozens of projects. We can count on two hands the number of times we implemented a full or partial DER. Based on this experience as well as our analysis, we now think we can cut more carbon with our moderate retrofit approach. Given the pressing nature of the climate crisis we encourage other practitioners in this field take a moment every now and then to re-evaluate your approach and when necessary make a change to maximize your ability to move towards your goals.

 

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