Last month, Harvard’s Center for Green Buildings and Cities released a brief report on energy production and consumption at HouseZero—the 1920s dwelling in Cambridge converted to the Center’s offices as an experiment in energy efficiency—during its first full year of operation. At the same time, the Center published an academic article about the prospects for offsetting the building’s embodied energy in time to help thwart climate change. It also put out a press release in which its director, Ali Malkawi, said he and his partners firmly believe “in the overarching importance of transparency.” And a spokesperson responded to a series of questions posed by RECORD, at one point noting that the Center plans to install additional photovolatic (PV) panels on the house’s roof to help it offset its operational and embodied energy—a goal the Center now refers to as achieving “carbon balance.” 

HouseZero, at 20 Sumner Road in Cambridge, has been promoted by Malkawi and others as an attempt to show how older houses can be retrofitted to reduce energy use. In an article last July, RECORD asked hard questions about the design of the experiment, in particular whether sufficient attention was paid to the house’s embodied energy—that is, the energy that was consumed by its renovation.

Malkawi has long asserted that the solar panels on its roof will produce more clean energy than the house needs, and the excess will be sold to the local utility, Eversource, reducing the amount of power that the company must generate. Among other things, RECORD asked whether a plan that requires 100 years (Malkawi’s own timeline) to begin reducing atmospheric carbon is the right way to address an urgent climate crisis. RECORD requested that Malkawi release his estimate of the house’s embodied carbon and data on how much excess clean energy the house produces, since without those numbers it’s impossible to determine whether the house is on track to meet its goals.

On the subject of how much energy the solar panels produced between June 2020 and May 2021, Malkawi offered caveats about faulty meters, “uncertainties in PV harvest measurements” and “possible system efficiency loss” before estimating that the panels produced 38.7 kWh/square meter, which works out to 13,777 kWh for the 356 square meter house. That is less electricity than the house used, meaning it had to buy electricity from the utility. Specifically, the house imported 5,482 kWh​, fully half of what an ordinary American residence uses in a year. To Malkawi, this was not as disappointing as it might seem because, he noted, the PV system was not fully functioning for three periods (totaling about ten weeks) during the year. Even so, because production may exceed use at certain times, particularly on sunny days, “the building has been able to provide excess generated energy to the local power grid,” the press release states. But the spokesperson declined to say how much electricity was sent to the grid—an essential number for evaluating the house’s ability to meet its goals—asserting that its electric company, Eversource, does not provide that information. The spokesperson added that a new meter was installed last August to provide a “higher degree of PV harvest reporting” and that evaluating the system would require 24 months of uninterrupted operation.

The report notes that “the building was not occupied” during the year in question due to the pandemic, suggesting that energy consumption was lower than it would be during periods of normal use. However, because HouseZero is a laboratory operating 24/7, it contains technical equipment that would not be found in a typical office (or house) and which consumes significant amounts of power. Excluding that power would have made the house a net energy producer. Thus, Malkawi saw the overall news as favorable (he called it “positively promising” in the press release), with a few provisos. According to the press release: “HouseZero demonstrates that surplus onsite renewable energy generation can offset emissions associated with the primary building structure (which does not include emissions from technical equipment), provided that the photovoltaic (PV) production can achieve optimal efficiency as designed, and the building performance is maintained.” Responding to questions, the spokesperson mentioned a plan to install “additional PV panels with the aim to balance HouseZero’s carbon” and called the house’s performance “an opportunity to continue to make improvements.”

The academic article by Malkawi and six co-authors, (all associated with the Harvard’s Graduate School of Design, Snohetta, the architecture firm that designed HouseZero’s renovation, or Skanska Norway, the lead engineer), peer reviewed and published in the journal Building and Environment, under the title “Zero-carbon Balance: The Case of HouseZero,” runs to more than 12,000 words. It offers no definitive conclusions about the performance of HouseZero.

Much of the article describes the use of a life cycle assessment (LCA) to estimate the building’s embodied carbon. (A life cycle assessment analyses a project’s environmental impacts from early in the construction process until the end of the building’s useful life.) After a long discussion of the difficulties of coming up with a reliable number, the authors offer their best estimate: 174,000 kilograms of CO₂ and its equivalents.

But, they note, “The ‘flexible’ definition of LCA system boundaries and the limited consensus on the methodology used for rating and certification systems have resulted in incompleteness across most LCA studies.” In this case, only emissions “associated with the primary building structure” were considered.

The authors note the irony that “high-performance, zero carbon emissions buildings” will likely produce “higher embodied CO2e emissions.” They describe two attempts to reduce HouseZero’s embodied carbon emissions: First, by using concrete in which slag replaced the usual Portland Cement, the embodied energy associated with concrete was, they say, reduced by up to 58 percent. Second, wood fiber insulation was used as an alternative to mineral wool and polyisocyanurate insulating materials because of its lower embodied carbon.

The report listed major items that are excluded from the LCA, including plumbing and electrical systems. Though excluding such things in an LCA may be standard practice, it does reduce the real-world applicability of data obtained from HouseZero.

The report calls for additional research to make embodied carbon computations less uncertain. It stated: “More transparent and consistent accounting of carbon emissions is needed for all building life cycle stages.”

Ultimately, the article suggests that HouseZero’s primary goal—to offset operational and embodied carbon by producing cleaner electricity than the local utility—may be unattainable once the embodied carbon associated with every aspect of the building is accounted for. The authors state: “As more precise data becomes available for buildings systems beyond the typical system boundary of current LCA methods, it is possible that achieving net-zero carbon balance ... will become more difficult across the building sector.”

In addition, the article describes what would happen if the local utility begins generating clean electricity, making it impossible for HouseZero to send cleaner electricity to the grid. “If zero-carbon grid emissions are achieved by 2050,” the article states, HouseZero “cannot achieve a net-zero carbon balance.” That’s because the only way the house can offset its embodied and operational carbon is by making cleaner electricity than Eversource.

But if the local utility begins producing electricity without emitting carbon, that would be good news for everyone.