Summary of “Zero-Carbon Balance: The Case of HouseZero”

The research paper “Zero-carbon Balance: The Case of HouseZero” provides readers with a case study of the data, methods, and scenarios used to calculate HouseZero’s carbon balance over a 100-year estimated building life (the carbon balance for a 60-year estimated building life is also calculated in the full paper). To do this, the team calculated the total embodied carbon emissions from as-built data, simulated operational carbon emissions, and projected carbon offsetting from onsite renewable energy over the entire life of the building. To make the findings comparable to other studies, the paper reports the embodied carbon emissions per square meter of occupied space, which results in 488 kg CO2e/m2 for the 100-year estimated building life. This calculation includes carbon emissions generated from the extraction, manufacturing, construction, and replacement of all elements considered to be within the building’s physical system boundary. This incorporates the building’s material systems (e.g., building assemblies including the foundation, structure, enclosure, and interior partitions and finishes) as well as technical systems that are not as typical in most studies (e.g., energy well, heat pump, photovoltaic panel and balance-of-system, and elevator). When temporal and physical system boundaries are aligned to current benchmarking studies, the normalized embodied CO2e emissions of HouseZero are much lower – less than half – at 233 kg CO2e/ m2.

Researchers also found that achieving zero carbon balance is dependent on how much carbon is emitted by the current and future energy supply from the electrical grid. The carbon balance results showed that HouseZero can achieve net-zero carbon balance with its simulated energy consumption and onsite power generation under current energy grid carbon-intensity scenarios when a zero-carbon emission grid is not achieved by 2050. However, if zero-carbon grid emissions are achieved by 2050, the project cannot achieve a net-zero carbon balance. These results highlight the sensitivity of the carbon balance analysis when designing a zero-carbon emissions building, the need for transparent CO2e accounting, more uniform systems boundary definitions, and the importance of data related to the future building and energy grid scenarios prescribed in the LCA study.

Other highlights from the paper:

The whole building LCA showed that around 42% of the building’s embodied CO2e emissions were generated by technical systems beyond what is typically defined as a building’s LCA system boundary (33% when batteries are not included in the LCA calculation).

For the technical systems, the majority of emissions (61-73%) are associated with renewable energy products, such as photovoltaic panels and batteries.

For the technical systems, 54% of emissions are associated with future product replacement, compared to 24% for material systems. These future-use stage scenarios have a high level of uncertainty.

The project’s low energy use intensity (EUI) indicates that the building integration and coupling of passive-active systems intended for low operational energy use and optimal thermal comfort produces a meaningful reduction of operational energy use, and therefore positively impacts the building’s net-carbon balance.

And, in addition to potential operational CO2e emissions reductions, these building-integrated designs have a co-benefit of reducing CO2e emissions that are often disregarded and unaccounted for, associated with HVAC equipment such as ducts and fans, which also have a high occurring replacement.

Early lifecycle assessment was used to identify materials with high embodied emissions, such as concrete. Low-carbon solutions were carefully considered as some low-carbon concrete products may pose adverse health effects. Human health concerns led to selecting high slag concrete over other waste-stream aggregates. The use of high-slag-content concrete reduced the embodied CO2e emissions per unit by 44-58% compared to relevant benchmarks.

Similarly, thermal insulation was also identified as a significant contributor to the total embodied CO2e emissions. Wood fiber insulation, which had a lower embodied CO2e per unit, was used as an alternative to mineral wool and polyisocyanurate insulating materials.

Read the full paper, “Zero-carbon balance: The case of HouseZero” in Building and Environment.