Making the case for Embodied Carbon in renovations

Carbon Wise recently created a case study to investigate the life cycle benefits of deconstructing an existing structure and then undergoing a deep retrofit, as compared to building a new net-zero structure.

“What we were hoping to prove was that it’s better from both an energy and embodied carbon perspective to retrofit a home, instead of the more conventional practice, which would have been to demolish the home and build new,” writes Elisabeth Baudinaud, principal of Carbon Wise. “The numbers confirm this assumption.”

This Vancouver home was the subject of CarbonWise’s case study. Photo courtesy CarbonWise.

The Vancouver-based energy and life cycle assessment consultants conducted the Life Cycle Assessments (LCAs) on a one-story detached home of 2,960 ft2 with a basement originally built in 1958, in North Vancouver, BC.

“In the past, operational emissions have always been the focus of green building policies,” writes Baudinaud. “However, there is an urgent need to address embodied emissions, particularly as new or renovated high-performance homes result in reduced operational emissions. Over time and as we add more materials (i.e., more insulation, triple-glazed windows, etc.) to reach energy efficiency targets, embodied emissions will account for an increasing portion of the total emissions and provide a valuable opportunity to further decrease overall emissions from the building sector.”

A gut renovation was determined to be the best choice for minimizing both operational and embodied carbon emissions. Photo courtesy CarbonWise.

 

The case study home was originally built using 2×4 wood frame construction, with an attic truss roof, on an unfinished concrete basement. The proposed upgraded home design included upgrades to a 2×6 exterior wall and a new roof design incorporating scissor truss construction and a section of flat roof. Additional floor space was added to the above-grade floor as an exposed floor area. Basement geometry was unaltered with some minimal additional material added to suit the new layout above and changes in rough openings. New framing and insulation were added to the below-grade structure.

The home’s existing roof was near the end of life and any new design would require a major reconstruction of the roof. However, a decision to reuse the slab and foundation walls proved impactful in the analysis, as these elements were found to represent the largest and most easily realized carbon savings in this project.

The intention behind the renovation was to improve the energy efficiency of the home to meet the CHBA net-zero standard (CHBA NZ or NZ). The homeowners were not only sensitive to the operational emissions of their home, but also the embodied emissions from the materials that would be used to accomplish the net-zero renovation.

As part of the renovation, Vancouver firm Unbuilders de-constructed and salvaged the lumber used in framing the parts of the home that would be rebuilt, for use in the retrofitted home. Photo courtesy CarbonWise.

 

Carbon Wise was brought on as a consultant to conduct a Life Cycle Assessment (LCA) of the home that included the deconstruction (C1) of the original building by Unbuilders, and that connected that deconstruction process to the production and construction (A1 to A5) of the retrofitted home. Carbon Wise compared those results with a Life Cycle Assessment of a business-as-usual scenario, in which the original home would have been demolished and a new, high-performance home built from entirely new materials. In both cases, they included optional photovoltaic (PV) systems necessary for net-zero operation.

Results and Conclusions

For homes with poor operational energy performance, net energy savings can be realized in a relatively short timeframe through either a demolition and reconstruction or deep energy retrofit. The carbon pay-back period was three and a half to five years in this study.

The carbon advantage of choosing a deep energy retrofit is also clear, though. Because of embodied carbon savings, the deep retrofit maintains a lower level of net carbon emissions. A deep retrofit, particularly one using carefully selected low-carbon materials can result in significantly below-average embodied emissions (120 kg CO2e/m2) as opposed to the average of 193 kg CO2e/m2 for a new home in the City of Vancouver

“Another important thing to note is the high embodied emissions of solar PV systems which, in British Columbia, should not be seen as the solution to climate change if the home is already using electricity from renewable sources,” adds Baudinaud, noting that solar panels have a relatively long, 35-year payback when the carbon used to manufacture them is taken into account.

The authors note that their conclusions will vary in regions where fossil gas is used or where electricity is not made from renewable sources.

For more information and to access the full case study, click here.

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