A New Way for the Housebuilding Industry to Look at Embodied Carbon

A critically important study concludes that we have been doing it all wrong.

House Construction

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A new report issued by Natural Resources Canada, "Achieving Real Net-Zero Emission Homes," could change the way the homebuilding industry looks at carbon. Prepared by Builders for Climate Action, it is written for the Canadian scene but the concepts could and should be applied everywhere.

Embodied Carbon has been called the blind spot of the buildings industry and, more recently, a hidden climate challenge. I have described it as "the carbon burp that comes from extracting, manufacturing, transporting, and assembling building materials." It is just beginning to appear on the radar of the North American construction industry; see the Rocky Mountain Institute dip its toe into the issue with its recent report.

While embodied carbon may be getting a tiny bit of attention from architects and the commercial construction industry, the homebuilders have probably never heard of it. They are still working with building codes that regulate operating energy efficiency and haven't noticed that we have a carbon crisis, not an energy crisis. Embodied carbon is hard to define and explain, and probably harder to regulate; this new report is the best stab at it that I have seen to date.

Chris Magwood in front of "Canada's Greenest Home."
Chris Magwood in front of "Canada's Greenest Home.". Lloyd Alter

I have often complained that "embodied carbon" is a terrible name because it is not embodied, it's in the atmosphere. I suggested that it be called Upfront Carbon Emissions. The authors of the report, Chris Magwood (known to Treehugger readers as a pioneer in the issue of embodied carbon), carbon analysts Javaria Ahmed and Erik Bowden, and Jacob Deva Racusin, don't think much of it either and have come up with yet another name.

"Even if all operational carbon emissions (OCE) from Canadian buildings reach net-zero, the substantial volume of emissions from the production of materials used to build Canadian homes will continue to be a leading source of housing sector emissions. These material-related emissions are commonly known as 'embodied carbon,' but would perhaps be more accurately labeled as 'material carbon emissions' (MCE). This project focuses on the MCE aggregate of greenhouse gas emissions from processes involving raw material harvesting, transportation, and manufacturing of a product."

The key point of the report is that the industry and the codes have to stop just measuring energy consumption and start looking at the full carbon picture. "This study makes clear it will require seriously addressing MCE by embracing low-carbon and carbon-storing materials and designs, while recalibrating efforts on the operational side by concentrating on total GHG metrics rather than energy use metrics."

The report then goes through a study of different housing types in different Canadian climates and models them under different tiers of the Canadian building codes. We will skip all that here and stick to the universal themes and findings. They try to keep things relatively simple by dividing materials into four categories.

High Carbon Materials (HCM): "Readily available and commonly used in residential construction. Though this selection represents a worst-case scenario, it also represents a scenario not uncommon in the home building industry." Includes XPS foam insulation, spray foams, brick.

Mid-Range Carbon Materials (MCM): "This set of materials is readily available and represents a fairly typical residential building constructed in today’s market that intentionally avoids the worst materials from an MCE perspective." Includes mineral wool, fiber cement siding.

Best Available Carbon Materials (BAM): "Selected to represent a building that could be constructed today using widely available mainstream products with the lowest MCE. This is the best material selection set for homes that could readily be built in large-scale quantities today." Includes cellulose, wood siding.

Best Possible Carbon Materials (BPM): "These materials were selected to achieve the best possible MCE results from existing materials. Some of these materials are not yet available in the mainstream market ... A home constructed from this combination of low-carbon and carbon-storing materials has negative MCE emissions, meaning it stores more carbon than it emits. This represents a potential for the housing sector to become a national carbon sink." Includes straw bale, wood siding.

 Visual summary of highest and lowest material carbon intensity and carbon emissions
Visual summary of highest and lowest material carbon intensity and carbon emissions. Builders for Climate Action

The cost differential between choosing Best Available and High Carbon materials isn't huge, but the difference in Material Carbon Emissions is profound. And it's not rocket science—the authors used a new Material Carbon Emissions Estimator that Natural Resources Canada is releasing to the public later this year, but there are not a lot of different materials in residential construction and most of the carbon impact is in the insulation, cladding, and concrete.

Measure What Matters, and That's Carbon Use Intensity

Carbon Use Intensity
Carbon Use Intensity. Builders for Climate Action

Perhaps the most important insight for the industry at large is the concept of carbon use intensity (CUI). Instead of just measuring building energy efficiency as is done now, CUI is based on calculating the Material Carbon Emissions and adding the Operational Carbon Emissions. But in an all-electric home, these vary according to the carbon footprint of the source of electricity. So once again, forget about energy efficiency and think about carbon, which you get by multiplying the energy consumption by the source emissions. This will obviously result in a CUI that will vary from region to region, but it's the number that matters.

"The Carbon Use Intensity metric would enable more accurate accounting for [greenhouse gas emissions] from the homebuilding sector, and would also allow for regionally appropriate ways to reach CUI targets. In those jurisdictions with available clean electricity, the focus for improving CUI would be more weighted to material emissions, while in jurisdictions with emissions-intensive energy sources, CUI reductions could be achieved by addressing material and operational emissions in conjunction. Anywhere in the country, designers and builders could respond to any national, provincial, or regional CUI regulations while pursuing a CUI strategy that meets the needs of their clients and the climate with as much flexibility as possible."

So, in Vermont, with its clean renewable electricity, you would concentrate on lowering the material carbon emissions; in coal-fired Wyoming, you'd focus on the operational carbon emissions. I have not seen another model that takes such a big-picture view of the full carbon problem.

This Changes Everything

Highest Carbon Materials
Highest Carbon Materials. Builders for Climate Action

Look at the difference between a two-story house built in Toronto with high carbon materials here:

Medium Carbon Materials
Medium Carbon Materials. Builders for Climate Action

Compare it to a house built with medium carbon materials. They are almost indistinguishable, mostly with changes in insulation and a different mix of concrete, and the Material Carbon Emissions are about a quarter as high.

Best Available Materials
Best Available Materials. Builders for Climate Action

Go wild with the best available materials and the house actually is carbon negative. This may be a bit much for the housing industry, but they could go with the medium carbon materials without missing a beat. They just don't know about this, and they don't have to because it's not regulated. It is not even discussed.

Forget About Energy and Focus on Carbon

This is the main lesson. This is what matters, and why that Carbon Use Intensity calculation is so important.

There are likely to be 1.6 million houses built in the United States this year; according to the Census, the average size is 2,333 square feet. Based on data from this report, that works out to 64 tons of Material Carbon Emissions of CO2 per average house, or 102 million tons of CO2 from the housebuilding industry, burped into the air this year, the equivalent of 22 million cars driven for a year. Much of this could be eliminated without much difficulty if only the industry were actually aware of it.

Of course, there are many other issues that have to be discussed, from urban planning and ending sprawl, or house size, and whether we should be building single-family houses at all. But this is the American housing industry we are talking about, so those issues are not easily resolved. This issue of embodied carbon can be dealt with now.

I can't overestimate the importance of this report, "Achieving Real Net-Zero Emission Homes." It was written for Canada, but the ideas and the lessons should be applied everywhere.

View Article Sources
  1. Magwood, Chris, et al. "Achieving Real Net-Zero Emission Homes." Natural Resources Canada, 2021.