Total Carbon Footprint of a North American Home Is More Than You Think

Measuring the energy efficiency of a house is pretty easy: You have your heat loss calculation before it's built and you have the gas and electric bills after. Figuring out the operating carbon emissions is straightforward as well. But what about the embodied carbon—the upfront carbon emitted when building a house?

Embodied carbon is not so straightforward and is rarely even thought about, even though it should be. There were 1.7 million single-family housing starts in the U.S. in 2021 and 63,456 in Canada, where there is a higher proportion of multifamily units. That's going to add up to a lot of carbon going into the air right now. Now, a new Canadian study, "Emissions of Materials Benchmark Assessment for Residential Construction" (EMBARC), quantifies how much carbon is at stake and what we can do about it.

This is important because we have a global emissions budget, the total amount of carbon dioxide that can be added to the atmosphere or a ceiling that we have to stay below to keep global heating below 1.5 degrees Celsius (2.7 degrees Fahrenheit). Every ounce or gram of carbon we add to the atmosphere now counts against that ceiling. That's why the carbon emitted while building houses matters.

This is often called embodied carbon, but I prefer the term upfront carbon emissions (UCE) because that's when they are emitted. These can end up being substantially more than the operating emissions over the life of a building, especially if the houses are all-electric. Upfront emissions are unregulated and are generally ignored, but they are emissions happening now and are critically important.

There has recently been a lot of research into the UCE or material carbon emissions (MCE) of large buildings, but not much research into low-rise residential construction. In the new EMBARC study, a team led by Chris Magwood of Builders for Climate Action, in collaboration with Passive Buildings Canada, looked at over 500 as-built single-detached, semi-detached, and townhouses. While the data and the houses are Canadian, they are not dissimilar from homebuilder houses in the U.S. in cooler climatic regions.

The researchers examined 59 different floor plans and entered data into a spreadsheet application called BEAM (building emission accounting for materials), which counts up the emissions from raw material acquisition (A1), transportation to the manufacturing facility (A2), and manufacturing (A3), or "cradle to gate" emissions. These are the MCEs.

Interestingly, they do not include A4 (transport to site) or A5 (the construction process) which I would have thought were significant contributors to the full upfront carbon picture. When asked about this, Magwood told Treehugger:

Two reasons they weren't included: They are much less significant than might be expected (3-6% of total emissions), and it's impossible to estimate them accurately. I did a deep dive for my thesis and found that the assumptions in the LCA softwares I was sampling were typically 50 to 150% wrong in their estimations compared to an actual analysis of how materials move around. So although it is definitely important to consider A4 and A5, it's not something that a broad study or a tool can do with anything approaching meaningful accuracy.

They also didn't include mechanical, electrical, plumbing, surface finishes, fixtures and appliances, millwork and carpentry, or exterior site work because there was insufficient reliable data. The researchers note these could double the impact of home construction.

The average house had 40 tonnes of carbon dioxide equivalent (CO2e) of MCE, the lowest had 9.5 tonnes, and the largest, a 15,880-square-foot monster home, had a whopping 827 tonnes of CO2e. And remember: These should probably be doubled to reflect the total contents of the house.

Unsurprisingly, single-detached homes had significantly higher MCEs than semi-detached or townhouses. They are typically bigger and have more surface area to clad, and generally have more complex forms.

Material choices have a massive impact on the MCE, with a third of the MCE being concrete from the foundations and slabs, with insulation close behind. These numbers may reflect the designs of Toronto-area houses, which almost all have full basements and are often wrapped in XPS foam board sheathing to increase the level of insulation.

Note how the report states no carbon credit is applied for the use of wood:

"There remain important and unresolved concerns with current accounting methods related to virgin forest products like lumber. Some of these concerns include uncertainty about the amount of carbon released from soils during logging operations; the amount of carbon returning to the atmosphere from roots, slash and mill waste; the amount of carbon storage capacity lost when a growing tree is harvested; and the lag time for newly planted trees to begin absorbing significant amounts of atmospheric carbon dioxide."

The study then looked at best available material (BAM) substitutions, such as better concrete mixes, cellulose insulation, and engineered wood cladding versus brick—none of which would surprise an average homebuyer. But also best possible material (BPM) substitutions, such as using straw insulation, compressed strawboard walls, and drywall "changed to sheets of compressed recycled drinking boxes, based on products available in the USA and Europe."

Switch to the best available materials and—BAM!—a 51% reduction in material carbon per square meter. These are changes that could be made by any builder without a significant increase in cost if only any builder was actually aware of the issue. Switch to the best possible materials, which would be a stretch in the marketplace, and the house goes carbon-negative thanks to the straw.

The study also didn't look at making any design changes, "massing changes, solar orientation, window sizing, air tightness or mechanical systems." This is surprising given the participation of Passive Buildings Canada, as they could point out that simple forms and careful window placement can make a significant difference. (See our discussion of radical simplicity.)

The study concludes by noting that the impact of MCEs is substantial—significantly more substantial than even shown in this study because of the elements that were excluded. They suggest "the measurement of MCE for new homes should become standard practice in order to collect more accurate and complete data and help to drive voluntary emission reductions and inform future regulatory interventions."

Why wait for future regulatory interventions? Magwood previously discussed carbon use intensity, which measures what really matters now—carbon. Almost all of our regulations today cover only one-third of this picture: the energy use of a building. The gas company won't like it if we start measuring the energy source emissions, but they certainly matter. And of course, there are the MCEs.

The housebuilding industry is huge, and as this study demonstrates, we are only thinking about a very small part of the picture. We have to fix this now or we will just blithely sail through our carbon budget without even knowing how or why.