Depending on size, materials, and how those materials are sourced, constructing a new house likely emits on the order of 15 to 100 tons of CO2. That's a lot, but only a fraction as much as an inefficient house might emit over its lifetime.

To know how much carbon is emitted by building a house, says David Hsu, MIT associate professor of urban and environmental planning, you need to know the embodied energy of a house: how much energy was used to make all the materials that go into a home. It’s a complicated question, because a house contains not only common building materials like concrete, brick, and steel, but also aluminum, wood, glass, copper, asphalt shingles, and all sorts of plastics, from vinyl flooring to weatherproofing house wrap.

“All of those things involve greenhouse gas emissions,” Hsu says. “The cement and steel are probably the two biggest categories we know of because they're relatively easy to measure. But a house is a combination of pretty much every industrial material you can imagine.”

Steel and concrete are particularly problematic emitters because creating them requires heating raw materials to high temperatures, and the energy to do this typically comes from fossil fuels. As a result, the creation of cement for concrete is responsible for 7% of the entire world’s carbon emissions (1), while steel creates 2.3 tons of carbon for every ton of metal produced (2). Among the other materials, aluminum is a particularly high emitter, causing 3% of the world’s direct industrial CO2 emissions (3).

In the 2009 book Sustainable Energy—Without the Hot Air, British physicist David MacKay added up all the materials in a typical three-story home and concluded that such a house would have an embodied energy of 42,000 kilowatt-hours (kWh) (4). In the United States, producing 1 kWh creates around 0.85 pounds of CO2 (5). At that rate, building the house in MacKay’s calculation would produce around 16 tons of carbon dioxide. Another estimate, from 2021, pegged a three-story home at 26 tons of embodied CO2 (6). And a 2021 project by the environmental consultants Circular Ecology found that their test home would’ve created 78 tons of CO2 to build under normal circumstances, but that the number could be brought down to 32 tons by smart design strategies (7).

It’s no surprise the estimates vary so much. There’s plenty of wiggle room in calculating the carbon footprint of a single material from mining to production to a person’s home, much less performing the same calculation for everything in a house. Plus, as Hsu notes, the true amount of carbon created by a given home could vary wildly depending on which materials were used, in what quantities, and where they came from.

To reduce a home’s carbon footprint, researchers are working on lower-carbon ways to make important construction materials. Meanwhile, architects can design structures that reuse materials such as recycled timber and avoid especially high-carbon stuff like aluminum. “Choosing to build with timber is a really interesting route now,” Hsu says, “because the timber comes from a tree that's sequestered carbon in the first place, so presumably you're building with carbon from the atmosphere. By using that material, you're not using some other carbon-emitting material.”

Once you’ve made the decision to build a new home, Hsu sees two big choices that govern how carbon-intensive it will be. First is the size of the house, since a mansion might require vastly more building material than a cozy cottage. New homes in the U.S. have grown 45% in size since the 1970s, requiring much more material per home (8).

The second part is location. If someone builds their new house close to work and family, they could reduce their carbon emissions from transportation. For someone who builds a second home across the country or across the sea, however, the air travel required to go there would wipe out any carbon savings the homeowner gained through smart building practices.

Of course, construction is simply the beginning. Once complete, the home begins its decades-long lifespan. During those long years, its carbon footprint will be measured not by embodied energy but by operational energy: how many kilowatts it takes to meet the residents’ heating, cooling, electrical, and other needs, and how much CO2 is the result. If a home lasts a hundred years, its ongoing energy efficiency will prove far more meaningful than how much CO2 was released by building the house.

Thank you to Patrick McCann of Farmington, New York, for the question. You can submit your own question to Ask MIT Climate here.

Published December 9, 2022.

FOOTNOTES

1 PBL Netherlands Environmental Assessment Agency. "Trends in Global CO2 Emissions: 2016 Report."

2 Energy Transitions Commission. "Mission Possible sectoral focus: steel." January 2019.

3 International Energy Agency. "Aluminium tracking report—September 2022."

4 David MacKay. Sustainable Energy—without the hot air. UIT Cambridge Ltd., Cambridge, UK, 2009.

5 U.S. Energy Information Administration. "How much carbon dioxide is generated per kilowatthour of U.S. electricity generation?" Accessed December 9, 2022.

6 The Conversation. "Embodied carbon: why truly net zero buildings could still be decades away," by Ljubomir Jankovic, November 11, 2021.

7 Circular Ecology: "COP26 House - An Embodied Carbon Exemplar." November 4, 2021.

8 University of Michigan Center for Sustainable Systems: Residential Buildings Factsheet. Accessed December 9, 2022.