More Building Matters
The United States has set ambitious decarbonization goals by 2050—but what role will the homebuilding industry play? Well, with nearly 1.6 million units being constructed in the U.S. each year, hopefully a big one. Decisions made now for new construction will affect both the upfront carbon spent building the home, and the carbon that is released by operating and living in the home in the decades to come.
In this post, we will share the findings from a recent case study conducted by IBACOS for the U.S. Department of Energy (DOE) Building Technologies Office. The study’s intent is to help homebuilders benchmark their current performance and better understand the largest sources of carbon emissions in their homes. Equipped with this information, builders can select materials and construction practices that reduce carbon emissions and bring us another step closer to our decarbonization goals.
Why it matters
According to 2022 data from the U.S. Environmental Protection Agency, carbon emissions from commercial and residential buildings account for 36% of total U.S. emissions—with residential buildings representing just over half of this amount. This sets the stage for a considerable opportunity to help decarbonize the economy.
In April 2024, DOE released its roadmap for the building industry titled, “Decarbonizing the U.S. Economy by 2050: A National Blueprint for the Buildings Sector.” That roadmap lays out a vision of federal actions for increasing building energy efficiency, reducing on-site emissions, and minimizing embodied life cycle emissions.
As large production homebuilders adapt to these emerging realities, there is a need to understand and assess their current performance against “net-zero carbon” goals to enable better decision-making around available paths forward and help them to deliver on their commitments to decarbonize their businesses in environmentally, socially, and fiscally responsible ways.
To help with this process, the IBACOS/DOE case study introduces an approach to assess carbon emissions associated with a typical single-family detached home currently being constructed by production homebuilders in communities across the United States. The intent is to help homebuilders benchmark their current performance and better understand the largest sources of carbon emissions in their homes.
Two types of carbon emissions
There are two primary components of a home’s associated carbon emissions—the embodied carbon emissions and operational carbon emissions.
Embodied carbon emissions refer to the greenhouse gas emissions that result from the extraction of raw materials that go into building products, manufacturing of the products, transportation to a job site, construction of a building, the maintenance and repair of that building, and the eventual disposal of building products. These are also referred to as upfront emissions because they are released before a building is even occupied.
Operational carbon emissions refer to the greenhouse gas emissions associated with the heating, cooling, and energy and water use of the building.
For the purposes of this study, operational carbon emissions associated with the water use of the building were not included.
Same home plan, different climates
The study compared the embodied and operational carbon emissions in a typical new home in both cold and warm climates. Specifically, the research considered new, single-story, 1858-sq.-ft. production homes located in International Code Council Climate Zones 5 (cold) and 2 (warm).
The house plan was designed to roughly meet the 2015 International Energy Conservation Code requirements for the thermal enclosure and whole-building air leakage, with a slab-on-grade foundation in Climate Zone 2 and a basement foundation in Climate Zone 5.
Embodied carbon emissions calculations are based on actual material take-offs for foundation, exterior walls, interior walls, floors and roofs, windows, interior finishes, and site elements. Emissions calculations include upfront construction of the home, projected emissions over a 28-year maintenance period, and replacement of materials such as asphalt shingles, paint, and carpet.
Operational carbon emissions are based on modeled annual gas and electric use of the home using the National Renewable Energy Laboratory’s (NREL’s) Building Energy Optimization (BEopt) tool, converted to global warming potential using emissions factors available through NREL’s Cambium database. Operational carbon emissions were also calculated for a period of use from 2022–2050.
How to pencil out the math
Analyzing carbon emissions in residential buildings is an emerging field that uses software tools to calculate embodied and operational carbon emissions based on the specific products and materials in homes. Carbon emissions are typically presented in tons (or metric tonnes) of carbon dioxide equivalent (CO2e). While these carbon calculators each have a unique methodology to calculate carbon emissions, they all share a similar approach based on established life cycle assessment techniques and most rely on data provided by Environmental Product Declaration (EPD) documents available from product and material manufacturers and suppliers to assess and calculate the global warming potential of a building.
Case study results
1. Concrete and insulation decisions can lead to higher embodied carbon emissions.
The figures below show embodied carbon emissions associated with the same model home plan in a cold and warm climate. Primary differences between the two houses include foundation type (basement versus slab on grade) and insulation levels. Interior materials and finishes, and exterior cladding materials are similar.
As shown in the figures, the same home in a cold climate has approximately 50% higher total embodied carbon emissions than in a warm climate, primarily due to the larger volume of concrete used in the foundation and to the increased insulation levels in the thermal enclosure.
2. Natural gas means higher operational carbon emissions than electricity alone.
The operational carbon emissions calculated for this study were based on the home’s modeled annual energy use. For the cold climate home, natural gas was used for heating, water heating, and cooking while in the warm climate home, all equipment and appliances used electricity. Gas produces higher direct carbon emissions than electricity when used in these applications, leading to much higher operational carbon emissions from the cold climate home.
Breakdown of operational carbon emissions for a typical new production home constructed in a cold and warm climate, by end use (carbon emissions factors provided by NREL’s Cambium database). Home was constructed in 2022 and operates from 2023–2050.
Putting it all together
According to the EPA’s 2022 Greenhouse Gas Equivalencies Calculator, for a builder constructing 10,000 homes in a given year to the 2015 building code standard, the cumulative amount of embodied and operational carbon emissions released from these homes over the next 28 years is equivalent to the emissions from about a single year’s production of six natural gas power plants.
Given the current rate of approximately 1.6 million housing starts, if a net-zero emissions economy is to be realized by 2050, the residential construction community has a significant role to play in reducing these carbon emissions.
Actionable steps
Builders can consider taking several important actions right now to reduce the carbon emissions associated with their homes. With nearly 1.6 million units being constructed in the United States each year, the biggest immediate impact a builder can having on meeting U.S. climate goals is to select the materials and construction practices that reduce upfront embodied carbon emissions.
Additionally, building energy-efficient homes, such as homes that comply with the U.S. Environmental Protection Agency’s ENERGY STAR Residential New Construction program or DOE’s Zero Energy Ready Home program, will provide ongoing reductions to operational carbon emissions that will continue to lower as utilities adopt cleaner sources of energy.
As the industry continues to adapt to these emerging realities and as greater attention and awareness is focused on building carbon emissions, new programs and guidance will emerge to support a transition to a “net-zero carbon” building sector.
Two such initiatives include the HERS Carbon Index released by the Residential Energy Services Network (RESNET) to assess a home’s operational carbon emissions, and the new Standard RESNET/ICC 1550 that will enable the building industry to assess a home’s embodied carbon emissions. Ultimately, standard tools and processes will be needed to simplify the upfront analysis of home designs and to enable builders to make more informed decisions for their businesses.
The following immediate actions can help builders start to address the needed reductions in embodied and operational carbon emissions associated with their homes:
Embodied carbon emissions
- Create specific goals for embodied carbon reductions, and then consider alternative products and construction practices for the biggest areas of embodied carbon emissions in the home.
- Request information from your suppliers and vendors on the environmental impacts of their products, and ideally get an EPD for each of their products.
- Reduce construction waste and hold trade contractors responsible for compliance. Working with off-site manufacturing suppliers even modular manufacturers is one way to reduce construction waste.
Operational carbon emissions
- Improving building energy efficiency will impact the long-term operational carbon emissions, so consider improvements to your building enclosure, mechanical systems, appliances, and other areas of your homes to reduce operational energy use.
- Work with electric utilities to support homeowner participation in energy efficiency and demand flexibility programs, to help reduce peak load issues and support greater integration of renewable energy sources to the electric grid.
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Ari Rapport is a sustainability professional and former HERS Rater based in Pittsburgh, PA. Anthony Grisolia is Managing Director of Innovation Programs at IBACOS. This report was a collaboration between IBACOS, the National Renewable Energy Laboratory (NREL), and Building America Technical Support. Images courtesy of authors.
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9 Comments
Build smaller homes. Use less concrete. Stop relying on foam insulation.
(Thank you for this article. I look forward to reading the full NREL report.)
Clearly we should all be moving to warmer climates. If we can't manage it during our working lives we can always do it in retirement.
What's really needed, in addition to the items mentioned, is for most new housing to be multi-family. That has a greater impact than using greener materials on smaller, single-family homes.
Michael
I grew up as an "Army Brat", spent half (or more) of my childhood
in military housing (multi-family). Nothing short of threatened jail
time would convince me to go back to it now! I suspect my attitude
is fairly typical for older adults who have achieved some economic
independence. What's really needed is a never-ending advance in
technology which will eventually negate the need to worry about
things like carbon emissions.
I understand; my career has almost entirely been designing, building or renovating single-family homes, and that's mostly what I've lived in, so I understand the appeal. I'm just saying that despite the appeal, we aren't going to make a serious dent in the climate crisis by building slightly-better-than-average homes. (Even Pretty Good Homes.)
Technology may save us in the end, but we would also do well to reign in our unending appetite for consumption, including of energy and carbon-emitting materials and activities. I'm not saying it's realistic, but it's looking like we are going to pay the price for not being able to control our gluttony.
Multifamily absolutely!
I am currently finishing a 4-floor 6000 gross square foot multifamily. It's a slab on grade and the entire foundation (5 foot tall precast Superior Wall) came on a single semi-trailer. That's less concrete than what would go in the poured-in-place foundation of a modestly sized single family -which is multiple 20-ton concrete trucks when concrete footings are included. Same goes for siding, insulation, roofing etc. And later on, space heating.
Gstan, I've lived all my adult life in multifamily. I've never lived in army barracks but I imagine that these were very lightly built. With proper air sealing and sound insulation -neither of which is that expensive- sharing wall or ceiling with a neighbor is uneventful. I once spent a few nights in a rural area where each home had an acre or two of land. There was a neighbor who enjoyed late drinking parties and shooting cans; I couldn't wait to return to my condo in the hood!
Yesterday the NYT had a nicely illustrated summary about the fast-approaching tipping points of climate change. From reading this and other mainstream stories, my guess is that sea level rise will pick up pace to over a foot per decade before 2100. It happened not that long ago -easy to find by googling "meltwater pulse".
Much of the planet is already experiencing a housing crisis. I am trying to picture what it's going to be like when hundreds of million need to be relocated from coastal regions with new housing built for them. And that will be on top of the emissions from the unlivable buildings that are taking water -those will generate plenty of emissions even if there is enough time and resources to put them in a landfill.
And for context, we, American, are already responsible for over 15 tons of CO2/capita/year, or three times the world average.
This link to yesterday's NYT piece may work for non-subscribers:
https://www.nytimes.com/interactive/2024/08/11/climate/earth-warming-climate-tipping-points.html?unlocked_article_code=1.Ck4.e05G.ulHOGLaxAp4C&smid=url-share
Seemingly a frost protected shallow foundation and cold climate heat pump on the northern example would have provided a more fair comparison. I’d guess those are excluded to stay true to the norms of production builders.
Mike: Thank you for bringing up the elephant in the room! I'm certainly biased having one leg in the urbanism world, but seemingly if we’re serious about sustainability, dense, walkable, bikeable, and transit rich development needs to be the new normal, and car dependent single family homes should be acknowledged as the inherently unsustainable luxuries that they are.
Multi-family urban living is great for a lot of people.
But not for me and a lot of people like me.
I'm just mentioning this because we sometimes slip into the slightly fanatical tendency to insist that "my solution is the one right solution".
So, in addition to encouraging multi-family housing solutions, we do need to find better ways to do suburban and rural single-family as well. (And GBA is a great place to learn how to do better single-family!)
Arguably the most important thing is to find ways to upgrade the existing low-income housing stock, urban, suburban and rural. After all, we can design the perfect multi-family, or even single-family, project. And we can build 1 million of them over the next year. But the many millions of grossly under-performing existing homes will still be there, still grossly under-performing. Bringing those homes up to a higher standard would not only address the serious climate change issue we're facing. It would directly improve the lives of the low-income people living in them.
Back to fanaticism though. I'm a vegetarian and have been most of my life. I've read studies that show that vegetarian, and especially vegan, diets have a much lower carbon footprint. I could be standing on my soapbox insisting that everyone should eat just like me. But I don't do that, first of all because I think other people should have the choice as to what they eat. Part of advocating for a free society is that people should have more choices. Secondly, I don't do it because I know it's unrealistic and being fanatical about it is probably counter-productive.
I have a young relative who is very active in the bicycle community and has done a lot to get her city to put in more bike lanes. I admire her for her efforts. But sometimes she slips into that fanatical mindset as well and forgets that some people simply can't use her solution. For example, people of limited mobility are not going to be able to get on a bike and bicycle across town in the rain to get to their doctor's appointment, or carry bags of groceries home. These people will need to access services and shops available only on pedestrian streets. How are they to do that? Their needs must be worked into the solutions as well. Maybe that means providing an exception for cars bearing handicapped placards, or perhaps a shuttle bus cruises up and down the street.
We need inclusive solutions. And note that I used the plural form, solutionS.
If you made it this far thanks for listening to my rant.
I think a variety of housing forms makes sense. Multi-family, townhouses, and single family. What doesn't are the suburban developments which have no access to services, education, or employment without using a car, are too dispersed for public transit, and have disproportionate infrastructure costs.
What I struggle with in my own practice is the question as to whether the small changes we make in carbon reduction in our buildings lead to reductions that make any difference, or whether we are just doing them for (perhaps legitimate) moral reasons. Yesterday I saw that last year's emissions from wildfires here in Canada produced about 2bn tonnes of CO2. That dwarfs any improvements in carbon reduction we are likely to see from changing building practices. It seems increasingly clear that worldwide we are not getting to grips with the problem of emissions, nor are we likely to.
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