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[Editor's note: This article has been enhanced with additional product details since it first appeared in print.]
As a custom homebuilder in North Idaho in 1978, I wasn’t thinking about carbon, I was thinking about saving energy. To build my first superinsulated house, I used double, staggered 2x4 studwalls stuffed with fiberglass pink stuff and wrapped with newfangled plastic white stuff—Tyvek. Driven by skyrocketing fuel costs (sound familiar?), I was trying out innovative techniques and materials to make my houses energy efficient.
Some 40 years later, energy efficiency has become just one part of constructing green, resilient, and sustainable buildings, which the Environmental Protection Agency defines as “… creating structures … that are environmentally responsible and resource-efficient throughout a building’s life-cycle from siting to design, construction, operation, maintenance, renovation and deconstruction.”
New building codes require tighter, better insulated buildings, which use less energy to heat and cool and, as a byproduct, put less CO₂ into the atmosphere. This category of CO₂ savings is referred to as “operational carbon.”
Now, however, there’s a growing awareness of reducing a second—many believe, more critical—type of CO₂ emissions known as “embodied carbon” or “upfront carbon,” which consists of the total CO₂ emitted when we extract, manufacture, transport, and install all the materials that go into our buildings. (See “Carbon and the Carbon Cycle.”)
What is eye-opening to those of us who have struggled to squeeze out every extra Btu through energy-efficient construction (as with Passive Houses, for example) is that the amount of embodied carbon in a new building is huge. In fact, the quantity of upfront carbon released into the atmosphere to construct even a zero-energy home can equate to decades of the CO₂ emitted (operational carbon) by using that house. Builders striving to make a large and immediate reduction to CO₂ going into earth’s atmosphere must find ways to lower the amount of embodied carbon going into their buildings. Looked at another way, construction material emissions are “today” emissions; they are released into the atmosphere even before the building is built. If our goal is to reduce emissions now, these are the emissions we must focus on.
In this article, I’ll look at design and material choices that designers, builders, and remodelers can use to reduce the amount of embodied carbon in both their new and remodeled buildings. I will also explore strategies that won’t force you to abandon your favorite building systems such as SIPs, ICFs, or studwalls.
Key Strategies
There are three key strategies to lower—to zero and even negative—the amount of embodied carbon in our buildings: reusing infrastructure, designing to minimize carbon emissions, and using lower-carbon materials. And since reducing upfront carbon is an additive process, you can use these approaches individually or in any combination to reduce a building’s overall carbon footprint.
Use existing infrastructure to lower embodied carbon. When it comes to building with low embodied carbon, the best thing you can do is reuse a building or its parts. It may not be cheaper, but whole-building renovation and reuse have been calculated to save up to 75% of embodied carbon emissions compared with constructing a new building. This is because most embodied carbon resides in the foundation and the structure—especially if they are concrete and steel. By retaining those, that carbon is already accounted for.
If you cannot reuse the whole building, look to salvage and reuse its materials—brick, metals, broken concrete, and wood. Reclaimed materials, in general, have a much lower embodied-carbon footprint than new materials because the carbon to manufacture them has already been spent. Even the additional carbon impact of salvaging materials and making them fit for reuse is often lower than manufacturing new materials.
For example, not only does reclaimed wood siding save the energy that would have been spent cutting, transporting, and processing new siding, but the tree you didn’t cut down is still doing the work of capturing and storing (sequestering) carbon. Another example is reusing broken-up concrete slabs for landscape, riprap, or even just backfill, which eliminates the carbon that would have been emitted hauling and dumping the waste, as well as saving money on fees.
Lower embodied carbon by design. From the start, building designs should aim for low-carbon, carbon-neutral, or even negative-carbon outcomes. Stated simply, your design should incorporate materials with the least embodied carbon, and a significant reduction in upfront carbon can be made by reducing the amounts and changing the makeup of three materials: concrete, insulation, and cladding/interior surfaces—in that order.
Since most of the embodied carbon is in the structural components, the design should strive to achieve maximum structural efficiency. One example is to use optimum value engineered (“advanced”) wood framing, which saves wood, money, and embodied carbon.
In addition, designs should strive to minimize waste. Perhaps your residential designs already incorporate 2-foot modular layout using common materials like 4x8 plywood, 12-foot drywall, and precut structural members. Sam Rashkin, architect for the Department of Energy Building America program, cleverly suggests adjusting the roof pitch so that roof rafters can be sized to use standard lumber lengths to eliminate waste. A centralized subpanel might reduce copper runs. Kitchens and baths can be grouped near each other and water heaters placed nearby to reduce piping and heat loss through plumbing runs (see “Architectural Compactness and Hot-Water Delivery” by Gary Klein, Jan/20). Split HVAC systems can replace steel ducting with a much smaller volume of copper and reduce the heat losses associated with ducts.
Designing a building so it can easily be remodeled—future-proofing—can lower the carbon footprint of a building over time. For instance, designers can incorporate clear spans in their plans so spaces can later be reconfigured by moving nonbearing walls without significant demolition, effort, and waste. And using structural components such as modular wood interior wall panels that are screwed in place and can easily be taken apart and used again can guarantee a longer building life and fewer future emissions.
While the building envelope is critical for the energy performance of the building, the façade and roof are more expendable. These building elements are under constant assault from rain, snow, ice, and sun, and necessarily need repeated maintenance and repairs. The use of durable, local materials not only reduces the cost and frequency of repair but also reduces the use of material replacement and its associated carbon footprint.
Select low- or negative-carbon materials. Four material categories contribute substantially to a typical residential building’s carbon footprint: concrete (35.5%), insulation (15.3%), cladding (12.5%), and interior surfaces including flooring, wall and ceiling materials (12.2%). When deciding on materials, you want to choose low-carbon or even negative-carbon alternatives. Replacing steel or concrete in the structure with wood or using wood cladding instead of cement or vinyl can reduce the embodied carbon in your project.
When choosing materials with low embodied carbon, you’ll often encounter conflicting claims, because the science continues to evolve and because some manufacturers “greenwash” their product’s carbon footprint. To understand a product’s impact from a life-cycle perspective, we now have Environmental Product Declarations (EPDs)—documents that transparently report objective, comparable, third-party-verified data about products and services.
However, EPDs for an entire project can be hard to find and overwhelming to a designer or builder who wants to build with a low carbon footprint but also wants to get on with the job. Fortunately, there is an increasingly wide range of software tools and strategies available to help in design and material decisions. BEAM, EC3, One Click LCA Planetary, and EC Calculator are a few. In writing this article, I chose BEAM and will discuss it below.
Also, try to use materials with high recycled content, especially metals. The carbon footprint of virgin steel, for example, is five times greater than that of high-recycled-content steel. Again, this is because the impact from raw material extraction is accounted for only the first time that material is processed. Subsequently, the recycled material includes only the reprocessing impacts.
You can also use fewer finish materials. One way is to showcase structural materials as finish. Using polished concrete slabs as a finished floor saves the embodied carbon from carpet, tile, or vinyl flooring, not to mention noxious and toxic adhesives and coatings. Finishes may help with the acoustics and thermal conditions inside living spaces. Yet, they have short lifespans due to wear and trends in fashion. The additive consequence of replacing these elements numerous times over the life of a building can have a measurable impact. So, finishes should include low-carbon materials and allow for the easy recovery of those materials for recycling or reuse.
Negative carbon. In some cases, it’s possible to select materials that not only have a low carbon footprint but that also remove and store carbon from the atmosphere, a process known as carbon sequestering. For instance, some concrete mixes actually absorb and store small amounts of carbon. Others add CO₂ captured in other industrial processes (such as capturing CO₂ in coal-fired power plants) into the mix.
Our buildings can also be designed to remove and store embodied carbon, becoming carbon “sinks” that can help reverse the accumulation of the CO₂ catastrophically warming the planet. In their book, Build Beyond Zero: New Ideas for Carbon-Smart Architecture (Island Press, 2022), from which I learned about many of the strategies presented here, Bruce King and Chris Magwood reenvision buildings as one of our most practical and affordable climate solutions.
Using materials made from what today is considered “agricultural waste”—products such as wheat or rice stalks that are commonly burned—can make a big impact on a project’s carbon footprint because they sequester carbon that would otherwise go into the atmosphere as methane when they’re allowed to rot, or as CO₂ when burned to make electricity. Wood may be the first material to come to mind, but other options include straw or hemp-based materials, say for insulation, which—unlike wood—not only store carbon but are annually renewable. And cellulose insulation, which has been successfully used for decades, is a no-brainer choice, with its negative carbon footprint.
Specific Materials Overview
In this next section, we’ll take a brief look at concrete, steel, and insulation materials from an embodied-carbon viewpoint.
Concrete. For all the benefits of concrete, the “moldable rock” used since it was invented by the Romans, it must be the primary target in our efforts to lower embodied CO₂. Worldwide, the cement sector represents about 7% of CO₂ emissions. In most cases, concrete is the biggest source of embodied carbon in virtually any new building project—representing 20% to 50% of the total material carbon emissions (MCE) for a low-rise building. The good news is that there are a growing number of ways, both in design and material composition, to lower concrete’s impact on a project’s total upfront carbon.
For one: Use less. Engineers love safety margins when designing, so if you make them aware of the impact concrete has on a project’s carbon footprint, they may, for example, be able to specify smaller footings, or recommend alternative foundation systems, such as concrete piers, steel helical piers, treated posts, or just thinner stem walls. Also, the strength of concrete is largely a factor of the amount of cement in the mix. A 6-sack mix may be needed for a foundation spread footing, but is it needed for a 4-inch-thick sidewalk? For that matter, will 3 inches work instead of 4 inches?
Try using the knowledge of your concrete supplier; it often can specify low-carbon mixes that use additives such as fly ash, slag, calcined clays, or polymer fibers. The cement industry realizes the impact its product has on the environment and is making an effort to come up with lower embodied-carbon solutions.
Newer substitutes such as hempcrete and carbon-neutral CMU block can work in many applications. Carbicrete of Montreal, with funding from the Quebec government, has developed a method of making concrete without cement by replacing it with a by-product of steel production, steel slag.
Steel. The steel used to reinforce concrete is also a huge contributor to a building’s carbon footprint. Of course, concrete needs something to give it tensile strength.
One alternative is to use rebar made from recycled steel. An even better alternative is to use fiberglass rebar, which has been around for several years but is only now getting attention as a sustainable choice. As a steel replacement, fiberglass rebar weighs less, costs less to ship, and even allows the use of unwashed sand and salt water in the mix because corrosion and subsequent spalling is no longer a problem. And there are many fiber additives that strengthen concrete. Again, your ready-mix supplier probably can provide low-carbon mixes—just ask.
Insulated concrete forms (ICFs). For those builders who do not want to abandon the many positive benefits of ICFs, fiberglass rebar and fly-ash-enhanced cement can reduce the carbon content in what is a relatively high-embodied-carbon building-envelope system. ICFs such as Logix Platinum Series, which uses BASF Neopor low-carbon-content foam, can also reduce a project’s carbon footprint. And concrete form blocks, such as Nexcem, made with wood chips or other natural fibrous materials, eliminate high-carbon foam while offering a comparable resilient, energy-efficient building system.
Structural insulated panels. If you are a committed SIP builder, and I’m one of them, you have a growing number of options to reduce the amount of embodied carbon in the structural insulated panel. A SIP is a composite sandwich composed of two skins laminated to an insulative “spacer,” typically 4 to 6 inches of expanded polystyrene (EPS) or extruded polystyrene (XPS).
OSB is the most common skin material, but there are lower-embodied-carbon substitutes for OSB including “boards” made of compressed straw stalks of wheat or rice or other agricultural carbon-sequestering “waste” material. Cementitious materials such as magnesium oxide (MgO) boards are being used as skins (TitanWall is one example among many), and although they may have comparable carbon footprints to OSB, their moisture, fire, mold, and insect resistance can allow wall assemblies to eliminate additional layers—such as WRB, cladding, and gypsum drywall—and thereby lower the overall carbon footprint.
Straw combined with other low-carbon insulation and vapor control layers is increasingly chosen as the base for structural frame panels, using both site-built panels (see New Frameworks example, below) and commercially available panels. Straw Bale SIP Walls by NatureBuilt, which have 1-inch-thick cement- and lime-plaster skins and straw filler between, and EcoCocon (see below) are two available panels.
Wood framing. If you traditionally frame using 2x4, 2x6, or larger stud- or timber-framed walls, you are already on a path to a lower-carbon-footprint building. “Wood is good” because the material takes significantly less processing energy to extract, transport, and process (mill and kiln dry). And the carbon in the lumber, cladding, flooring, and so on is stored (sequestered) until the wood burns or decays—which returns the carbon to the atmosphere. However, the complete carbon cycle isn’t as clear as it looks, since there are consequences to removing trees that could still be capturing carbon if left in the forest, and it’s not clear how the roots of cut trees and branches and slash contribute to atmospheric carbon.
Of course, you can simply use less wood by using advanced “optimum value engineering” (OVE, also commonly called “advanced framing”): 24-inch-on-center stud spacing, single top plates, box headers, and other wood- (read: carbon- and money-) saving tactics.
Insulation.
There is a marked difference between glass-fiber materials or petrochemical-based materials, such as closed-cell spray polyurethane foam at 409 kg CO₂ net emissions, and bio-based products that store carbon. Some bio-based materials can contain more atmospheric carbon in the physical substance (that gets stored and therefore not emitted to the atmosphere) than was emitted in producing the material. For instance, cellulose is carbon negative at -66 kg CO₂; hempcrete at -187 kg CO₂; and straw bale with a whopping -238 kg CO₂ net emissions.
BEAM Carbon Calculator
As mentioned earlier, there are several software tools available to help designers and builders calculate the amount of embodied carbon in the materials, assemblies, and buildings they build. But as a builder who wants to spend time building, I want a tool that I can use out of the box without a large learning curve. BEAM, which stands for Building Emissions Accounting for Materials, is a user-friendly, climate-science- and methodology-based software tool, built by a team at Builders for Climate Action.
You can get a free copy or make a donation to the Builders for Climate Action website (buildersforclimateaction.org/beam-estimator) and log in to use the BEAM Estimator. The tool is a sophisticated Google Docs online spreadsheet that is, relative to other calculating tools, simple to use, especially for builders because it’s based on 12 construction categories: footings and slabs, foundation walls, structural elements, and so on up to the roof.
With BEAM, you can compare embodied carbon in materials, such as different types of insulation (see screenshot of chart, above); you can build assemblies and compare them (see sample at left, top); and you can compare whole buildings built with different materials (see sample of results, below).
BEAM has a concise user guide to help you get started and comes preloaded with all the residential EPD data the creators could locate, and they continue to add more as it becomes available, which is a good reason to donate. You can toggle between metric and imperial measurements, a huge relief for U.S. builders unfamiliar with metric measurements like kilogram per square meter.
You will need to have a good understanding of building design and construction to navigate the assembly sections and make appropriate selections. Within each assembly section are categories of materials that will be appropriate for your project and likely many that will not be. It is up to you to build assemblies that are feasible and meet all the energy-performance and legal requirements for your project. BEAM doesn’t provide any warnings or suggestions about appropriate selections.
It’s worth noting that BEAM Estimator is a work in progress; nevertheless, the results are the best results possible given the current state of Life Cycle Analysis. Data in EPDs and the resulting outcomes are not 100% accurate numbers, so users should view them as guides to their selections of low-carbon materials. But as the authors make clear, especially for the three largest carbon-footprint categories of materials, reducing material amounts or making lower upfront carbon substitutions is more important than a few percentage points of error. Saving carbon now is much more critical than saving carbon over the next 30 years.
