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Green Building News

A Better Path to a Low-Carbon Future?

Building materials capable of reducing up-front carbon to zero are available, code-compliant, and affordable, study shows

A detailed study of the carbon impact of different building materials began as an extension of Chris Magwood's work at the Endeavour Centre, which included construction of this house. Photo courtesy of the Endeavour Centre.

Building superinsulated houses with low energy needs and highly efficient mechanical systems is the best way for builders to lower greenhouse-gas emissions that contribute to global climate change, right?

Maybe not. A group of builders and designers led by the director of a sustainable building school in Canada has concluded that energy efficiency is only part of the answer, and that accounting for embodied carbon in the materials used to construct houses is much more important than previously believed.

The group, Builders for Climate Action, has published a report stressing the importance of accounting for carbon emissions in the effort to address climate change. Buildings, the report says, can be significant carbon sinks rather than a big part of the carbon problem. (A copy of the report is available here.)

“The response to building-related emissions has been to focus solely on energy efficiency,” the report’s introduction says, “but this may result in initiatives and policies that will raise emissions rather than lower them.”

The authors warn that net-zero building codes will not address the carbon-emission problem adequately “within a meaningful time frame.”

The group argues that buildings contribute to carbon emissions in three ways:

  • Up-front embodied carbon: Emissions that result from harvesting, manufacturing, and transporting building materials.
  • Energy efficiency: The amount of energy buildings use (their energy-use intensity, or EUI).
  • Fuel-source emissions: Carbon emissions that can be attributed to the fuel used to heat, cool, and power the building.

To study the question, the report’s introduction explains, the group modeled two hypothetical buildings (a single-family house and a multifamily) made with a wide range of conventional and alternative building materials. The models included different levels of energy efficiency using two different fuel sources, natural gas and electricity. Then the group ranked the impact of up-front embodied carbon, fuel-source emissions, and energy efficiency on the overall carbon footprint. Data on carbon came from the Environmental Product Declarations published by material manufacturers.

Low-carbon and carbon-trapping materials proved more important than energy efficiency in reducing emissions in the 30-year time frame following construction. And, their report argues, building materials capable of reducing up-front carbon to zero are available, code-compliant, and affordable.

“Zero upfront emissions is a realistic option for the sector requiring no changes in codes or construction methodology and creating vast emission reductions across the sector,” the report says.

Other conclusions:

  • Switching to clean, renewable energy provides reductions in carbon emissions of 70% to 75% when compared to natural gas. Investments in renewables at the grid level are the most meaningful.
  • Choosing the right materials can reduce up-front carbon emissions by 150%, allowing designers to “completely transform” the carbon footprint of the buildings.
  • Increasing energy efficiency beyond code minimums (at least in Ontario, Canada) is the “least effective and most costly means of reducing building emissions.”

Study began with a master’s thesis

The study, which was published in late November, grew from a master’s thesis by Chris Magwood, director of the Endeavour Sustainable Building School in Peterborough, Ontario.

In a telephone interview, Magwood said his work at the school prompted him to dive a little deeper into the question of how buildings affect carbon emissions. The results of his work surprised him, and he later formed a group of local builders, architects, and building officials to help him get the word out.

Preliminary results were presented at a conference of the Northeast Sustainable Energy Association in Boston last spring. Magwood has since been invited to speak about his research at more than a dozen conferences. Reactions have ranged from excitement about the opportunity to address carbon emissions to defensiveness from energy-efficiency advocates.

“The whole climate change thing can be really overwhelming and depressing,” he said, “and it can feel like there’s nothing to do. When you look at results like this, it’s like, ‘Oh, there’s a pretty meaningful way that people in the building field can go about making what looks like a pretty major contribution with fairly minor adjustments in what we do.”

Time is a key consideration

Net-zero-energy houses may have a big impact on carbon emissions down the road, but we need much lower carbon emissions now if we are to stave off the most dramatic and damaging effects of global climate change.

Emissions that are avoided today, the report notes, do more to slow climate change than emissions that are averted in the future. In the next decade, it adds, up-front embodied carbon will have a greater impact than carbon emissions due to operating the buildings—heating and cooling them, and running their various systems.

The study modeled four hypothetical building configurations for both single-family and multifamily house types to illustrate the impact of building materials on the up-front embodied carbon emissions, or UEC. A building with a high UEC [total net carbon emissions of 241 kilograms of carbon dioxide emissions per square meter (kgC02e/m2)] incorporated such materials as high-carbon concrete, extruded polystyrene insulation, brick cladding, steel framing, vinyl windows, tile and carpet flooring, and clay-tile roofing.

At the other end of the scale, a building that has net carbon storage of 137 kgC02e/m2, is made with insulating concrete forms, SCM concrete (concrete in which some of the Portland cement has been replaced by other materials), straw and wood fiberboard insulation, wood-cladding interior wall panels made from compressed straw, wood-framed windows, linoleum, and FSC-certified softwood flooring, and a cedar-shake roof.

When these numbers are scaled up to reflect total low-rise construction in the U.S. during 2017 (241 million square meters), total carbon emissions came to 54 million tons. Adding operational carbon emissions (heating and cooling) boosts the total carbon footprint on a house between now and 2050. But a carbon-storing building that runs on renewable energy will have a net carbon savings of 107 tons—a negative carbon footprint.

“The results of this study demonstrate that we are capable of making low-rise residential buildings with a net-zero embodied carbon footprint, and that we can even surpass this threshold and create buildings that actually have net carbon storage rather than emissions,” the authors note.

Plant-based building materials are available

The study recommend a number of plant-based building materials because they prevent the release of stored carbon for the life of the building. There are many of them already on the market, including sustainably grown timber, cellulose, straw bales, hemp fiber, and medium-density fiberboard made with rice straw.

Products made from agricultural residue—like rice-straw MDF—are especially attractive because the raw materials are already produced in huge quantities. In 2016, for example, more than 2 billion tons of grain straw were grown globally, capturing almost 8 billion tons of CO2.

Using products that store carbon affects a number of other areas, the report says, including smog and ozone generation; the acidification of lakes, rivers, and oceans; and the generation of hazardous waste. The report calls these “co benefits.”

The report also notes that none of the carbon-storing materials it modeled contains compounds that are included on the International Living Future Institute’s chemical Red List of banned substances.

“As the indoor environment quality of buildings is of growing concern, the move to a materials palette that includes more carbon-storing options appears likely to correspond with
improvements in occupant health and safety,” the study says.

Emissions from operating the building

Houses that were heated with natural gas were responsible for significantly more emissions than those heated with air-source heat pumps running on the Ontario grid—depending on the house, CO2 emissions could be 20 times higher with natural-gas heating. In itself, that’s not a surprise, but the report also found big differences in carbon emissions between code-compliant houses and those that were net-zero ready.

For example, a code-compliant single-family house would produce 160 kgCO2e/yr, while a single-family that was net-zero ready contributed less than a third of that, 50 kgCO2e/y. A code-compliant house that was airtight would produce 100 kgCO2e/y. In other words, air-sealing a house cut carbon emissions in half.

The results were similar for the modeled multifamily house: a code-compliant building produced 750 kgCO2e/y while the net-zero-ready multifamily produced 190 kgCO2e/y. An airtight, code-compliant house emitted 330 kgCO2e/y, less than half that of the code-compliant building.

By contrast, the single-family, code-compliant house heated with natural gas emitted 3000 kgCO2e/y; the multifamily 15,000 kgCO2e/y.

The airtight houses were modeled at 1 ACH50 with no upgrades to insulation or window quality, demonstrating the significant impact that air-sealing alone has on energy consumption and carbon emissions.

That said, the modeling concludes that reducing up-front carbon emissions by choosing the right building materials had a bigger carbon impact than higher energy efficiency over a 30-year span. Improving the building envelope from code-minimum to net-zero-ready reduced average carbon emissions by 60 tons over 30 years, while cutting embodied carbon at the time of construction doubled those reductions and did so immediately.

“We can build and operate the best conventional UEC building for 30 years with fewer emissions than just the upfront embodied emissions of the high UEC building,” the report says.

Change is possible now

Three of the most important building materials in models with reduced carbon emissions are cellulose insulation for walls and attics, wood-fiber exterior insulation, and concrete mixes with a high percentage of supplementary cementitious materials (SCM) and reduced Portland cement. All are available today.

“Designers and builders could realistically move to implement this type of zero-up-front-carbon building with few impediments, and in doing so dramatically alter the embodied carbon emissions of the building industry, bringing residential UEC climate impacts close to zero,” the report says.

Energy efficiency still counts, but has its greatest impact in a longer time frame.

Magwood said he hoped no one would interpret the study to mean that building highly efficient houses wasn’t important. “I definitely don’t want this study to steer people away from energy efficiency,” he said. “I just want to steer people toward understanding that there’s a balance. If you pursue energy efficiency as the single, solitary goal, you can have this unintended consequence.”

While the original research was focused on new construction, Magwood said he’s undertaking a new study that would cover retrofits. His hunch is that there’s an even bigger opportunity to make a climate impact there because some of the most carbon-intensive parts of new construction—the foundation, for example—typically are not part of a retrofit. That new report, he said, could be available in about six months.

—Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.

13 Comments

  1. Eric Habegger | | #1

    Holy Hell, this is an important bit of news here! There is going to be years worth of back and forth as this information is slowly digested and people overcome some natural defensiveness over misplaced priorities. Hopefully that resistance can be overcome sooner rather than later. We've all probably misordered some of those priorities over the years, including myself. I'm very happy that one thing I did do right in my own renovated home was to use dense packed cellulose, which I did myself. Now about that propane gas fireplace I installed, best not to think about that...

    1. User avater
      Dana Dorsett | | #2

      Don't sweat it Eric- consider of the PassiveHaus projects with 2 feet of EPS under the slabs, or houses built with ICF or high-R polyurethane SIPs. :-) Some decorative propane burning for ambience is small spuds compared to most of that stuff.

      Installing a decent efficiency woodstove might have been lower-carb (assuming sustainably harvested wood fuel), but has other local air pollution issues beyond mere carbon footprint.

      I've always been a big fan of using reclaimed/recycled building materials where appropriate, even when it's not a big money saver. Reclaimed rigid foam board is usually a big net win on all fronts. Cast iron radiation for hydronic heating systems can be too, but not always architecturally appropriate for newer (or new build) homes.

      Better heat pumps and a greener grid are really the cheaper greener path forward for existing houses. Deep Energy Retrofits on the building shell become cost-prohibitive, and often requires high footprint materials to meet the energy use targets.

      1. Eric Habegger | | #3

        "Better heat pumps and a greener grid are really the cheaper greener path forward for existing houses. Deep Energy Retrofits on the building shell become cost-prohibitive, and often requires high footprint materials to meet the energy use targets."

        That's probably true for the most part. It does help to be handy, which I am. Even though I'm not a professional builder that changed the equation a lot. Your advice probably applies to most here who are just looking for advice that they want implemented by someone other than themselves. I'm sure that applies to most people so your advice is mostly true.

  2. Kohta Ueno | | #4

    Preliminary results were presented at a conference of the Northeast Sustainable Energy Association in Boston last spring. Magwood has since been invited to speak about his research at more than a dozen conferences.

    If anyone wanted to see the NESEA talk, the slides are available here: http://nesea.org/session/carbon-drawdown-now-turning-buildings-carbon-sinks

    And the full video here: http://nesea.org/buildingenergy-boston-2019-keynote-session

  3. User avater
    Jeff_LDC | | #5

    I have to admit, I only skimmed the article (pressed for time), but have read similar articles, and the repeating theme seems to be this, "Builders lack of access to more sustainable products in the main-stream market".

    1) Much too difficult to find the Plant Based construction materials, and

    2) once one does, they typically find them to be exceedingly expensive due to lack of efficiencies of scale, which is due to lack of availability, therefor lack of demand.".

    It seems this may be a good place for government subsidies to come into play (for lack of a better option)... with it being so difficult for any new product to break into the building market, if we could offer them (the alternative products) initially for a cost similar to that of main-stream products, sales just may be able to increase to the point that the prices come down to no longer require subsidies.

    Otherwise... it seems we may be stuck in this rut for decades or more, which we really cannot afford at this point in time.

  4. Tyler Keniston | | #6

    This article makes me wonder how far down the carbon accounting rabbit-hole we should go. I think the answer is we should keep going.

    Will we eventually arrive to a place that resembles something of an 'intuitive' understanding— perhaps what hippie-environmentalists came to 50 years ago while spinning in the mud on caps and stems— that 'natural' and local is better, because most things manufactured and transported consume resources, energy, and release toxins and/or carbon? Probably not. But it does seem that the more accurate accounting we do, the more we find relatively new notions being disproved (like the notions that gave birth to the Passive House concept, and perhaps even net-zero to some extent).

    How accurately are we accounting for carbon in every facet of our infrastructure? When we say 'renewable energy' are we properly assigning the carbon (and other) impacts to that? We can't use a zero anywhere.

    I do think we're trying and people are on the track. It's daunting, however, and can sometimes lead one to throw their arms up and admit defeat for lack of accounting skills.
    I'll just dig a hole and buy a blanket.
    Maybe a space blanket with low-e.

  5. Scott Paulson | | #7

    I enjoyed the article and think it drives home some very important points when it comes to carbon accounting. I also agree with other comments, that exploring this further and having a better understanding is important.

    However, there is one question that keeps coming to mind when looking at carbon accounting in building materials.

    We have a serious issue with soil quality being depleted by our current farming practices. I'm not sure that turning plant based by products into building materials is the answer. We need to be composting this material and getting back into the soil. It seems that making these plant byproducts into building materials will just make our soil problems worse. Was this considered in the study?

    It becomes a bit overwhelming trying to look down the path of each decision further and further, by this seems like an important consideration.

    1. User avater
      Dana Dorsett | | #10

      >"We have a serious issue with soil quality being depleted by our current farming practices. I'm not sure that turning plant based by products into building materials is the answer. We need to be composting this material and getting back into the soil. It seems that making these plant byproducts into building materials will just make our soil problems worse. Was this considered in the study?"

      You may be misunderstanding the soil carbon issue. Composting crop residues is just the frosting on the cake, not the bulk of it. The carbon sequestration cycle of compost is extremely short- few years at best. It's the ROOT structures are what puts plant-scavenged carbon into the soil long enough to actually matter- the deeper rooting the species, the better in terms of lengthening overall carbon cycle.

      Building lifecycles are much longer than compost carbon cycles. Using crop residues for construction material (such as rice straw MDF or hay bale insulation) sequesters that carbon for the life of the building, usually measured DECADES if not centuries.

      Depletion of stored soil carbon has more to do with soil tilling, erosion, and to a lesser degree chemical fertilizers. Composting the crop residue on the surface doesn't particularly help in a big way, but it does some. Deep rooted fast growing prairie grasses can store carbon 8-10' below the surface in their root structures, where most of it will remain for a century or longer. Making hay out of the above-ground part of the plant for insulation would be a good thing.

      Large woody grasses such as bamboo have somewhat limited ability to sequester carbon in soils, but do great job at sequestering it in building materials. It usually grows on somewhat poor soil for other agricultural purposes. Most of the root carbon of bamboo is in top foot of soil, delivering a much shorter carbon cycle than say, switchgrass (or the lifecycle of a house.) But it's carbon scavenging rate is quite fast compared to coniferous trees commonly used for structural wood, and much more could be done with structural bamboo even in the temperate zone countries.

      1. Burninate | | #11

        Any time we talk about composting, I go back to what I ultimately concluded after studying how people actually compost and trying to sell them fancy bins:

        People talk about aerobic and anaerobic decomposition, and the One Weird Trick they learned to ensure it stays aerobic, and they universally conclude that this trick is enough, over a wide variety of tricks & effort levels and composting situations, without measuring anything. It just conveniently happens to be the exact thing they're willing to do. These people are certainly not all simultaneously correct.

        Nearly all decomposition in a big compost pile is anaerobic decomposition, until proven otherwise. Anaerobic decomposition produces significant methane emissions. If methane is generated, it traps approximately 80 times as much heat in the atmosphere over 20 years as if we had generated CO2 instead (eg: by burning the equivalent waste, or composting it aerobically if such a thing were practically feasible).

        The degree that biological carbon fixation has a methanogenic component to its decomposition cycle is likely far more important than anything else about it, if we're growing things for the purpose of fixing carbon. These things must be measured, extensively, to know whether we're even improving the situation at all. We have not yet begun to bother caring about them.

        1. User avater
          Michael Maines | | #12

          Burninate, you seem to be confusing composting with rotting. Proper composting, by definition, is aerobic decomposition. You can identify a well-managed compost pile because it does not smell bad. A properly made compost pile also breaks down quickly, while anaerobic decomposition is relatively slow.

          I make several cubic yards of compost each year so I'm pretty familiar with the differences. I acknowledge that a lot of home-based compost probably is rotting, not composting. Commercial-scale composting is actively aerated for odor control and fast decomposition, so it's definitely not anaerobic, though from what I've observed it usually includes already-rotting organic matter as an ingredient.

  6. User avater
    Michael Maines | | #8

    To learn more about carbon accounting, you might want to join the webinar that Chris Magwood, author of the study in this article, is doing on March 5:

    http://endeavourcentre.org/endeavour-workshops/

    "Basic Carbon Accounting for Buildings will help you to understand embodied carbon and how to dramatically lower it in your building projects. During the webinar, you will learn how to:

    Understand embodied carbon emissions within life cycle analysis
    Find and select tools for measuring embodied carbon, including LCA software, EPDs and free databases
    Read the results from LCA tools and EPDs
    Understand the strengths and weaknesses of tools, EPDs and free databases
    Generate specific results based on your building design
    Understand biogenic carbon and carbon storage in materials
    Calculate the carbon storage potential in biogenic materials
    Create a net emissions or storage calculation for your building
    And more…"

    1. Robert Swinburne | | #9

      Carbon accounting
      I am also not a person (I'm an architect! - mom would have been so proud) with much (none, actually) interest in coming up with a number. I prefer to look at the big picture and do everything I can in the projects that I am involved with. I am fortunate to work with like minded builders. Living and practicing in Vermont allows me huge opportunities that I'd be a fool to ignore in terms of materials and products.
      I realize that the few inconsequential projects that I do can be leveraged through social media and publication to help others think about their choices with an eye toward "doing the right thing"
      Personally, I'm sure I can do better in terms of how much damage I am doing to our planet such as diet and air travel and driving miles and I will. I promise. But I am a bit (only a bit as I am naturally an Eeyore) encouraged by what I can accomplish by setting an example in my work.
      Perhaps I will hire Michael Maines to do the math for me. But probably not.

  7. Mark Fasano | | #13

    Reading this made me wonder which would ultimately have better net environmental impact: retrofitting a 95-year old brick home to try to make it as energy efficient as possible with conventional insulation/weatherization materials, or knocking it down and building a new home with the materials described in the report?

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