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.
- 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.
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