Embodied Carbon: The Three Little Pigs’ Choice of Materials
This article introduces why our focus on embodied carbon is so critical, if we want to maintain a chance to meeting the world’s climate goals.
In the story of the three little pigs we learned that the best and safest house is the one built out of brick. The one the wolf can’t destroy. There are two reasons why we should pay more attention to the other two: Wood and straw, both have much better insulation properties, or R-Value than brick. You need less energy to heat or cool them. Even more impressive however is their potential to sequester carbon, also called negative embodied energy or embodied carbon. Basically, these materials take carbon out of the atmosphere and lock it up as walls, roofs, and floors. Plant-based materials have absorbed a significant amount of CO2 before being harvested (such as wood, hemp or straw) and require little energy to extract, manufacture and transport to the building site. They have a low or even negative “Global Warming Potential.”
The World Green Building Council’s definition of a net zero carbon building is a building that is highly energy efficient and fully powered from on-site and/or off-site renewable energy sources. The Council proposes a framework for 2030 in which all new construction should achieve net zero. This is utterly misleading.
Focusing solely on operational energy has the potential to set us even further back from meaningful reduction of greenhouse gas emissions by 2030
In this decade, any new building that is “highly energy efficient” will only emit 12% of its lifetime operational emissions (assuming a 100-year life span of the building). Even significant reductions in energy consumption of 50% or a switch to renewable energy will only result in a small impact during this critical time frame of the next 10 years. Instead, the embodied energy or upfront emissions in the material and equipment to build this new energy-efficient building will actually increase total emissions, potentially even by more than what the resulting operational energy reduction can make up for in the short term. In other words, passive house technology to reduce operational energy was a pretty good idea in 1990, when the first passive house was designed in Darmstadt, Germany. Today, where immediate reduction in greenhouse gas emissions is absolutely critical, it is not enough. We need to combine this ambition with a reduction in “upfront” emissions, or embodied carbon, the accumulated greenhouse gas emissions resulting from harvesting, manufacturing and transportation of building material.
In his thesis, “Opportunities for Carbon Dioxide Removal and Storage in Building Materials”, Chris Magwood, Director of the Endeavour Centre wanted to quantify this claim. He compared different material choices for two sample construction projects and found that “achievable reductions in embodied carbon could provide more than four times the overall greenhouse gas reductions than energy efficiency improvements to reduce operational carbon between 2020 and 2050.” He found that, at the current scale of US residential construction, annual carbon storage in residential buildings as modeled could reach 30,000,000 tonnes, the equivalent of 10 coal-fired power plants.
I found Chris’ thesis super-fascinating, but also really difficult to read and more than once got lost between metric and imperial systems of weight or volume, BTU, kgCO2e, EPDs, GWP, ….., it is complicated to trace emissions from materials, but before discussing the various tools and mechanisms to do that, here are a series of basic graphics that try to visualize why a critical look at materials is important for this 2030 milestone.
But the single-most alarming analysis of embodied carbon is this one: Chris compared different construction methods for an imaginary 1,000 square foot house in Ontario, a climate comparable to upstate New York. Energy-efficient construction using materials such as spray foam insulation may lower your utility bills, but they really hurt the cause of reducing emissions, especially in the critical years to 2030, where little of the emission reduction from energy savings is captured. Even by 2050, you would still do better by our planet, if you just built to code.
The calculation gets a lot better, if you are using an air source heat pump instead of natural gas. But again, if you just put in super high-performing foam insulation in your passive house, you added over 90 tons of CO2 to the atmosphere before you even saved anything.
It also shows however that when using plant-based materials, buildings can actually become carbon sinks. The CO2 the plants captured while growing is stored in the building for its lifetime. The graphic below compares carbon impacts of different materials for insulation.
While a panel of extruded polystyrene (the blue stuff) emits approximately 38.5kg of CO2 equivalent, achieving the same insulation value with straw bales, would actually store 41.8kg of CO2e in the panel. This does not sound like much and it isn’t (It’s the equivalent of driving 100miles according to the EPA’s calculator). But it’s also just one panel. It adds up the bigger your building gets. To achieve the same R-value, the straw bale panel would have to be much thicker, so it does get really complicated to compare materials, but it is clear that there is great potential in bio-based materials and that a sole focus on R-value and operational energy may hurt our ability to achieve urgently needed reduction of greenhouse gas emissions in the coming decade.
I started to rework the image of the three little pigs using our design for a carbon neutral single-room dwelling. The building is 30 by 10 feet. 300 square feet in total. The walls will be 12" thick using a double stud wall construction. We plan to use hempcrete for the insulation, a mixture of hemp and lime. I will write more about that, but for now, let’s just say it sequesters an enormous amount of carbon.
A number of software tools are in various stages of development and use to help architects quantify embodied carbon. Few integrate “negative carbon” or carbon storage into their database and that’s because there aren’t yet enough standards and research how to account for it. The numbers in the graphic are derived from various sources. (For Hempcrete: Tradical, a UK-based provider of hempcrete; for Cellulose: Sustainable Minds Transparency Report; for Mineral Wool: Institut Bauen und Umwelt, accessed through the EC3 Carbon Calculator). For the framing, I used the Inventory of Carbon and Energy developed at the University of Bath. Assuming average values for timber, the frame (just the walls) could store 900 kilogram of carbon dioxide equivalent, when using sustainable forestry sources.
The EC3 tool, incubated by the Carbon Leadership Forum, is an open source tool that utilizes building material quantities from construction estimates and/or BIM models and a database of digital, third‐party verified Environmental Product Declarations (EPDs). It’s a great tool, but for our tiny project and our specific goal of looking at negative carbon, I didn’t find it to be the ideal single source. That might be changing in the future thanks to people like Chris Magwood.
