I am embarrassed to admit this but until just recently I had no idea what building “net zero” or “carbon-negative” or “climate-positive” or “carbon-neutral” was. I thought I would never use those terms. Today those terms and others are about achieving a balance between the carbon emitted into the atmosphere, and the carbon removed from it.
When I started to learn more about and tried to spread the word, it soon became apparent that most people in the construction weren’t much more aware of the meaning of those terms and why they’re important to us than I was.
So when this article from the Climate Action Lab at Wally Farms, came across my desk about a project diary to build a carbon-negative dwelling, it caught my attention.
Here is the article, written by Kaja Kühl:
In architecture, it is about thinking of building materials as carbon sinks. Hemp is such a sink, because of the high amount of carbon it sequestered while growing. Estimates claim that hemp absorbs 2 metric tons of CO2 per acre, 5–10 times as much as trees because of its fast growth.¹ This CO2 is then stored in the building.
A number of software tools exist at various stages of development and/or use to help architects quantify embodied carbon in all kinds of materials. Few integrate “negative carbon” or carbon storage into their database. We worked with a beta version of the BEAM Estimator by Builders for Climate Action to input materials for envelope and framing.
In its current form — a public beta version — it is a google sheet and pretty simple, which is its biggest strength in my view.
Its ease of use makes it a great tool to quickly look up the difference between two materials and to get a general sense of embodied energy without having to study complex scientific Environmental Product Declarations (EPDs). Too often, we make material decisions based on aesthetics, warranty, functionality and of course — price. A quick look at this database lets you add the embodied carbon impact as a factor for choice.
The BEAM estimator focuses on “cradle to gate” — the material emissions associated with producing a material (A1-A3 in a life cycle analysis).
Here are some of the reasons why:
Product emissions represent between 65–85% of the full life cycle emissions for most building materials, making A1-A3 the biggest source of emissions to address. Product emissions are emitted now. They are emitted into the atmosphere before the building is built. To stay within 1.5 to 2.0˚ warming we need to reduce emissions now. Energy efficiency within the building will only help reduce CO2 in the atmosphere over a long time period. There are meaningful differences in A1-A3 emissions between competing products, making it relevant to pay attention when choosing.²
I can see the need for a more comprehensive and larger database like EC3 if you work on a big project and want to understand the embodied carbon in different concrete mixes for instance or variations of mineral wool but for our tiny scale and the specific intention to use as much natural material as possible, BEAM was perfect. (I also want to mention here again, Pathfinder, an application specific to landscape architecture, just to give more examples of what’s available. I haven’t tested it, but it definitely has the prettiest interface)
Negative Embodied Carbon
So obviously the best part of using the BEAM estimator was inputting our buildings’ dimensions and seeing its embodied carbon value drop below or close to zero.
BEAM does this by subtracting the carbon-storing capacity of a material from the carbon emissions in phase A1-A3 using values from Phyllis 2, a database containing information on the composition of biomass. When the carbon-storing value is higher than the emissions to harvest and process a material, its embodied carbon is negative.
To generate a value for net emissions, BEAM subtracts the carbon sequestered by a material when growing from them A1-A3 emissions as reported in EPDs.
Most notable, it doesn’t do this for timber products, citing that there is too much uncertainty in the accounting currently. This includes uncertainty about the amount of carbon released from soils during logging operations; the amount of carbon returning to the atmosphere from roots, slash and mill waste; the amount of carbon storage capacity lost when a growing tree is harvested; and the lag time for newly planted trees to begin absorbing significant amounts of atmospheric carbon dioxide.
I would add that even with best intentions, trying to secure material in the past two years hasn’t always been easy and so being able to trace every piece of framing material or board of plywood would have been impossible. In our project all timber was assigned a somewhat generic value assuming it meets standards of the American Wood Council.
Comparing Apples to Apples
Having tried to decipher the scientific language of three different EDPs for three different cladding products recently, where each used a different functional unit, I appreciate how the BEAM estimator does the work in translating everything in a way that allows me to compare apples to apples. I don’t see how paying attention to embodied energy can become mainstream in design and construction if it wasn’t for this ease of use.
The project input on which calculations are based is perhaps too simple for a small project like ours. With 13 inch thick walls, there is a difference between the amount of material on the interior of that wall vs the exterior. This isn’t a significant difference in most cases, but in a small project like ours, we decided to measure the wall area for hempcrete at the center of the hempcrete, but use the exterior dimensions for the wood fiber boards and the siding. Other than that, you can assign different percentages in the spreadsheet. We did this for instance to differentiate between wood floor and tiled floor.
Another thing to keep in mind is that it is an estimator. It’s not a perfect or super-accurate tool to calculate your exact carbon footprint. Instead, it is best at translating highly technical Environmental Product Declarations (EPDs) into one format and in one place.
And while you may not end up with an exact number for your design’s carbon footprint, it is great to assist in material choices and see the difference between cellulose and spray foam insulation for instance.
Apart from being really pleased with the look of the windows, we were surprised to find that European windows on average have a lower embodied energy. Would love to know why, but as mentioned above, the source of energy for processing will increasingly make that difference. And as I will write about in the next episode, the difference even holds true for transportation emissions.
That said, I wish BEAM could incorporate better referencing of the sources for the data. While the simplicity is great, sometimes it would be informative to where the difference in embodied carbon comes from.
Using the three EDPs of cladding material again as an example, one thing that became very apparent was that the global warming potential (GWP) of different materials was in part dependent on the source of energy used to process the material.
Even when sustainably harvested, it makes a big difference if your lumber mill is powered by renewable energy or a coal-fired power plant. Most people immediately think about transportation emissions associated with where things are coming from, but increasingly, the bigger difference will be in sources for energy generation.
Soooo…. Here is what we found
Both homes’ embodied energy is about 500kg of CO2 equivalent, approximately the equivalent of using 56 gallons of gasoline.
The concrete foundation, while not big, was really hard to make up for in both micro homes. This again is a question of scale. We would have not used a lot more concrete for a three-story house, but would have installed a lot more biogenic material to balance the high emissions of the concrete.
Hempcrete
The hempcrete did not disappoint, storing the greatest amount of carbon in both structures. Cellulose is also impressive without the additional health benefits that hempcrete provides. Ultimately, paying closer attention to embodied energy, be it through using more bio-based materials or materials processed with renewable energy is critical to reduce emissions in the critical short term setting buildings on a path to reducing emissions in their design and construction.
CLICK HERE to read the entire article by Kaja Kühl and supporting sources
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