Building Energy Efficient and Carbon Smart with Prefab Straw Bale
GREEN BUILDER WHO ARE motivated to reduce carbon emissions tend to focus on increasing energy efficiency to reduce the overall carbon emissions of a building over its lifespan. This is certainly a worthy goal, but in its pursuit, we often use materials that actually contribute to greater immediate carbon emissions. The conventional mantra has been that these up-front emissions are dwarfed by the lifespan emissions and are therefore “worth it.” However, with more energy efficient buildings, the embodied carbon in the materials begin to represent a more significant portion of a building’s overall carbon footprint. And as the move towards renewable energy sources continues to progress, the amount of carbon that will be “saved” in the future also dwindles.
This points to the importance of building both energy efficient and carbon smart. Instead of emitting vast amounts of carbon today in the hopes of a “payback” in a decade or more, why not choose materials that sequester carbon instead of emitting it?
Straw bale building has long offered the promise of high performance with exceptional carbon sequestration, but most builders aren’t interested in stacking and plastering bales with the attendant labor costs, re-training, and sourcing issues common with site built straw bale walls.
However, prefabricated straw bale wall panels – often called S-SIPs for Straw bale Structural Insulated Panels – have begun finding their way into the marketplace, bringing with them the lower costs and labor inputs of prefab systems without any carbon penalty.
The problem with adding more insulation to conventional buildings is that we enlarge the carbon footprint in an effort to reduce the carbon footprint because most insulation materials are very carbon-intensive.
The chart above shows the embodied carbon of a number of common insulation materials, compared equally at a value of R-28. The materials range from a low figure of 8 kilograms of CO2e (total emissions expressed as a volume of carbon dioxide) for straw bale insulation to a high of 38.5 kilograms for extruded polystyrene (XPS) for a 4x8 foot wall section. Compared by embodied carbon, straw bales have a fraction of the impact of other insulation materials. On this basis alone, it is a great choice.
But if we consider the amount of carbon sequestered in the straw, the comparison becomes particularly noteworthy. Here we see that an R-28 straw bale wall actually sequesters 42.8 kilograms of CO2, erasing its small amount of embodied carbon and driving the figure into negative territory. The cellulose in the straw is 37-51% carbon by weight, and if the straw is prevented from decaying or being burnt, that carbon won’t return to the atmosphere for decades or centuries.
Scaling up, a 2,000 square foot home insulated with fiberglass would emit 792 kilograms of CO2, and XPS walls would emit 1,732 kilograms of CO2. By contrast, the straw bale walls would actually sequester 1,926 kilograms of CO2. If used to replace these two manufactured insulation materials, building with S-SIPs would save 2,718-3,658 kilograms of CO2 for just one house.
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Scaled further, the 740,000 single-family dwellings built in the US in 2015 could have reduced their carbon footprint by 2-2.7 million metric tons. These are not insubstantial numbers, especially considering that straw bale walls have the same level of energy efficiency. Instead of waiting 6-10 years to pay off the initial carbon debt – assuming a quality build and occupants acting as expected – we could be making better buildings reduce carbon emissions immediately.
This would be a mere academic exercise if it wasn’t practical to use S-SIPs on a large scale. But the technology is simple, cost-effective and exceptionally well-suited to regional production hubs. It’s certainly feasible to envision rapid expansion of the technique, especially with the inclusion of straw bale construction in the IRC in 2015, making prescriptive use of the material straightforward in many jurisdictions.
S-SIP producers are currently working in the UK, Europe and Ontario, and others are in prototype stages. All forms of S-SIPs feature a wooden frame “box” that is filled with straw bales, often to standardized sizes.
Some manufacturers use plaster for exterior and interior finishes, as is common with site built bale walls. However, others use permeable sheathing such as structural wood fiber board or gypsum products. Even at current small-scale production levels, these walls are cost-competitive with conventional options, and more R&D and higher volumes will drive the price lower.
S-Sip walls take a vast agricultural byproduct – US annual straw production is enough to build all new houses without increasing acreages – and make it a potentially valuable asset in the drive to reduce the carbon footprint of our buildings.
It’s a carbon revolution lying in our farmers’ fields, ready to be put to use!