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|Embodied Energy in Strawbale Houses|
Embodied Energy in Strawbale Houses
by Dr. Owen Geiger
There is growing awareness of how much energy goes into housing – both embodied energy, which will be addressed in this article, and the amount of operating energy that is required to heat, cool, illuminate, run the appliances and maintain the building over its lifetime.
The definition of embodied energy given by Your Home Technical Manual 1, an excellent online source of sustainable building information, is: “Embodied energy is the energy consumed by all of the processes associated with the production of a building, from the acquisition of natural resources to product delivery.”
Why should we be concerned? The Worldwatch Institute estimates that 40% of the world’s materials and energy is used in buildings. Another source, architect Edward Mazria, believes the total is closer to 48% 2. Energy use impacts everything from the air we breath (pollution induced particulates, sulphur dioxide), to global warming (the production of cement, for example, is responsible for about 7% of all carbon dioxide emissions), to leaking oil tankers fouling our oceans and beaches, to resource depletion and reduction of biodiversity, and to entanglement in foreign wars to ensure access to energy.
Building with bales is a step in the right direction. The potential for strawbale building to slash energy consumption, including embodied energy and operating energy, is substantial. Straw bales have very low embodied energy of about .24 MJ/kg, because it doesn’t take much energy to bale the straw that is left in the fields from the harvest of grain.
However, for strawbale houses that use a lot of high embodied energy materials, such as aluminum, concrete, and steel – a “high impact house” – the savings in embodied energy in the straw bale walls are rather minor when compared to the total amount of embodied energy that goes into building the entire house. This is because wall systems comprise only a small portion of a home. Building overly large homes in the typical North American fashion compounds the problem. We should build as sustainably as possible and consider the amount of embodied energy that goes into building a complete home, not just the walls.
For example, let’s consider a strawbale house with a concrete floor, concrete foundation, concrete driveway and sidewalks, lumber interior walls and floors, gypsum wallboard on interior walls, wood trim, fiberglass ceiling insulation, steel trusses, plywood floor and roof sheathing, asphalt shingles, PVC or copper plumbing, aluminum windows, linoleum, synthetic carpet, plastic laminate countertops, particle board cabinets, brass or steel or zinc hardware, paint… I think you get the picture – the overall amount of embodied energy used to build this particular strawbale home will be very high, despite the benefits of using straw in the walls.
Let’s take a closer look. Several studies estimate that an average 200 square meter (2,153 square feet) house contains about 5.0 GJ (GigaJoules) per square meter or a total of 1,000 GJ of embedded energy in the building materials. If we assume the exterior walls account for 10% of the total structure, then the walls in a conventional structure contain about 100 GJ. It is difficult to determine just how much embodied energy is in a bale wall because of the variation of wall finishes, etc., but just as an example, let’s assume the bale walls contain 10 GJ of embedded energy. So the bale walls in this example save 90 GJ in comparison to a conventional house. As you can see the house still contains 910 GJ of embodied energy. The savings of 90 GJ amounts to just 9.8%.
Calculating the embodied energy of building materials is not an exact science and estimates vary widely due to the complexities involved, although the consequences are very serious, as we’ve seen. The exact values are less important than the general principle involved – that you can lower the total amount of embodied energy that goes into building a home by using low embodied energy materials.
Ann Edminster compared the embodied energy in a low impact strawbale house to that in a high impact strawbale house. 4 The high impact house had 20 times the embodied energy of the low impact house. This supports my argument that bale walls by themselves do not necessarily lead to a low embodied energy house if the other elements of the home are not taken into proper consideration.
A similar example could be made about the ongoing energy use in an inefficient “leaky” strawbale house. Picture a strawbale house with super-insulated bale walls, but with inadequate ceiling insulation, low quality doors and windows, and excessive air leaks. Heat will take the path of least resistance and escape through the ceiling, air gaps, doors and windows, even though the bales provide excellent insulation. Again, we need to consider the entire building to achieve the desired result.
There are several ways that strawbale can reduce embodied energy in a house: downsizing the heating system, eliminating or downsizing the cooling system, and, of course, reducing the embodied energy in the walls.
The tables below compare high embodied energy materials to low embodied energy materials. All data are based on Measures of Sustainability 5, except where noted. Values for the earthen floor and clay aliz are assumed to be zero, based on hand-mixed earth from the site.
Table 1 Comparison of Floor Materials by Floor Surface Area
From Table 1, note how earthen floors substantially reduce embodied energy by replacing a wood-framed floor and synthetic carpet – zero embodied energy versus approximately 1993 MJ/m2, even without accounting for any beams, posts, footings, joist and post anchors, nails, screws, construction adhesive, and carpet padding! It is interesting to know that earthen floors in the Taos Pueblo in New Mexico have lasted for over 600 years, so clearly durability is not an issue. Earthen floors eliminate the need to replace linoleum and carpet every 15-20 years, and do not rot or offgas VOCs.
A similar comparison could be done between straw bale walls and walls made of high embodied energy materials, such as wood studs, fiberglass insulation and gypsum wallboard. However, you soon encounter problems, because the difference in wall thickness changes the usable floor area, and other such variables. But my basic assertion is still unchanged: Low-tech materials (straw bales in this case) have less embodied energy than manufactured materials, and therefore a home built of low-tech materials will have less embodied energy than a home built of manufactured materials. The difference is even greater if you compare straw bale walls to double stud walls.
Table 2 Comparison of Various Building Materials by Volume
Table 3 Comparison of Various Building Materials by Weight
There are many techniques for creating buildings that use far less energy than conventional buildings, often at little or no additional expense. Possibilities include: build small, maximize the solar design, reuse building materials (this can save up to 95% of the embodied energy that would otherwise be wasted), choose durable and low maintenance building materials, use local natural materials that are minimally processed (straw, earth, etc.), create built-in furniture such as bamboo shelving and cob benches, and add fly ash or rice hull ash to concrete.
One example: Walls made of fired clay brick and Portland cement mortar contain much more embodied energy than pressed earth blocks and mud mortar. Adobes, pressed earth blocks, cob or local stone are all excellent choices for interior walls inside of a strawbale home (and for exterior walls in some climates). They could be built on a rubble trench foundation to reduce costs, and consumption of energy and materials. These materials use far less embodied energy than wood-framed walls with wallboard, plus the added thermal mass helps stabilize indoor temperatures, lowering heating and cooling costs.
I believe our environment is at a perilous crossroads and all of us need to do more to reduce energy consumption. This isn’t something we should wait for decades to address as the environment continues to deteriorate. It is up to us as ethical builders, designers and homeowners to make reasoned choices. For the sake of the world and all of humanity, let’s act now before it’s too late.
Dr. Owen Geiger is the Director of the Geiger Research Institute of Sustainable Building and a correspondent for TLS. www.grisb.org
2. Turning Down the Global Thermostat, MetropolisMag.com,
3. Worldwatch Paper #124: A Building Revolution: How Ecology and Health Concerns Are Transforming Construction, http://www.worldwatch.org/pubs/paper/124/
4. Straw Bale Construction: How Environmentally Friendly Is It?
5. Measures of Sustainability,
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