Embodied energy is the energy required to extract, transport, process, install, and dispose of, or recycle the materials that make up the building.
For this study, the total embodied energy was not used to compare the two construction types, only the energy for the material manufacture was used, because energies used to transport, install etc. would in most instances be the same for both construction types and cancel each other out, and because almost 70% of the total energy invested in a building's construction (Embodied energy) is embodied in the materials themselves, one can compile a rather accurate comparison with using just the energy used for the material manufacture alone, however, when referred to the energy used to manufacture the materials I will refer to the Embodied energy.
Materials which will be more or less the same in quantity / volume eg. ceiling boards, cornices, skirtings, floor slab & finishes, because of the same floor area, have been omitted for they will have no impact on the embodied energy outcome.
Construction
To compare the two types of construction, I started with a 12000mm x 6000mm brick building and included 2 bedrooms, a bathroom, open plan kitchen & living and a garage. To justify the comparison, I designed the strawbale dwelling with the same rooms and exactly the same floor area for each room, but because of the bales rather wide (approx. 480mm) module width I ended up with a 13200mm x 7250mm external envelope for the straw bale dwelling. This made quite a difference on the material volume of the roof and roof trusses. Both the dwellings received one plaster coat, but vary in thickness. Conventional brick wall plaster width vary from apprx. 12mm 18mm, compared to the 30mm plaster coat for strawbale walls because of the greater surface un-evenness among other reasons.
The foundation details differs from conventional brick buildings. The straw bales are laid on a bed of stone so they will not retain moisture. A cement screed is cast in the bottom of the trenches on the conc. footing to be sure that any water that might find its way into the trench would be directed away through the weep holes on the sides. These are the only bricks used in the strawbale dwelling, thus embodied energy values for mortar and bricks are a lot less for this type of construction. The foundations for strawbale buildings are shallower (200mm deep), thus less conc. Is used as well.
The last main difference is, of course, the wall material which differs hugely in the amount of embodied energy to produce & install them. Embodied energy for straw bales is 31MJ/m3 compared to the 5200MJ/m3 of stock bricks, thus because the walls have the greatest material volume of all the building components, it is understandable that the strawbale dwelling will have a much lower total embodied energy value than its rival as is indicated below.
Conclusion
Straw is a viable building alternative, plentiful and inexpensive. Straw-bale buildings boast super-insulated walls simple construction, low costs, and the conversion of an agricultural byproduct into a valued building material. Properly constructed and maintained, the straw-bale walls, plaster exterior and interior remain water proof, fire resistant, and pest free. Because only limited skill is required, a community house-raising effort can build most of a straw-bale house in a single day. This effort yields a low-cost, elegant, and energy-efficient living space for the owners, a graceful addition to the community, and a desirable boost to local farm income. I think, especially in this country, residential straw bale buildings could be a very sustainable viable alternative to residential architecture.
Written by Jere Botes, architect & founder of http://www.dreamhouses.co.za, a website dedicated to provide home builders, home renovators, home owners & developers with free professional advice on all aspects of home design, building & diy. Reproductions of this article are encouraged but must include a link pointing to http://www.dreamhouses.co.za.
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