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Chug, Andrew, everyone - The figure for embodied energy depends of course on what you include. Would straw have been baled anyway? What distance are the bales being transported? I addressed this issue in an approximate way (there is, I think, no other way to address it...) in my PhD thesis last year. The relevant text is below - I hope the formatting comes out OK... My own calculations are for "typical" conditions in the southwest of England, where I was living at the time. The actual local figures will depend heavily on the haulage distance. atb, Mark 2.1.2: Low ecological impact:Straw is a natural agricultural waste material, requiring no industrial processing. As all energy and resource inputs into crop growth are aimed at maximising the yield of the grain, the energy and environmental costs associated with use of the crop stalks are minimal, consisting only of those resulting from baling and transportation.[i],[ii],[iii] As the energy costs of baling would, in almost all cases, be incurred anyway, the embodied energy of straw used in building may therefore usually be assumed to equal that involved in haulage of the bales to site. For a typical 400-bale strawbale house (approximately 150 m2 wall area, compatible with a 100 m2 dwelling floor area) in Britain, this is unlikely to exceed 3.5GJ, or about 250kg CO2 emissions[iv]. This clearly represents a considerable saving over traditional building materials such as brick, stone or concrete, and a smaller (but still significant) saving over “natural” materials such as timber or recycled products such as cellulose insulation. [This claim, however, assumes that no re-baling is required. This is indeed the case where adequate bales are available, but some builders have not always found this to be the case and have therefore re-baled on site[v]. Even so, the energy consumption in baling is low – approximately 0.34 GJ for 400 bales[vi], corresponding to about 25kg CO2 emissions.] To put these figures into context, let us compare the approximate embodied energy values and carbon dioxide emissions corresponding to 3 different full wall assemblies of 150 m2 area[vii] and similar insulation value. The results are given in table 2.1 overleaf. While no serious attempt at precision is possible at present with this type of calculation, it is clear that, even under the worst realistic conditions, strawbale construction compares favourably on these criteria with the principal competing low-U-value wall designs. At the end of the building process, in contrast to much modern practice, the majority of the building waste can be recycled or used as mulch, reducing the load on municipal landfill and incineration facilities. Edminster[viii], in commenting on the results of her thesis on this subject, notes however, that misleading “greenwash” exists in sections of the strawbale community as elsewhere. Largescale use of concrete in foundations, steel in frames, and cement in plastering, combined with large building dimensions, can result in a structure just as resource-hungry (in terms of both materials and embodied energy) as any conventional building. At the same time, she notes the potential of the technology for minimal impact when thought through carefully. In an article for an audience mostly composed of enthusiasts, she notes: The results of my research and analysis show a startling spectrum of environmental impact severity among approaches to designing with straw bales. Examining only measurable indices of impact – embodied energy, water consumption, and waste potential – ten- and hundred-fold degrees of impact can be identified between different design options. In some cases the impacts exceed those of conventional building systems. It thus became evident that the potential for lessening environmental impacts through deliberate design choices is substantial.[ix] Table 2.1: Comparison of embodied energy in different construction methods
Strawbale construction, therefore, provides an opportunity for unusually ecologically sound building, but does not guarantee it: a thorough environmental audit of the whole process is still necessary if ecological excellence is to be achieved. [i] Edminster A V (1995), Straw Bale Construction: Patterns for a Sustainable System, Masters Thesis, Department of Architecture, University of California, Berkeley, CA, USA (also available as: Investigation of Environmental Impacts: Strawbale Construction, design AVEnues, Pacifica, CA, USA) [ii] Hofmeister R (1994:winter), Straw-Bale Construction: How Does it Stack Up?, The Last Straw, issue 5, Tucson, AZ, USA [iii] Pierquet P (1997:winter), How Good?: Embodied Energy of Insulation Materials, The Last Straw, issue 17, Tucson, AZ, USA [iv] Calculations based on 2 trucks carrying 200 bales each, and each with a 60-mile round trip [v] van der Zoom H (2004:06), unscripted talk, International Strawbale Building Conference, Kalø, Denmark [vi] maximum figure obtained from calculations based on (i) data provided on bale manufacturers’ websites, and (ii) observation of baling on a field in Carmarthenshire. [vii] Calculations based on unit embodied energy costs from various sources tabulated in: ed. Thomas R (1996), Environmental Design, London: E & F N Spon [viii] Edminster A V, op cit [ix] Edminster A V (1996:winter), Strawbale Design: Towards an Ecological Accounting, The Last Straw, Tucson, AZ, USA, issue 13 Andrew Webb wrote: Closely related to the question is embodied energy, if that is a help to them. Straw bale has 0.24 MJ/kg or 31 MJ/m3 according to http://www.canadianarchitect.com/asf/perspectives_sustainibility/measures_of_sustainablity/measures_of_sustainablity_embodied.htm |