A sustainable alternative -
Conventional vs Strawbale construction
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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 it’s 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.
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