Clay - Straw
operated by J.F.Burri
Clay-staw
As the name indicates it, earth - straw is
a mixture of earth and straw used in completion of walls as an
ecological insulation.
As the majority of insulators, it is not a strong material, it
needs so there a structure which gives the rigidity of the construction,
a wood structure rather.
The qualities? A low price (even free), the clean vibration of
the material, the possibility of being realized without particular
competence.
Mine in this domain being limited to a realization, I gather
here the information stemming from different builder and in particular
from Jean-François Burri who used it abundantly in his
houses. Although a part important to know how to make them is
difficult to communicate in a text because it results from the
practice.
Contrary to the cob, it does not seem that earth - straw was
used in the traditional constructions.
Earth and straw
What earth agrees? A big variety if it contains
some clay. Rather which does not agree? The ground of surface
(steppe black soil), the pure clay, the too sandy ground. So
if you make an excavation to implant your construction, separate
the ground of surface to be used for places to be cultivated
of that of the base to be used in construction (adobe, cob, earth
- straw). If there is too much clay, one can add some sand.
Straw of wheat, barley, rye agree, or quite other vegetable stalks
of which lock of the air( (lavender, hemp..). The straw should
not be chopped but such whichfrom the bales.
The barbotine
It is necessary to obtain a very liquid mud
to mix with the straw. Sieve the ground if it contains pebbles
to eliminate those of (about) more than 5 mm. Pour it in rain
into some water. Indeed shake.
In what bowl? A bucket or one bathes for a small quantity, a
hurdy-gurdy bathtub or as JFB a buzzard in cement.
How indeed to mix? In the mixer of painting on a drill or an
agitator tinkered with an electric engine. Shake abundantly to
obtain a very homogeneous juice. One can also add of the cows
shit.
How to judge good proportions? According to Alain Klein, one
fills a small jar of glass yoghourt, he should spread out on
a circle 10 cms in diameter if one pours it on a horizontal glass
pane. If there is too much water, it spreads out more, so one
adds of the earth. Or conversely.
Buzzard in cement, axis and pales of the mixer and the engine under the blue plastic/td
The mixture
One pours the barbottine on a straw heap everything
moving it with forks (several persons are welcome)
What quantity? Enough little, a bucket or two for an oblong bale
of straw. The less one puts of mud, the more the material will
be insulating but the less it will have of rigidity. It is necessary
that by moving it the straw scrapes still a little, so one judges
in the ear. Indeed turn and return to homogenize the mixture.
Let rest 24 hours in heap or at least one night to let the straw
become soaked. So on a construction site one prepares in the
evening the quantity to operate the next day.
Coat also the inside of the structure wood with the barbottine
to avoid a future unsticking between wood and earth - straw.
Application
Move last time the straw, take packages of
it and fill the space between banches (boards or panels) by trying
to align fibers perpendicularly to banches (in the direction
of crossing of the wall finally about). And from time to time
a layer in opposite direction.
Indeed compress every layer with measure, especially in corners,
possibly compress by rising inside.
What thickness of wall enters banches? For a correct insulation,
count between 20 and 30 cms according to the density of the mixture.
Every 30 - 60 cms in height, one can put a bracket in length
in the middle of the wall to stiffen it.
Being made a height of about 50 cms then please , go back raise
banches. The surface of the wall will air some days.
Alterations and finish
The wall is very dry in some
weeks. It is possible that there is a collapse of the material
with a space in the top of the wall. So one put again this same
mixture. If the wall is sheltered from the rain, one can leave
it as it is, especially if it is dense in earth. Or coat it with
barbottine in several layers.
Make well renter the first sleeps between fibers. Let dry between
every layer so that the following one fills cracks. If there
is a lot of clay in the earth, one can add a fine sand to prevent
cracks. Or put one plastered by sand and air lime colored or
not with earth. Idem for the inside.
First layer of earth
To finish
Although there one can think about straw,
earth - straw is a product of the stiffest very strong. To remove
its illegal hut JF Burri sawed it in two parts to transport every
half with the mechanic shovel: to cut the earth - straw walls
he had of to use a powerful electric saw. And even one of these
halves fell in the transport without breaking or unsticking the
completion.
Contrary to what one could think, earth - straw is insensible
on the fire and the rodents can not there penetrate.
A single negative point - and it is frequent with free materials
-: the preparation and the application are rather long, so it
is adapted to the selfbuilder who has time and would enough be
expensive by paying the workforce to the professional rate.
According to the quantity of earth
Density 0,3 - 1,2 (0,9 in 1 for JFB)
Estimated conductivity value 0,1 - 0,45
Cutting in
a wall:
It
is dense
the straw
is not
crushed.
One sees
ends of
bamboo
being used by JFB to reinforcing
the wall
More
by the Web
Earth
- straw in Belgium, Germany...