Clay - Straw

operated by J.F.Burri


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.

malaxeur à barbottine

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.



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

 vitrage et terre-paille 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.

enduit de terre sur terre paille

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

coupe dans un mur de terre pailleCutting in
a wall:

is dense
the straw
is not
One sees
ends of
being used by JFB to reinforcing
the wall



by the Web
Earth - straw in Belgium, Germany...