Building with earth - Gernot MINKE (1)

of grains can also be used, but given their
higher density, they exhibit inferior insulating
properties. The more porous the mixture,
the lighter it is and the greater its thermal
insulation.
element. Cutting can be managed by a variety
of manual or mechanical methods.
Preparing the mixture
Loam and straw is mixed together either by
Compressive strength (N/mm 2 )
According to the German standard DIN
pouring the slurry over the straw or by dip-
18951, loam with lightweight aggregates is
ping the straw into the slurry. The straw
called lightweight loam if its density is less
shoots must be totally surrounded by loam
than 1,200 kg/m 3 . If straw is used as the
filler, it is called lightweight straw loam,
while sawdust or wood shavings are
referred to as lightweight wood loam.
slurry. Chapter 10, p. 83 describes how this
mixture is handled subsequently for various
applications.
Sand 0- 4
Bentonite : Sand
= 1 : 9
Kaolinite : Sand
= 1 : 9
Silty loam : Sand = 6 : 4
Porous mineral aggregates are called light-
Thermal insulation
4.18
weight mineral loam. Since these three
One widely held misconception is that straw
types of lightweight loams differ in their
loam used as infill in medieval timber-
properties and methods of manufacture,
framed houses in Europe provided sufficient
they are described separately.
thermal insulation. If 10 parts of cut straw
Rich clayey slurry is used to produce these
are mixed with thick loam slurry made of 2
lightweight loams. The process of making
parts of dry clayey loam and 1 part of water,
slurry depends upon the specific loam mix-
this will give a mixture with a dry density of
ture that has been found, and can be per-
about 1,300 kg/m 3 and a k-value of about
formed either manually or mechanically,
0.53 W/mK. Thus, a typical element of this
as described in chapter 3, p. 38.
material with a thickness of 14 cm covered
In theory, it is also possible to use loam that
with 2 cm lime plaster on both sides gives a
has been blown up or expanded with
U-value of 2.1 W/m 2 K. On the other hand, if
foam-creating substances to form air-filled
a U-value of 0.5 W/m 2 K is to be achieved
pores. To date, tests with loam have failed
(as generally desired or required by building
to do produce corresponding results.
codes in most central and northern European
countries today), then this wall would
Lightweight straw loam
have to be 0.95 m thick. Even if the straw
content were to be increased threefold, this
General
material is unacceptable for a thickness of
Lightweight straw loam is a mixture of
14 cm.
straw and loam with a density of less than
In practice, it is almost impossible to achieve
1,200 kg/m 3 . If this density is higher than
a density less than 500 kg/m 3 , since the
1,200 kg/m 3 , it is called straw loam. There is
straw is softened by moistening caused by
4.19
worldwide debate over which type of straw
the mixing process, and is compacted when
is most suitable, and it should be tested in
placed in the formwork.
each case. For loam plaster, however, barley
There have been claims of lower density (as
straw has proven to be suitable, since it is
low as 300 kg/m 3 ), but these are not usually
usually softer than the other straws. More
correct, since they are often based on or
important than the kind of straw is the
produced by inaccurate testing methods.
structure of its shoots. In order to increase
Typically, a small brick-size formwork is
thermal insulation, straws with rigid shoots
loosely filled with a straw loam mixture. This
are preferred, since they do not deform eas-
is then weighed after drying and divided by
ily, and hence keep air trapped inside.
the volume of the mould, which can lead to
errors of about 40%. The only accurate
Cutting straw
method of determining density is to saw-
The length of the straw shoots should be
cut a cuboid out of a larger block (especially
no greater than the thickness of the building
in height) so that the straws bent at the cor-
48
Improving the earth