Building with Earth: Design and Technology of a Sustainable Architecture

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.
According to the German standard DIN
18951, loam with lightweight aggregates is
called lightweight loam if its density is less
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.
Porous mineral aggregates are called light-
weight mineral loam. Since these three
types of lightweight loams differ in their
properties and methods of manufacture,
they are described separately.
Rich clayey slurry is used to produce these
lightweight loams. The process of making
slurry depends upon the specific loam mix-
ture that has been found, and can be per-
formed either manually or mechanically,
as described in chapter 3, p. 38.
In theory, it is also possible to use loam that
has been blown up or expanded with
foam-creating substances to form air-filled
pores. To date, tests with loam have failed
to do produce corresponding results.

Lightweight straw loam

General
Lightweight straw loam is a mixture of
straw and loam with a density of less than
1,200 kg/m^3. If this density is higher than
1,200 kg/m^3 , it is called straw loam. There is
worldwide debate over which type of straw
is most suitable, and it should be tested in
each case. For loam plaster, however, barley
straw has proven to be suitable, since it is
usually softer than the other straws. More
important than the kind of straw is the
structure of its shoots. In order to increase
thermal insulation, straws with rigid shoots
are preferred, since they do not deform eas-
ily, and hence keep air trapped inside.

Cutting straw
The length of the straw shoots should be
no greater than the thickness of the building

element. Cutting can be managed by a vari-

ety of manual or mechanical methods.

Preparing the mixture

Loam and straw is mixed together either by

pouring the slurry over the straw or by dip-

ping the straw into the slurry. The straw

shoots must be totally surrounded by loam

slurry. Chapter 10, p. 83 describes how this

mixture is handled subsequently for various

applications.

Increasing thermal insulation

One widely held misconception is that straw

loam used as infill in medieval timber-

framed houses in Europe provided sufficient

thermal insulation. If 10 parts of cut straw

are mixed with thick loam slurry made of 2

parts of dry clayey loam and 1 part of water,

this will give a mixture with a dry density of

about 1,300 kg/m^3 and a k-value of about

0.53 W/mK. Thus, a typical element of this

material with a thickness of 14 cm covered

with 2 cm lime plaster on both sides gives a

U-value of 2.1 W/m^2 K. On the other hand, if

a U-value of 0.5 W/m^2 K is to be achieved

(as generally desired or required by building

codes in most central and northern Euro-

pean countries today), then this wall would

have to be 0.95 m thick. Even if the straw

content were to be increased threefold, this

material is unacceptable for a thickness of

14 c m.

In practice, it is almost impossible to achieve

a density less than 500 kg/m^3 , since the

straw is softened by moistening caused by

the mixing process, and is compacted when

placed in the formwork.

There have been claims of lower density (as

low as 300 kg/m^3 ), but these are not usually

correct, since they are often based on or

produced by inaccurate testing methods.

Typically, a small brick-size formwork is

loosely filled with a straw loam mixture. This

is then weighed after drying and divided by

the volume of the mould, which can lead to

errors of about 40%. The only accurate

method of determining density is to saw-

cut a cuboid out of a larger block (especially

in height) so that the straws bent at the cor-

48 Improving the earth

Sand 0 - 4 Bentonite : Sand

= 1 : 9

Kaolinite : Sand

= 1 : 9

Silty loam : Sand = 6 : 4

Compressive strength (N/mm

2 )

4 .18

4 .19