Building with Earth: Design and Technology of a Sustainable Architecture

49 Improving the earth

ners as well as the air spaces left around the

edges of the mould are eliminated. The

larger the sample, the greater the accuracy,

since there is always some edge erosion

during cutting and handling.

Due to the above-mentioned errors, unfor-

tunately, densities of as low as 300 kg/m^3

tend to be assumed and the k-value com-

puted accordingly. Since, in reality, densities

are typically about 700 kg/m^3 in built sec-

tions, the k-value of this is 0.21 W/mK, from

which, for a 30-cm-thick wall plastered on

both sides, the U-value can be derived as

0.6 W/m^2 K. This value of heat transmission

is double the value that can be claimed by

assuming a density of 300 kg/m^3.

The following points are to be kept in mind

when working with lightweight straw loam,

for lightweight straw loam has certain un-

deniable disadvantages in comparison with

pure loam:

1. In a moderate or humid climate, fungus
   growth occurs after only a few days, emit-
   ting a characteristic strong smell. This can, in
   extreme cases, give rise to allergies. There-
   fore, good ventilation during construction
   must be provided so that building compo-
   nents dry out quickly. After the walls have
   dried completely, which might take several
   months, or even a year or more, depending
   upon thickness and climate, the fungus
   stops producing spores. However, spore for-
   mation may be reactivated if water perme-
   ates the walls either from the outside
   through leakage, or from inside through
   condensation. Fungus growth can be inhib-
   ited by adding lime or borax, but this has
   the following disadvantages:

• binding force and compressive strength
  are significantly decreased,


• hands become irritated while working
  with this mixture.


• Walls thicker than 25 cm may appear dry
  on the surface, even though they are rotting
  within (see chapter 10, p. 83).

2. The surface strength of the mix for a wall
   with a density of less than 600 kg/m^3 is usu-
   ally too low to effectively grip nails or dow-
   els, as is often required. Since two layers are
   necessary, plastering is more laborious,

sometimes with some reinforcement in

between.

3. When drying, vertical settling occurs,
   leading to gaps on top of wall elements
   (4.21). These must carefully be filled later on
   in order to prevent heat and sound bridges
   and air infiltration.


4. Working with this material is fairly labori-
   ous. Without special machines for mixing
   and transportation, the labour input for a
   typical 30-cm-thick wall is about 6 h/m^2
   (20 h/m^3 ). This is four times the labour
   required for typical brick masonry work.
   The disadvantages mentioned above can
   be avoided if porous mineral aggregates are
   used instead of straw, as discussed in the
   following section.
   The potential advantages of lightweight
   straw loam are the low material costs
   involved, and the fact that it can be worked
   without investments in special tools and
   machinery. It is especially appropriate, hence,
   for do-it-yourself construction.

Lightweight mineral loam

In order to increase thermal insulation,

porous mineral aggregates can be added to

loam as an alternative to straw; these

include expanded clay, foamed glass,

expanded lava, expanded perlite and

pumice. It is possible to achieve a shrinkage

ratio of 0 (i.e., to eliminate shrinkage alto-

gether) by choosing the right proportion of

aggregates. All other techniques of earth

construction require consideration of shrink-

age.

In comparison with straw loam, the vapour

diffusion resistance is two to three times

higher and, therefore, the probability of con-

densation of water within the wall is low

(see chapter 2, p. 29).

Another advantage of the material is that

the mixture can be pumped into a form-

work, thereby greatly reducing labour input.

As investments on machines are higher,

this method is recommended only for

larger construction projects. The densities

generally achieved vary from 500 to 1,200

kg/m^3.

Straw Weight Compressive strength

[%/mass] [kg/m^3 ] [N/mm^2 ]

0

1

2

4

8

1882

1701

1571

1247

872

2.2

1. 4


2. 3
   1.1
   0.3

4.20

4 .18Compressive

strengths of loams and

sand with the addition

of 6% lime

4 .19Suggested appro-

priate stabilisers for loam

in relation to their plastici-

ty (CRATerre, 1979)

4.20Reduction of the

compressive strength of

loam by adding cut straw

(5 cm)