Building with Earth: Design and Technology of a Sustainable Architecture

tion 2.27 shows the drying process of differ-

ent loam samples compared to other build-

ing materials. In this test, conducted at the

BRL, brick-size samples were immersed in

3 mm of water for 24 hours and then kept

in a room with a temperature of 23°C and

relative humidity of 50% in still air condi-

tions. Interestingly, all loam samples dried

out after 20 to 30 days, whereas baked clay

bricks, sand-lime bricks and concrete had

not dried out even after 100 days.

Effects of vapour

While loam in contact with water swells and

weakens, under the influence of vapour it

absorbs the humidity but remains solid and

retains its rigidity without swelling. Loam,

hence, can balance indoor air humidity, as

described in detail on pp. 15 –18.

Vapour diffusion

In moderate and cold climates where indoor

temperatures are often higher than outside

temperatures, there are vapour pressure

differences between interior and exterior,

causing vapour to move from inside to out-

side through the walls. Vapour passes

through walls, and the resistance of the wall

material against this action is defined by the

“vapour diffusion resistance coefficient.”

It is important to know the value of vapour

resistance when the temperature difference

between inside and outside is so high that

the indoor air condenses after being cooled

down in the wall.

The German standard DIN 52615 describes

the precise test procedure used to deter-

mine these values. The product of m with

the thickness of the building element s gives

the specific vapour diffusion resistance sd.

Still air has an sd-value of 1. Illustration 2.28

shows some of the μ-values determined by

the BRL for different kinds of loam. It is

interesting to note that silty loam has an μ-

value about 20% lower than that of clayey

and sandy loams, and that lightweight loam

with expanded clay weighing 750 kg/m^3

has a value 2.5 times higher than that of

loam mixed with straw and having the

same overall density.

Chapter 12 (p. 98) describes how painting

reduces the permeation of vapour through

walls.

Equilibrium moisture content

Every porous material, even when dry, has a

characteristic humidity, called its “equilibrium

moisture content,” which depends on the

temperature and humidity of the ambient

air. The higher temperature and humidity

levels are, the more water is absorbed by

the material. If temperature and air humidity

are reduced, the material will desorb water.

The absorption curves of different loam mix-

tures are shown in 2.29. The values vary

from 0.4% for sandy loam at 20% air

humidity to 6% for clayey loam under 97%

air humidity. It is interesting to note that rye

straw under 80% humidity displays an equi-

librium moisture content of 18%. In contrast,

expanded clay, which is also used to achieve

lightweight loam, reaches its equilibrium

moisture content at only 0.3%. In 2.30, four

values of loam mixtures are shown in com-

parison to the values of other common

building materials.

Here, one can see that the higher the clay

content of loam, the greater its equilibrium

moisture content. Additionally, it should be

mentioned that Bentonite, which contains

70% Montmorillonite, has an equilibrium

moisture content of 13% under 50%

humidity, whereas the equilibrium moisture

content of Kaolinite under the same condi-

tions is only 0.7%.

The graph shows that silty earth blocks or

adobes (no. 4 on the graph) reach a mois-

ture content five times higher than a sandy

loam plaster (no. 9 on the graph) at a rela-

tive humidity of 58%.

It should be noted that for the humidity

balancing effect of building materials, the

speed of absorption and desorption

processes is more important than the equi-

librium moisture content, as explained on

p. 14.

29 Properties of earth

2.24

2.25

Drying time t (d)

Water content

Shrinkage

at 20/81

at 20/44

at 20/81

at 20/44

2.26

2.24 Water spraying test
apparatus developed at
the BRL
2.25Loam samples
before (left) and after
(right) being exposed to
weather for three years
2.26Linear shrinkage
and drying period of lean
loam mortar (clay 4%, silt
25%, sand 71%) with a
slump of 42 cm accord-
ing to the German stan-
dard DIN 18555 (Part 2)

0

0.5

1

1.5

2

2.5

Water content W (%) Linear shrinkage (%)