Building with Earth: Design and Technology of a Sustainable Architecture

32 Properties of earth

Decrement factor and time lag
“Decrement factor” and “time lag” refer to
the way the exterior wall of a building
reacts to damp and to the period of delay
before outside temperatures reach the inte-
rior. A wall with a high thermal storage
capacity creates a large time lag and heat
decrement, while a wall with high thermal
insulation reduces only temperature ampli-
tude.
In climates with hot days and cold nights,
where average temperatures lie within the
comfort zone (usually 18° to 27°C), thermal
capacity is very important in creating com-
fortable indoor climates. In 2.32, the effect
of material and building shape on interior
climate is shown by readings taken from
two test buildings of equal volume con-
structed in Cairo, Egypt, in 1964. One was
built of 50-cm-thick earth walls and mud
brick vaults, and the other of 10-cm-thick
pre-cast concrete elements with a flat roof.
While the diurnal variation of the outside
temperature was 13°C, the temperature
inside the earth house varied only by 4°C; in
the concrete house, the variation was 16°C.
Thus, the amplitude was four times greater
in the concrete house than in the earth
house. In the concrete house, temperatures
at 4 pm were 5°C higher than outside,
whereas inside the earth house, they were
5°C lower than outside temperatures at
the same time (Fathy, 1986).

Thermal expansion
The expansion of a material caused by rais-
ing its temperature is relevant for mud plas-
ters on stone, cement or brick walls, and for
lime or other plasters on earth walls. The
coefficients of linear expansion measured
by the BRL for heavy loam range from
0.0043 to 0.0052 mm/m·K; for mud brick
masonry up to 0.0062 mm/m·K; and for
sandy mud mortar up to 0.007 mm/m·K.
Soft lime mortar has a value of 0.005
mm/m·K, and strong cement mortar 0.010
mm/m·K, the same as concrete (Knöfel,
1979 and Künzel, 1990).

Fire resistance

In the German standard DIN 4102 (Part 1,

1977) loam, even with some straw content,

is “not combustible” if the density is not less

than 1700 kg/m^3.

Strength

Binding force

The tensile resistance of loam in a plastic

state is termed its “binding force.”

The binding force of loam depends not only

on clay content, but also on the type of clay

minerals present. As it is also dependent on

the water content, the binding force of dif-

ferent loams can only be compared if either

water content or plasticity are equal. Accord-

ing to the German standard DIN 18952

(Part 2), the loam must have the defined

“standard stiffness.” How this is obtained

is described in this chapter on p. 24.

The samples to be tested have a special

figure-8-shape made from a mixture of

standard stiffness. The samples are filled

The comfort zone for Cairo

Indoor air temperature Outdoor air

temperature

Time of day

Temperature °C

The comfort zone for Cairo

Indoor air

temperature

Outdoor air

temperature

Time of day

Temperature °C

2.32

2.34

2.35

Measurements in mm

Solid loam

up to 0.5

N/mm^2

Loam with

fibres upt to

0.3 N/mm^2

Binding force

Loamysand Lean loam Nearly richloam Richloam V. richloam Clay

after Niemeyer, DIN 18952

2.33