Building with earth - Gernot MINKE (1)

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 interior.
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 amplitude.
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 comfortable
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 constructed
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).
Temperature °C
Temperature °C
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
Indoor air temperature
Time of day
Time of day
The comfort zone for Cairo
Indoor air
temperature
Outdoor air
temperature
The comfort zone for Cairo
Outdoor air
temperature
2.32
Loamy Lean loam
sand
Binding force
Measurements in mm
Nearly rich
loam
2.33
2.35
Solid loam
up to 0.5
N/mm 2
Loam with
fibres upt to
0.3 N/mm 2
Rich V. rich Clay
loam loam
after Niemeyer, DIN 18952
Thermal expansion
The expansion of a material caused by raising
its temperature is relevant for mud plasters
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).
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 different
loams can only be compared if either
water content or plasticity are equal. According
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
2.34
32
Properties of earth