Building with Earth: Design and Technology of a Sustainable Architecture

31 Properties of earth

on the inside due to the vapour barrier.

In this case, the wall remains damp for a

longer period than it would without a

vapour barrier.

Influence of heat

The common perception that earth is a

very good material for thermal insulation is

unproven. A solid wall of rammed earth

without straw or other light aggregates has

nearly the same insulating effect as a solid

wall of baked bricks. The volume of air

entrained in the pores of a material and its

humidity are relevant for the thermal insula-

tion effect. The lighter the material, the

higher its thermal insulation, and the greater

its humidity level, the lower its insulating

effect.

The heat flowing through a building ele-

ment is defined by the overall heat transfer

coefficient U.

Thermal conductivity

The heat transfer of a material is charac-

terised by its thermal conductivity k [W/mK].

This indicates the quantity of heat, mea-

sured in watts/m^2 , that penetrates a 1-m-

thick wall at a temperature difference of

1°C.

In 2.31, the different k-values according to

DIN 4108-4 (1998), indicated by a 1, are

shown. 2 are measurements of Vanros,

3 and 4 of the BRL.

At the BRL, a lightweight straw loam with

a density of 750 kg/m^3 gave a k-value of

0.20 W/mK, whereas a lightweight expand-

ed clay loam with a density of 740 kg/m^3

gave a value of 0.18 W/mK.

Specific heat

The amount of heat needed to warm 1 kg

of a material by 1°C is called its “specific

heat,” represented by c. Loam has a specific

heat of 1.0 kJ/kgK which is equal to 0.24

kcal/kg°C.

Thermal capacity

The thermal capacity (heat storage capacity)

S of a material is defined as the product of

specific heat c and the density r:

S = c ρ[kJ/m^3 K]

The thermal heat capacity defines the

amount of heat needed to warm 1 m^3 of

material by 1°C. The heat storage capacity

Qs for a unit area of wall is S multiplied by

the thickness s of the element:

Qs= c ρc [kJ/m^2 K]

Heat intake and release

The speed at which a material absorbs or

releases heat is defined by the thermal dif-

fusivity b which is dependent on the specific

heat c, density r and the conductivity k:

b = √c ρk [kJ/Km^2 h0.5]

The larger the b-value, the quicker the pen-

etration of heat.

2.28

2.31

2.30

Clayey loam (clay = 28%, silt = 34%, sand = 38%)

Silty loam (clay = 12%, silt = 78%, sand = 56%)

Sandy loam (clay = 15%, silt = 29%, sand = 56%)

Straw loam 450 kg/m^3

Straw loam 750 kg/m^3

Straw loam 950 kg/m^3

Straw loam 1250 kg/m^3

Loam with expanded clay 800 kg/m^3

Loam with expanded glass 500 kg/m^3

Loam with expanded glass 750 kg/m^3

Clayey loam plaster

Silty loam plaster

Cowdung-loam-lime-sand plaster (12/4/3/20)

High hydraulic lime plaster

Lime plaster

Lime-casein plaster (10/1)

Lime-linseed oil plaster (20/1)

( ) Volumetric proportion

Vapour diffusion resistance coefficient μ (–)

1 Spruce, planed

2 Limba, planed

3 Earth block, clayey

4 Earth block, silty

5 Cement plaster

6 Lime-cement plaster

7 Lime-casein plaster

8 Silty loam plaster

9 Clayey loam plaster

10 Solid brick

11 Clinker brick

12 Porous brick

13 Lime-sand brick

14 Porous concrete

Water content (g/dm^3 )

Relative humidity (%)

.

..

..

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

0 0.2 0.4 0.6 0.8 1.0 1.2

Solid loam

Lightweight loam

U-value (W/mK) specificweight(kg/m (^3) )