Agriculture Reference
In-Depth Information
Insulating wool fibres are characterized by
their low density and thermal conductivity λ
expressed as W/m/K. The lower this value,
the harder it is to transmit energy via thermal
conduction and therefore the better the
insulation.
When considering the performance of
hemp wool fibre, the following characteristics
need to be considered:
The thermal properties of the material.
and compared to the predictions obtained from
a theoretical approach based on a self-consist-
that λ varies from 0.0038 to 0.007 W/m°C
(equivalent to thermal resistances of between R
= 5.26 W/m/K and R = 2.85 W/m/K for
products with a constant width of 20 cm).
These thermal conductivities are very similar to
commercial synthetic insulating wool fibres
currently in widespread use in the construction
industry today.
Thermal conductivity is sensitive to and
influenced by water content, a fact that is often
overlooked. Water vapour in room air interacts
with the material. The nature of the wool fibres
and their ability to exchange water can lead to
the wool taking on condensed water. The
extent to which it does so is affected by the
ambient humidity and temperature. Water is a
very good conductor of energy (Table 17.1)
and the thermal conductivity of the wool will
therefore increase with increasing water con-
tent. Experimentally, increases in conductivity
of the order of 30% have been measured in
environments saturated with water. It is there-
fore essential that the installed wool fibre insu-
lation is allowed to breathe naturally in order to
regulate water exchange.
•
Thermal insulation is the principal func-
tion of hemp wool.
The mechanical properties of the material.
•
The performance of the hemp wool dur-
ing its conditioning and employment are a
direct function of these characteristics.
Thermal properties and insulating
potential of hemp wool
Let us deal first with the thermal conductivity
of hemp wool. Examination of the electron
micrographs of hemp fibres (Fig. 17.2) shows
us the relatively regular polygonal profile of the
fibre and its hollow centre. The other major
feature of hemp fibres that distinguishes it from
other insulating wools is its porous nature. This
porosity is responsible for the very low weight
and for the very low intrinsic thermal conduc-
tivity of the fibres.
The microstructure of hemp wool results
in a conductivity of 0.05 W/m °C for a wool of
40 kg/m
3
. This is equivalent to a thermal
20 cm (R = e/
Mechanical characteristics and
behaviour during installation
If we now consider the installation of the insu-
lation material, another aspect of importance
becomes clear: the mechanical characteristics
and behaviour of the wool. As we have just
seen, the porous microstructure is at the root
of the insulating properties of this material.
= 0.2/0.05).
Figure 17.3 presents the thermal conduc-
λ
tivity
λ
as a function of density (values measured
Fig. 17.2.
Scanning electron micrograph of hemp wool fibres.
