Insulation Materials Made with Vegetable Fibres - Nearly Zero Energy Building Refurbishment - page 424

However, some disadvantages related to vegetable fibres do exist, such as
dimensional stability and reactivity (Olesen and Plackett), mainly due to their
strong water absorption, hygroscopic swelling, low resistance to microbiological
attack. Most of the vegetable fibres suffer alkali attack which need special treat-
ment of chemical modification. For fibre-reinforced composites, ecological treat-
ment of fibres is essential because of their limited compatibility between the fibre
and the polymeric matrix. Specifically, controlling the moisture content of fibrous
material is essential, but difficult. The vegetable fibres should always be carefully
controlled since moisture into their fibrous insulation material is inevitable natu-
rally due to the rains, snow, etc. Accurately identifying and controlling moisture
levels in fibreous insulation in buildings is of the most importance in order to
ensure energy efficient healthy buildings. The control stresses the need for mod-
elling and simulation of heat and mass transfer which is presented below.
5 Modelling Heat and Moisture Transport in Vegetable
Fibrous Materials
Thermal insulation is an important element of energy conservation. To maximize
energy efficiency of the insulating materials, various factors that may affect the
efficiency of heat insulation should be taken into account. The knowledge of
simultaneous heat and moisture transfer through the insulators is very important.
Vegetable fibrous materials have a unique structure of complex geometry which
belongs to a typical kind of porous media. Therefore, heat and mass transport
theories in porous media provide a fundamental understanding of the thermal and
moisture transport performance for the vegetable fibrous materials.
5.1 Basic Concepts
A porous medium is defined as a material characterized by the presence of solid
matrix and voids which is presented by its porosity. Porosity e describes the
fraction of void space V v in the material of volume V as:
e ¼ V v
V
Porosity has been reported to be an important factor to the thermal insulating
performance (Farnworth 1983 ; Fan et al. 2004 ; Wu et al. 2007 ; Cheng and Fan
2004 ).
The tortuosity is another important dimensionless parameter which describes
the complex structure of the material and is defined as