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4 Water Vapour Diffusion
4.1 Moisture Diffusion through Building Materials
The diffusion of moisture through building
materials is a natural phenomenon. It has significant effects on the comfort condition in
built environment.
4.1.1 Summer
Moisture transfer through walls in summer
is usually negligible because the actual moisture transfer rate is quite small and the
corresponding heat gain is hardly significant.
4.1.2 Winter
During cold weather, vapour pressure inside
a building is usually higher than that outside. The difference in vapour pressure leads to
moisture diffusion through the walls from inside to outside (Figure 1). The result is the
general increase in thermal conductance of each wall element and hence the overall U
value. In case of the presence of thermal insulation at the wall, the insulation may be
permanently damaged in addition to its reduction in effective.
Figure 1 Condensation
4.2 Basic Diffusion Equation
The equation used in calculating water
vapour diffusion through materials is given as:
 (10)
where m = mass diffusion rate of water
vapour or vapour transmission
A = cross-sectional area for water vapour
diffusion
m = vapour permeability
defined as the property of material that determines the rate at which vapour passes
through it under the influence of unit pressure gradient across the material.
pw = vapour pressure
x = distance along the flow
path
Therefore, m/A is mass of
vapour diffusing through unit area in unit time and dpw/dx is vapour pressure
gradient.
If permeability is taken as
the average permeability coefficient applicable to the varying conditions along the flow
path of length L, and its value is assumed to be independent of vapour pressure and
temperature, Eqn. (10) becomes:
 (11)
where M = total mass transmission rate of
vapour, ng/s
A = Area of cross section
of flow path, m2
D Pw = difference of vapour
pressure between ends of flow path, Pa
L = length of flow path or
thickness of wall, m
m = average permeability, ng/(s m Pa)
The overall vapour resistance (Rvt) of an
assembly such as a composite wall of materials is the sum of the vapour resistances of its
component parts (Rvi, i=1,2,....,n):
 (12)
The flow equation using vapour equation is:
 (13)
 
Figure 2 Variation of temperature Figure 3
Prediction of Condensation
and vapour pressure
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