Water Vapour Diffusion
Home Up Psychrometry Water in Building Materials Condensation Water Vapour Diffusion Prediction of Condensation Methods of Preventing Condensation Vapour Barriers


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:


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:


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):


The flow equation using vapour equation is:



Figure 2 Variation of temperature Figure 3 Prediction of Condensation

and vapour pressure