Water in Building Materials
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2 Water in Building Materials


2.1 Effect of Humidity

Most material surfaces exert forces to attract water vapour molecules to stick onto their surface. The film thickness and the amount of water held in equilibrium with the surrounding atmosphere is roughly proportional to the relative humidity of the atmosphere as shown in Table 1.


Table 1 Relation between Relative Humidity and Water Film Thickness

Relative humidity

Water film thickness

Low humidity

Only one molecule thick

Moderate humidity

Polymolecular films

Close to 100% RH

Relatively thick films

Saturation (100% RH)

Condensation occurs


2.2 Degradation of Materials

Material exposed to excess moisture may experience reversible performance degradation or irreversible deterioration caused by:


2.2.1 Physical Changes

The absorption or release of water contents in construction materials causes

(a) differential expansion/contraction of the building components,

(b) an increase of stress at junction points of dissimilar materials and possibly the formation of cracks/gaps, and

(c) an increase in infiltration or rain seepage through cracks/gaps of materials.


2.2.2 Chemical Changes

Condensed water brings about chemical reactions which:

(a) rust metallic building components, particularly structural components,

(b) reduce the effectiveness of thermal insulation, and

(c) damage other components or interior finishes.


2.2.3 Biological Changes

Moisten building materials face an increased chance of attack by mould, fungi or insects.


2.3 Effect on Heat Flow

The effect of moisture on heat flow depends on the nature of insulation, the moisture content, and the temperature of the insulation. In some porous materials partially saturated with water, moisture moves to the cooler side by the temperature gradient with the material. The migration takes place through the continuous process of evaporation, vapour flow and condensation within the material. This contributes to heat flow in both latent and sensible forms.


2.4 Effect on Heat Storage

Moisture affects the thermal storage capacity of hyroscopic building material considered as composite of a given substance and water.