Moisture content

[edit] Introduction

The term 'moisture content' (or water content) refers to the amount of water that is contained in the pores (voids) of a material. It is usually expressed as the percentage by mass of the water present relative to the material’s dry weight. Understanding moisture content can be useful for a number of common materials such as wood, ceramics, soil and so on.

[edit] Calculating moisture content

To arrive at the moisture content of a material, the following formula can be used:

Moisture content (%) = (wet mass – dry mass) / dry mass

Moisture content can also be expressed as a ratio that can range from 0 (completely dry) to a material’s saturation value. Equilibrium moisture content (EMC) is the point at which a material neither gains nor loses more moisture.

In most substances, the amount of water present will vary due to conditions such as relative humidity and temperature. For example, in most cases, including wood, as the humidity increases, so does the moisture content, while a decrease in humidity leads to a decrease in moisture content.

Acceptable moisture levels in wood are typically:

Materials such as glass, most plastics and steel have no pores and so have almost zero porosity. This means they are literally devoid of having an internal surface area into which moisture can penetrate, be absorbed or be transmitted through.

[edit] Reversible and irreversible moisture content

Variations in the moisture content of materials can cause movement in buildings. In porous building materials, moisture content changes can cause reversible or irreversible movement.

Irreversible moisture movement is seen, for example, in bricks that have just been manufactured: just out of the kiln, clay bricks will be very dry and will start to absorb moisture immediately causing irreversible expansion. In contrast, calcium silicate bricks are cured by an autoclave process using heat and steam; they will be more saturated than normal bricks and, as their moisture content drops to achieve an equilibrium with that of the atmosphere, they will shrink. Both these examples illustrate why some building materials should not be used immediately for building.

Reversible moisture expansion is seen in some materials that are part of a building structure. They generally expand when wet and shrink when dry. In doing so, they can have dramatic and unfortunate consequences on a building if their behaviours are not understood and accounted for. They therefore require thoughtful and intelligent detailing e.g movement joints, to maximise the life – and aesthetics – of the construction.

High moisture content can cause problems, especially when allied with changes in temperature. Saturated brickwork under freezing conditions causes a phenomenon called ‘spalling’ where the face of the brickwork is badly damaged, usually requiring rebuilding or refacing. The mechanism responsible is freezing which causes expansion of the water present in the pores of the material, thereby exerting pressure on the front surface of the brick.

Wet rot is a generic name given to a range of conditions that can affect timber where there is the continual presence of moisture, perhaps caused by leaking pipework, poor ventilation (resulting in condensation), rising or penetrating damp and so on. For more information see: Wet rot.

Most moulds require relatively high levels of moisture in order to grow. The majority require an equivalent of at least 70% relative humidity to thrive and most large mould outbreaks in buildings, occur where porous, cellulose-type materials contain persistent liquid water or condensation. For more information see: Mould growth in buildings.

Soils can also change with moisture content. For example, ground heave is the upward movement of the ground usually associated with the expansion of clay soils which swell when wet. As the soil generally cannot expand downwards or sideways, the result is that the exposed upper surface of the soil rises up. For more information see: Ground heave.

With soils, moisture content can be estimated by satellites using microwave remote sensing. This is achieved by contrasting the difference between the dielectric properties of wet and dry soil. Microwave radiation is largely unaffected by the atmosphere and so can penetrate cloud cover. It can also penetrate vegetation through to the ground surface.