Fiber saturation point

The fiber saturation point (FSP) is a term used to describe the moisture content at which the cell walls of wood are entirely saturated with water, but no water exists in cell cavities. Below the FSP, wood shrinks when moisture is lost and swells when moisture is gained. The FSP of wood averages about 30% moisture content.

The FSP is an important concept in wood technology because it marks the point at which the physical and mechanical properties of wood begin to change as a function of lowering the moisture content. For example, during the kiln drying of timber, little shrinkage occurs until the water content drops below the FSP. An accurate knowledge of the FSP is important because it can increase the efficiency of the kiln drying process.

There are different methods for determining the FSP, and each method gives different values for the FSP. Traditional methods for determining FSP, such as those using shrinkage measurements, only indirectly measure this point and are prone to large standard deviations. One method for determining the FSP is differential scanning calorimetry (DSC), which measures the enthalpy of melting of water in wood.