Fibre saturation point FSP
Fibre Saturation Point (FSP) relates to the moisture content (MC) of timber. Saturation Point (SP) may also be used with reference to other materials such as soil, but in both cases it refers to the point at which pores or cells are 'full' with water.
The FSP of timber will vary between species but it is normally accepted as averaging around 30%MC (moisture content), that being the percentage ratio of the wet wood to the theoretical weight of the dry wood. This level of MC is significant because it is considered to be the point of reasonable movement ie when wood that is significantly above this MC level dries its physical and mechanical properties change, which can lead to deformation.
Wood is hygroscopic so moisture levels can change in relation to the localised environment. As wood dries it shrinks, and as it absorbs moisture up it expands, the severity of which depends on the amount of moisture lost or gained. When wood is at the FSP of 30% further drying is less likely to cause significant deformation and it is therefore considered more stable or predictable in terms of being worked with.
The point at which timber stabilises in balance with the moisture of its own environment (which is normally below the FSP) is referred to as Equilibrium Moisture Content (EMC). This figure will vary depending on the species but more importantly with the moisture and temperature of the local environment, this hygroscopic characteristic of timber can be beneficial to internal environments when exposed internally. Fore more information see: Equilibrium Moisture Content.
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