Where the water is: free water, bound water and the fibre saturation point

Why start with water

Most quality complaints a sawmill receives are, at root, moisture problems: cupped boards, flooring that opens gaps in winter, failed glue lines, stain under wrapping, mouldy pallets. In each case the wood was at a different moisture content from the one it met in service, and it moved, or was attacked biologically, while it adjusted.

Figures in this course come mostly from North American sources (the USDA Wood Handbook and US extension guides). Where a figure is European, or no European value was available, the lesson says so.

A short tour of the structure

Each wood cell is a hollow tube: the hollow is the lumen, the tube wall is the cell wall. In softwoods, long thin cells called tracheids make up over 90 % of the volume (1 to 10 mm long, more than 100 times longer than wide). Hardwoods split the work between vessels (conducting tubes) and fibres (strength). The cell wall is a composite of cellulose, hemicelluloses and lignin, laid down in layers; the thick S2 layer dominates its properties.

Water can sit in two very different places in this structure.

Free water and bound water

• Free water is liquid or vapour in the lumens and other cavities, held by capillary forces. Removing it makes the piece lighter but does not make it shrink.
• Bound water is inside the cell wall, held to the wall polymers by intermolecular attraction. Removing it takes more energy and draws the cell walls together, so the dimensions shrink and, in general, strength and stiffness rise. The Dry Kiln Operator's Manual (North America) puts the extra energy at about 5 % at 15 % moisture content and about 15 % at 6 %, compared with evaporating free water.

The fibre saturation point

The fibre saturation point (FSP) is the moisture content at which the cell walls are saturated with bound water while the lumens hold no liquid. Above it, dimensions and strength generally stop changing with moisture. Below it, wood shrinks as it dries and swells as it takes up water.

• The Wood Handbook says the FSP averages about 30 % moisture content, with differences of several percentage points between species and pieces. When the species value is unknown, use 30 %.
• Bergman (Wood Handbook chapter 13, North America) uses about 22 % for four entries in his shrinkage table: northern white cedar, western redcedar, and old-growth and second-growth redwood. This is not a rule for all cedars or all redwoods.

The 25 to 30 % band sometimes quoted is a convenient range, not a constant. A 2025 paper in the journal Cellulose (Fredriksson and co-authors) argues that the classic definition mixes two different moisture states and suggests the clearer name "property intersection point". For planning, 30 % remains sensible as an engineering convention with scatter, not a sharp physical switch.

A drying board is not uniform

As a board dries, the surface passes below the FSP first, while the core is still wet. The shell tries to shrink while the core resists. This gradient is the root cause of drying stress and surface checking (Lesson 6).

Worked example: where is the board in the model?

Use the simple model with FSP = 30 %.

In this model a green board at 80 % and one at 40 % have the same dimensions even though the first is far heavier. A board with its surface at 18 % and its core at 35 % is shrinking at the surface and not at all in the core, which is exactly the situation that produces stress.

Key points

• Free water sits in the lumens; bound water sits in the cell walls. Only loss of bound water makes wood shrink and gain stiffness.
• The FSP averages about 30 % moisture content (Wood Handbook, North America) with several points of scatter; four named entries are treated as about 22 %.
• Above the FSP, size does not change with moisture; below it, shrinkage is assumed to start and proceed with falling moisture.
• A drying board has a moisture gradient: its surface can be below the FSP while its core is still above it.