Wood moisture content, fiber saturation, and seasoning

One-line orientation

Wood gains strength and shrinks only as it dries below the fiber saturation point — the free-water-vs-bound-water distinction is the concept every moisture question turns on.

Key points

• Two kinds of water in wood:
  • Free water — in the cell cavities; adds weight only, no effect on structural quality.
  • Bound water — held within the cell walls/fibers; controls dimensional stability.
• Fiber Saturation Point (FSP): the moisture state where free water is gone but bound water remains. It is the dividing line for both shrinkage and strength gain.
• Strength and drying: wood strength increases as it dries — specifically as bound water leaves below FSP. Drier wood is less likely to shrink, warp, or twist.
• Moisture content (MC), measured as a percentage:
  • Green lumber — not yet dried.
  • Structural / dry — roughly ≤19%.
  • Exterior wood — typically ~12%.
  • Interior wood — typically ~8%.
• Acclimatization: let wood reach equilibrium moisture content with its environment before installation; a site acclimatization period of roughly 48 hours is commonly cited.
• Seasoning methods: natural air-drying, kiln-drying, or site acclimatization to reach moisture equilibrium.
• Endgrain: absorbs moisture readily — avoid exposing it; protect with measures like metal end caps.

Confusions / comparison

	Free water	Bound water
Location	Cell cavities	Cell walls / fibers
Affects strength?	No	Yes (below FSP)
Affects dimension?	No	Yes — shrinkage source
Removed	Above FSP	Below FSP

Drying method	How	Speed
Air-drying	Natural exposure over time	Slower
Kiln-drying	Controlled heat	Faster
Site acclimatization	Reach equilibrium MC in place	Pre-install step

Related

→ Wood structure, grain, and shrinkage (this topic): the directional shrinkage that bound-water loss produces · Lumber sizing and defects (this topic): warps and checks caused by uneven drying · Wood treatment and preservatives (this topic): moisture-driven decay and where treatment is required.