Moisture & Wood

Moisture in wood exists in two forms:
  1. Free water, liquid filling the wood cell cavities
  2. Bound water, liquid or vapor chemically bound by hydrogen bonding to the cellulose of the wood cell walls

As wood dries, the free water in the cell cavities is drawn away first. Once the free water is removed, the bound water is gradually released from the cell walls.

 
FIBER SATURATION POINT

          Fiber saturation point (FSP): The moisture content at which all of the free water is removed - the cell cavities are empty - but the cell walls are still completely saturated.

          This is a key concept in wood design since moisture affects the physical and mechanical properties of wood differently depending on whether the MC% is above or below the FSP.

  • MC% above FSP: physical and mechanical properties of wood remain constant as MC% changes
  • MC% below FSP: physical and mechanical properties of wood change as MC% changes

          The FSP varies for different species of wood, but is typically around 30%. Table M-1 lists FSP values for various wood species. The rate of change of physical properties is also dependent on wood species. 

Table M-1: Fiber Saturation Point
At Room Temperature

Species

FSP(%)

Ash, white

24.0

Birch, yellow

27.0

Douglas fir

26.0

Hemlock, western

28.0

Larch, western

28.0

Pine, loblolly

21.0

Pine, longleaf

25.5

Pine, red

24.0

Spruce, red

27.0

Spruce, Sitka

28.5

 The following demonstration is based on the properties of Douglas fir.

  • Demo 1: Wood Cell Moisture Content

 
SHRINKAGE

          As the above example demonstrates, wood changes dimensionally as it gains and/or loses moisture at levels below the FSP. It swells as moisture content increases and shrinks as moisture content decreases. This is because below the FSP all moisture is bonded to the cell walls, which act like sponges, swelling when water is added and shrinking as they dry.

          The anisotropic nature of wood causes it to shrink at different rates along each of its three principal axes.

  • The greatest amount of shrinkage occurs in the direction of the annual growth rings (tangentially).
  • Shrinkage across the annual growth rings (radial) is approximately half as much as the tangential shrinkage.
  • Shrinkage along the grain (longitudinal) is very slight, only about 0.1-0.2% from FSP to oven dry condition.

          The inequality of shrinkage along the three axes causes wood to deform as it dries. The amount and type of deformation of a piece of lumber varies depending on the orientation of the annual rings.  The first demonstration showed shrinkage at the micro level, the following demonstration illustrates the effects of shrinkage macro level.

Demo 2: Effects of Shrinkage on Cross Sections  

  • Lumber Deformation. Shrinkage distortion typical for three cuts of lumber is illustrated in this log cross section. Note the curvature of the annual rings in each cut and how that affects the distorted shape.

          Drying causes not only cross sectional distortion, but may also result in warping along the length of lumber. Various types of warp are due to discontinuities such as knots as well as the annual ring orientations.