Wood Moisture Shrinkage
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Wood holds water in two forms: free water in the cell cavities and bound water in the cell walls. Shrinkage only begins once all free water is gone and the wood drops below the fiber saturation point (about 28-30% moisture content), as the cell walls themselves start losing bound water and contracting.

Shrinkage is proportional to how much moisture content changes below the fiber saturation point, not the absolute humidity of the wood. Drying from 20% to 8% shrinks the wood more than drying from 15% to 12%, even though both examples lose moisture.

Longitudinal (along the grain) shrinkage is typically under 0.5% and often ignored in practice, while radial and tangential shrinkage (across the grain) can be several percent, because the cellulose microfibrils in the cell walls run mostly parallel to the grain and resist shortening in that direction.

Significantly. Species like beech and oak have high tangential shrinkage coefficients, while species like teak or mahogany shrink much less for the same moisture change, which is why shrinkage calculations need a species-specific coefficient rather than one universal number.

Wood continues to exchange moisture with the surrounding air after a project is built, so a panel built during humid summer months will shrink once winter heating dries out the room, and if the design doesn't allow that movement, cracking or joint failure follows.