Point of fibre repletion = status of humidity where only the membrane of a cell is filled with humidity but the holes do not contain any water. Wood swells if it delivers humidity below this point of fibre repletion to its environment.

If the humidity of wood does not correspond with the air humidity wood gives moisture to the environment or wood absorbs moisture. There dominates a decline of moisture between timber and the environment.

Example 1: Decline of moisture from wood to air

Actual humidity of wood 30 %, at a temperature of 20°C and a relative air moisture of 70 % the humidity of wood for balancing is 13,2 %. 

Result: Swelling

Example 2: Decline of moisture from air to wood

Result: Shrinkage

Example 3: Balance of moisture between wood and air

Actual humidity of wood 16%, at a temperature of 20°C and a relative air moisture of 80 % the humidity of wood for balancing is although 16 %.