another drying tidbit
Bill Tindall, E.Tn.
KD lumber has a property not often discussed. It is more stable toward moisture changes. Using some data for one wood species from the Forest Products Lab as an example, this wood will dry to 14% moisture content (MC) when the green wood is left for a long time (equilibrated) at 60 relative humidity (RH). This situation is typical for only air drying lumber.
If this wood was then dried to well below 14 MC in a kiln and then exposed back to 60 RH it would not rebound back to 14% MC. This wood would rebound to only 9% MC. It is thought that stuff in the wood that absorbs and desorbs water gets baked out in the kiln, but the actual explanation may also include chemical changes in the wood. A name for this phenomena is "hysteresis".
The bottom line is that wood that has been over dried for it final environment and then brought back up to a higher moisture content, a common practice in kiln operation, becomes less sensitive to changes in relative humidity and hence changes in dimension. Simply, it becomes more dimensionally stable, and more forgiving.
Those that have followed dimension changes of wood in their shop environment know that KD wood does not change dimension with changes in humidity as much as the tables predict. This hysteresis phenomena could be some of the reason why.