Originally Posted by Ned Milburn

One of our regular posters, "redir", recently posted a good explanation. The moisture held by the cells is cellular moisture, and this is likely the 6% that your are hearing. Relative humidity is absorbed and dispelled by the "spongy" fibres of the wood, rather than in and out of the cellular walls.

So, regardless of the RH of the air (to which the wood will reach equilibrium in relatively short amount of time), the cellular moisture content of the wood should be low before building. RH for building instruments is commonly near 40%, with some building in more or less humid environments depending upon the target market location.