"Absolutely beautiful. We did it with tongue and groove edges and a slight bevel on the face edges."
With tongue and groove - there is always a place for the wood to move slightly - the joint is a slip joint - even hardwood basketball courts have a perimeter 1/2" to 3/4" expansion zone. Now painting the end grain will help quite a bit - as wood looses a lot more moisture through the end grain as apposed to the face and edge.
Now if anything will dry something out - it is a wood stove! That typical dry heat will suck the life out of wood! Even perfectly dried wood that had been locked in place via glue will crack under the drying load of a wood stove! BTDT - I would go through 3 gallons a day just to try to keep up back in the days of wood heat ( I loved it - wife hated it!
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My point was, any wood that is built as furniture, i.e. chest, kitchen table or china cabinet - the % humidity will not be achieved just from being cut and stored out in the ol' barn. You will not get a piece of wood to 10% humidity sitting in an uncontrolled environment. Summer humidity levels can easily reach 90% and in the winter less than 20% - now having said all that, placing the wood in an environment that IS controlled will help quite a bit.
ANYTIME you buy wood for a new hardwood floor, the first thing you do is stack it inside the heated/cooled house for at least 2 full weeks - this wood has already been kiln dried, yet must get acclimated to the humidity of the "new" house!
I have seen many of solid oak floors - that even in the summer months, the cracks in the joints are so large - pennies will stick in them! And in the winter - well ....it is worse! ALL this because some contractor wanted to push the schedule and didnt give a crap!
The grain structure of the butternut does make for a beautiful sight for sure! I bet the bathroom is off the chart based on your description
Post a picture - I would love to see it!