You could look up the expansion of ash with changes in MC across quarter sawn lumber and get some idea of the change knowing the changes in the RH inside your house. I am betting it's pretty small. For a little insurance, you could lay a straight edge across the to supports, mark the holes and elongate them a bit perpendicular to the grain at the top. That is, wallow the holes out. Wallow being an ancient but highly scientific woodworking term.
MC equilibrates at about 1/5 of RH.