Why didn’t the 85 year old sloped ceilng’s T&G sheathing, from a room I gutted show any signs of rot from moisture mitigation.

It was only insulated with balsam wool and the sloped ceiling was covered with the original 1/2 inch fiberboard which is still up on the ceiling. I enclosed a picture.

I even remember when I ripped out the balsam wool from the bays, in the unvented attic, in the late 90’s, the sheathing underneath was fine, although, the attic floor had R19 foiled faced batts.  

FYI: My house has always had an asphalt roof.

What prevented this?

Is it because it was able to dry to the inside? 

I am ready to insulate the 2×4 joists with R15 kraft faced or unfaced, depending on your ideas. 

Thanks for your thoughts in advance. 

Wayne

P.S. Getting too old to do jobs twice!