Prefinish the "color" on the wood (tongues at least) before you nail it up......as the wood shrinks the tongue will be exposed at different seasons and the unfinished wood will show........
The whitewash could be a transparent stain or even a wiped/thinned-down paint........
After it's hung finish staining and protect it with a coat or two of good water-based poly.....(like Polycrylic/Minwax).....Clear/does'nt turn YELLOW over white.......Fast drying too
Kitchens are known for generating lots-a-moisture (DUH!)......so yes a continuous POLY barrier is probably needed......Building Felt strips would be ineffective on the ceiling for a moisture barrier.......BUT tell us more........Is this space insulated?.....what's above?roof or another room?