I’m building an outdoor office for my wife.  The building is new 2×4 stud construction on a concrete pad.  The building is 20’X10′.  The botton story has an 8′ ceiling with can lights.  The top story has a 4.5′ short wall with 2×4 rafters attached to a 2×6 ridge beam with a 7/12 roof pitch-A ridge vent runs along the ridge and freize boards are installed under the eaves.  The building is clad with 7/16 plywood, tyvek paper, and hardiplank siding.  Downstairs is insulated with R15 wall batting and R38 ceiling batting covered with a vapor barrier and pine shiplap boards.  The upstairs is accessed through a fold down attic ladder and will be used as storage for sensitive papers and office equipment.

The question:
I’m wanting to finish the attic so that it looks nice and I don’t have to sweep that place between the studs.  My worry is that when I clad the walls and ceiling with 1/4″ plywood on top of insulation that it will retain moisture and ruin what will be stored in the attic.  With the venting that I have, if I clad the walls over insulation and leave channels between the rafters for airflow will the upstairs retain too much moisture?  Oh I live in Western Washington State and there are two windows 24″x36″ each upstairs on the same side of the building

Thanks for the help.