Insulation, Radiant and Vapor Barriers - Insulating a inside dormer wall that connects to kneewall

We have a 1 1/2 story cape cod house with a "dormer" on the back that is the upstairs bathroom. The kneewalls on each side of the house butt up to the inside walls of the dormer. One side of the dormer is insulated from the kneewall and the other side where the bathtub is located is not . . . when I look in the kneewall I can see clear to the back of the tub/shower, etc. Should we insulated behind the tub where the wall is wide open into the exposed kneewall? We don't have any mold or anything in that upstairs bathroom, but in the winter the bathtub wall is cold and in the summer it's hot . . . will putting batt insulation inbetween the wall studs call any ventilation issues in that room? Thanks for any input.