We are going to have some finished heated space in our new pole barn, the two rooms will be a shop and then a studio for my wife, there is also a full bath next to the studio. The exterior of the barn is metal siding and I'm filling 2X6 cavities with kraft faced R-21 face stapled to the 2X6's. We live in Eastern Washington- wheat country, dry, hot in summer, cold in winter and not much rain, snow or humidity. I've been doing some reading about a vapor retarder in addition to the kraft paper- many sources suggest installing it- others not. It seems that a 6 mil poly barrier would limit moisture into the walls but might condense on the outside of the plastic next to the drywall and during the summer months could condense on the inside of the poly. Does the metal siding envelope have specific considerations I should be thinking about- or is this just an age old discussion that I won't find the answer to?