See after bullets.jklingel: This is an outside wall, R22, no vapor barrier other than what you see. The room is going to be used as a dog grooming room and will have moisture... •• OK. A vb may be helpful then. If interested, buildingscience.com has a lot of info on insulating moist rooms, like pools and weed gardens. If the strips are for noise, I don't think they'll buy you much. Can you double sheetrock it (let the fun begin....). Did you tape or goo the joints on the XPS? Is there a gap between the new wall inside it and the XPS? There should not be a gap, if possible.