I have a partially underground basement which has concrete walls painted with waterproof paint. I’m building 2×4 walls inside the basement that will be covered with drywall. What kind of insulation should I use, and do I need a vapor barrier? -Mike
Building codes usually require a vapor barrier (4 mil plastic sheeting) on exterior basement walls if the framing is attached to masonry or concrete surfaces, or if the wood framing butts up against the outer basement walls. You should then use fiberglass insulation that has a vapor barrier between the studs, with the vapor barrier facing in toward the living space. This will give you two vapor barriers, one before and one after the stud wall framing.
The other option is to set the 2×4 wall framing in 4” from the masonry or concrete basement walls, and use just the vapor barrier on the insulation facing toward the living space.
Also, be sure to use treated lumber for the bottom plate on the wall framing.
Good luck with your project,