Vapor barrier along concrete wall

Question

I'm turning a 20X40 ft pig pen into a woodshop. The pen has a 4ft concrete knee wall and then 2X4 framing on top of that to 8 ft. It has a 2X6 framed ceiling. The pen is on a concrete pad and entirely contained inside, but not attached to, a 40X120ft pole barn (not insulated). I plan on just framing (2X6) inside the knee wall boarder so I can put in good insulation and because the concrete wall has some damage. We live in MN so I plan on putting a vapor barrier under the drywall of the new framing. Do I also need a vapor barrier on the concrete wall? It is totally dry and won't get wet because it's inside the pole barn but the pole barn is not insulated. I will be doing blown in insulation on the pig pen ceiling.

I'm putting a new 2x6 framing inside the footprint and will be taking part of that to 11 feet but none will be all the way to the pole-barn ceiling. There is no vapor management on the 2x4 wall but that is down to just the studs with no sheathing on either side.

That shell stands about 8 inches in from the tin of the pole-barn. I only plan on using poly on the inside of walls and ceiling. I don't see any reason that I need to put any vapor barrier anywhere above the flat ceiling of the workshop. I think it's exposed enough to the rest of the pole-barn interior to take care of any amount of humidity that we would create from heating the workshop.

Answer 1

Looking up Stearns county in IRC N1101.7, I see that your climate zone is 6A. IRC R702.7 prescribes a bunch of options for the walls. Table R702.7(2) from the linked section indicates that just kraft paper facing to the inside is sufficient, so I imagine that's what you'll implement. As long as there's no soil on the other side of your concrete wall, I don't think you need to do much thinking on whether to diverge from the code. Just the interior side's vapor barrier is sufficient, where outer vapor barrier could counter-productively trap moisture.

With the good air circulation that I imagine you get above the ceiling, there's no reason for a ceiling vapor barrier. If you can imagine someday wanting to cap the top of the ceiling with some kind of sheathing, a ceiling vapor barrier is still theoretically pointless. The ceiling vapor barrier (installed on the interior face of the ceiling) would let you use the more attainable 1/300 from IRC R806.2 instead of the more demanding 1/150, but both theoretically work for 20 ft long cavities sandwiched between 2x6s (assuming that the joists bear on top of the walls with no blocking or rim board to obstruct air flow). Realistically, though, the ceiling vapor barrier will reduce any upper sheathing exposure to moisture, so it's probably worthwhile given the minimal cost.

To allow air circulation in the ceiling to dry any moisture that gets in there, the code calls for a 1" gap between the top of insulation and the bottom of any sheathing on top of the ceiling. See IRC R806.3.