Should I use a vapor barrier when lining a garage with OSB?

Question

I am looking at finishing my garage because it's unusable half the year. You freeze in the winter, and you sweat just by standing out there with the door open and fans blowing. I have three exterior walls to insulate, so R-20 Insulation, and I want to finish the rafters, so R-38+ for the ceiling.

I am in Michigan, so I know I need to use a vapor retarder between the insulation and drywall. But I don't want to use drywall. I want to use OSB, mostly for the "hang anything anywhere" aspect you don't get with drywall. Plus drywall in a workspace doesn't seem like a long lasting proposition. I still have plaster and lath in the main house, so eh, what do I know.

The big orange store sells the vapor retarder for $100 a roll and I need like 5 rolls for both the 3 walls and the ceiling. I'm getting conflicting info when it comes to using a vapor retarder barrier and OSB. Some are saying you need the barrier, others are saying you don't need the barrier if you are using OSB. So which is it?

Answer 1

You absolutely want a vapor barrier behind your wall finish. There will be significant dew and frost somewhere in the insulation blanket's thickness due to the temperature gradient, and unless you want your walls to become Slushees this is critical. It doesn't matter that you're installing OSB. I have lots of it in my garage, it's not an airtight layer.

I'm confused about your cost assessment, though. You need 4 mil poly, which at 10x100 feet you'd need probably 3 rolls tops, and it's half as much as you stated at the store you referenced.