4mil or 6mil vapor barrier?

[ikessky]

I'm looking to add a ceiling to my attached garage so that I can hold a little heat in when I'm working out there. Now, the open rafters that I currently have are spaced plenty far apart. In talking with my father-in-law, he said that they are plenty light and I probably shouldn't hang any drywall on them. So, my plan is to throw up a layer of vapor barrier and then hang 2" styrofoam board from the rafters (yes, I know it needs drywall to be up to code, but we are looking to remodel in the next few years, so it will all be redone anyway).
My question really is, is there any benefit to using a 6mil poly vapor barrier over a 4mil? Menards was out of the 6 when I went the other day, so I bought the 4. Price difference is only $7.

 

[Shocker]

Should be fine

 

[e-tek]

Won't make any difference here.
Good luck!

 

[walrus]

If you are going to use rigid foam, you don't need a plastic vapor barrier, the foam can be the barrier. Use caulk or canned foam to seal the cracks between the sheets.

 

[ikessky]

I figured for $30, the poly vapor barrier would provide a little more protection. Plus I can use the left over for a whole variety of other things.

I will see how this works out this year. Maybe I'll throw a layer of Reflectix over the rigid foam or something like that. My main concern is just to keep the heat from the garage from going up to the roof. The new garage will have actual trusses with drywall an fiberglass insulation in the ceiling. Maybe a couple layers so that it is super insulated and won't take much to heat it.