Is rigid foam enough?

[LemongrassLime]

So, I'm working on my basement in zone 6 (NY) with 2X4 walls framed an inch off the concrete. I'm planning to add rigid foam with against the foundation with mineral wool in the stud bays. My question is rigid foam enough or i'm going to need a vapor barrier on the inside. I've heard mixed opinions and now i'm confused. I don't want to trap moisture but i also don't need mold. Help a diy guy

 

[WrapAndTrap]

Using 2-inch rigid foam tightly sealed against the concrete acts as an effective vapor barrier in Zone 6, so you won't need an interior vapor barrier. Additionally, mineral wool inside the studs provides excellent insulation while allowing for breathability.

What thickness and finish of foam do you have in mind?

 

[slvr22_bd]

So, I'm working on my basement in zone 6 (NY) with 2X4 walls framed an inch off the concrete. I'm planning to add rigid foam with against the foundation with mineral wool in the stud bays. My question is rigid foam enough or i'm going to need a vapor barrier on the inside. I've heard mixed opinions and now i'm confused. I don't want to trap moisture but i also don't need mold. Help a diy guy

I think this is a solid plan, and that combo of foam and mineral wool is great for a basement in our climate. To directly answer your question, as long as you're using a closed-cell foam like XPS or polyiso that's at least 1.5 inches thick and properly sealed at the seams, that will serve as your vapor barrier. You absolutely do not want to add another plastic vapor barrier on the inside of the wall. That's a classic way to trap moisture! The goal is to keep the water vapor from the house from hitting that cold concrete wall and condensing, which the foam does. The mineral wool then fills the rest of the cavity for an R-value boost while staying moisture-friendly.

 

[juggern0t]

I agree with @slvr22_bd you're on the right track with foam against the wall and mineral wool in the bays. In zone 6, rigid foam is your vapor control layer if you've got enough thickness, it keeps the warm interior air from reaching the cold concrete. Just tape and seal your foam seams tight, insulate the stud bays, and you're good without an extra vapor barrier

 

[mamamew]

You should check the specifications of your foam, as it might be sufficient on its own. It's best to avoid using a plastic vapor barrier.
Although some building codes may permit it, doing so carries a significant risk of trapping moisture unless the building has no air conditioning. A special spray-on paint is available that can be used to achieve the correct vapor permeability rating.

 

[birds.of.a.feather]

Rigid foam against the concrete is already a vapor retarder, so you usually don't want to add plastic inside or you'll trap moisture. Just seal the foam seams, insulate the stud bays, and let the wall dry to the interior