Do I need a vapor barrier for spray foam between studs in old house?

[Marney]

I'm planning to use closed cell spray foam between the studs. The walls are original wood siding over sheathing and no existing vapor barrier that I can see. My contractor says closed cell foam acts as its own vapor barrier so I don't need to add anything else before drywalling. But I've read conflicting info online saying you should still put up poly sheeting just to be safe, especially in older homes.

Does the foam really handle moisture control on its own? and if it does, is there any downside to adding a vapor barrier anyway, or does that create a double barrier situation that causes more problems than it solves?
I'm in northern Illinois if climate zone matters for this

 

[coldbr3w]

@Marney nope, you don't need a vapor barrier over closed cell spray foam because it is impermeable and acts as its own vapor barrier
adding a separate vapor barrier would create a double barrier situation, which can trap moisture and cause rot, especially in older homes

 

[Marney]

Hi, that makes sense, thanks for clearing that up. I was wondering about that double barrier situation because it sounds like it could create more problems
So with older homes specifically, is the concern more about trapping existing moisture that's already in the walls, or is it about moisture that could get in from either side? trying to understand the mechanics of why it's worse in older houses then the newer constructions

 

[Alan]

In older homes the main concern is trapping moisture already in the walls. Adding a second vapor barrier over closed cell foam can trap any new moisture and lead to rot or mold, so it is safer to let the foam act as its own barrier.

 

[Vertesol Eustache]

I'm planning to use closed cell spray foam between the studs. The walls are original wood siding over sheathing and no existing vapor barrier that I can see. My contractor says closed cell foam acts as its own vapor barrier so I don't need to add anything else before drywalling. But I've read conflicting info online saying you should still put up poly sheeting just to be safe, especially in older homes.

Does the foam really handle moisture control on its own? and if it does, is there any downside to adding a vapor barrier anyway, or does that create a double barrier situation that causes more problems than it solves?
I'm in northern Illinois if climate zone matters for this

Your contractor is right
Closed-cell spray foam already acts as a vapor barrier, so you don’t need poly, and adding it can actually create a double vapor barrier that traps moisture.