Is a Vapor Barrier for a Subfloor with Foam Board Insulation necessary?

[melissaivysummers]

We are finally at the subfloor stage and are facing some confusion... We have filled all of the holes and primed to avoid rust. We were just about to lay out the studs for the subfloor and then fill in those areas with our foam board insulation. On top of which we will but laying the actual subfloor.

However we then discovered vapor barriers. Help me understand if these are necessary if we are using foam board insulation? If it is necessary, do you put it under the studs and insulation or over the insulation and below the subfloor sheets? Is it just to protect against condensation? Thank you for any help in advance!

[Danjo]

Not necessary. It won’t do anything except maybe hold water vapor in

[melissaivysummers]

Ok thanks! So why do people put it in their skoolies? Is it just for other types of insulation that you would want the vapor barrier?

[Danjo]

I think it only helps to trap moisture. I believe that the walls and floor should be able to breathe so evaporation can dry those enclosed spaces

[musigenesis]

The sealed and painted floor is itself a vapor barrier, so putting a flexible vapor barrier between the floor and the foam board wouldn't achieve anything.

[Jaybz]

As a general rule, you don’t want two vapor barriers in an assembly. Try to allow moisture to escape in at least one direction. Since you have a moisture barrier in the form of the metal floor as musigenesis mentioned, you really don’t want another. Even the foam breathes. Not much, but enough to make a difference.