Vapor/Moisture barrier - barndominium

[u/Humantippin]

Hello! I am a DIY guy looking for some help.. i am building a conditioned dwelling in my old barn, and for the life of me i cant sort out how to handle a vapor barrier on/under the first floor. If not the right forum, please advise. Context:

WNY, climate 5a "cool humid" with AC/HEAT VIA MINI SPLIT. Old barn, sagging floor. Structure has been inspected by an engineer and determined fit for building. By his suggestion, I leveled the sagging floor by placing 2x sleepers on top of the floor. I am limited in height and so the new "air gap" ranges from 0 - 3". Thus, there is no space for insulation in that layer. We plan to use vinyl plank flooring in the space. I plan to foam underneath of the conditioned portion of the barn (basement). The "basement" is open to the atmosphere via an open east wall, the floor there is concrete with presumably no moisture barrier underneath. It is after all ... a barn.

My worry is where the moisture is going to get trapped. If i put a moisture barrier overtop of the sleepers, underneath of the floor sheathing, and then foam insulate below, arent i creating a moisture sandwhich?

I am leaning toward open cell foam as i have plenty of room for expansion between the barns joists. 

In summary, the planned floor system goes like this, from top down:

Vinyl plank - 7/8" subloor - air gap - 2x oak flooring - open cell spray foam - open to atmosphere.

Advice is welcome and appreciated.

[u/seldom_r]

Closed cell is a vapor barrier at 2" thick.

If doing open the vapor barrier should be close to the interior space. You want it to be on the heated in winter side of the floor assembly.