Dairylander

Here's the scenario:
The room was originally a back porch - slab on grade. It was converted to a four season room. The HO says that the previous contractor put in a system of 2x4 joists, then plywood subfloor, then carpet.
The HO wants me to install prefinished hardwoods and I'm wondering whether I should use a liquid vapor barrier or plastic sheet goods, and if you have any favorite products that would be great.
I already spoke with the flooring manufacturer and they were no help. They just said, and I quote, "Yeah, you should use a vapor barrier, but I don't know what kind."
I'm also concerned that sealing the plywood may trap the rising moisture from the concrete and rot out the sub floor. I may have to find some way to ventilate the joist space.
Please let me know what you think.
Thanks.

the big 12 inch

You will definitly need to vent the joists by drilling holes or cut notches in every one and either venting to the outside or along the walls to create airflow...still,these type of installs always have moisture issues.I would alert your client to this condition.Good luck!