Insulating under a Converted Porch

[jlcalbre]

I am insulating under a room that was a porch and was converted into a finished space. My plan is simply to install the insulation and then close it in with plywood and caulk the edges to seal it up. Do I need to use exterior grade plywood, even though it is under the porch and has no real direct exposure to weather? Any other things to look out for? Thanks.

[marksr]

I would think most any covering would be fine. Does the ground below the old porch stay dry?