I live in Southwest Ohio. I have a porch that I am turning into a three seasons room with windows and screen. The floor of the porch is deck boards with gaps inbetween each. I need to lay down a sub floor so I can put down a wood or tile floor to finish the room. I need to know what is the best way to go about sealing under the subfloor from moisture and the best material to use for a sub floor. I was thinking of using plastic vapor barrier and marine plywood. Will this work? Is there a better way to go about this. The porch is about 2 ft above ground.