Vapor barrier for enclosed porch

I am in the process of enclosing a large roofed porch at our home in northern PA, using large insulated glass units installed between existing 6x6 posts. I plan to lay half-inch OSB sheets on top of existing decking, with vinyl flooring on top of that. The porch sits about 3 feet above a dirt surface. The enclosed porch will not be heated but will be used much of the year as a sunroom and will likely experience major temperature swings and possible condensation challenges. Should I place plastic sheeting between the existing decking and the OSB subflooring? Should I also place plastic sheeting on the ground beneath the porch? Any other advice regarding moisture/condensation?

 

Plastic should go on the ground only, assuming this room has foundation walls that the poly will wrap up and seal to.. If you want to insulate the floor it might help some... you can put 1X2 cleats on the sides of the joists (near the bottom) and then drop in plywood rips that are 14 3/8" or so... glue and staple to the cleats. Then fill the floor with foam and/or the insulation of your choice. Then lastly, apply your subfloor. Insulating the ceiling will help keep heat in when it's cool and slow down the heat radiating from the roof from making it unbearable. Be sure you add adequate roof ventilation. And I would suggest a wall unit if you can afford one. (Both heat and AC, like what you would see in most motel rooms). It will make all the difference in it being a usable room for ALL 3 seasons... not just parts of them.

I hope your insulated glass has a frame of its own. Depending on which direction it faces, the room could be too hot in the summer, tinting and ventilation would help. Or if you are worried the room may be too cold you probably want solar gain glass. Getting the right type of low e coating will help one way or another depending on your needs.