I'm looking to insulated the floor of my 3 season porch. Underneath I have 2x6 joists 16" on center. If I use the 3 1/2" batts there would be about a 2" gap between the insulation and floor. Is there a problem with this gap or should I just go with the 6 1/4" batts and squish them into the 5 1/2" space of the 2x6's? Menards sells the 3 1/2" batts with a foil face vapor barrier that I'm thinking would be a good idea since the porch is exposed underneath but I couldn't find anything thicker with that foil face. I guess I could go with thicker insulation and make my own vapor barrier but isn't dead air space a good insulator as well?