I live in NW NJ (mountainous region) where winters can go down to -10º. I am in the midst of framing an under deck (14x24)patio as a 3 season room with PS lumber & PLY. The flooring consists of concrete pavers over 6" fill (4" QP + 2" Mason Sand) and the ceiling is constructed of polycarbonate corrugated panels for rain diversion from the above deck to gutters & downspouts linked to an underground perforated pipe (6"D - 23'L) wrappd with weed fabric and sandwiched in a 2ft, 3/4" gravel bed. I have no plans to insulate the ceiling, however I may possibly install radiant electric under the pavers in the future. I do have a 4 person hot tub on the patio. My question is simply this: Since the enclosure faces North, is there any benefit to filling the 6" void in the knee walls with rigid foam (either 6" block if I can find it or a 6" build-up of smaller foam boards) in order to reduce the "wind chill" in the room? Will this create a mold growth situation? Above the knee walls will be removable screen (Spring/Summer)/ plexi (Fall/ Winter) panels. Again, goal is not to heat, but rather cut down on the effect of wind chill. Thanks in advance for your collective advice.

The inssulation will have no effect. Save your money, or put it in in anticipation of future heating. But if the room will become habitable space, it will probably require foundations extending to below the frost line for all the walls, and some sort of perimeter insulation in the foundation walls or under the floor. So, if you do heat the room in the future, you'll probably be ripping everything down and staring over anyway.

Richard, Thank you as always.