I have been looking for flooring for my fully enclosed new back porch with windows. It does not have heating or cooling other than a ceiling fan. It is 16 feet off the ground, attached to the deck and now has a plywood floor. I am looking to put down flooring that can withstand the temperature and humidity fluctuations outdoors. I have found that vinyl, linoleum, cork, bamboo and laminate are not recommended, and that the flooring adhesive also presents a problem with the temperature and humidity range. I heard about tongue and groove treated wood flooring, and was wonsering if that would have to be installed directly onto the joists(too late) or if it can be laid over the plywood subfloor.

Tongue and groove needs a subfloor anyway, so you are all set, but I have found that the new tongue and groove floors, if in fact you mean the laminate floors are very susceptible to moisture and lift at the joins. I don't care what grade it is, I have good grade and don't like what has happened in my dining room.

Why can't you use indoor/outdoor carpet which would be a lot warmer then anything else.

Ms tickle

Is your current plywood floor pressure treated? If not, nothing is going to work.

I have to do some checking, but why not porcelin, ceramic or stone?

Vinyl glued tile (12" x 12") works fine on a slab porch, so I don't see why it would not work for yours. Expansion can easily occur at the seams.

I can definitely see where a single piece floor might have problems.

Don't even consider laminate. It MUST be kept dry, One day with a window open and it rains you may have serious problems.

You said totally enclosed, if that is correct then I see no reason that you couldn't use regular 1x4 tongue and groove pine flooring. See no reason not to install over plywood sub flooring. I wouldn't nail it very tight because of the greater swings in humidity. I wouldn't use treated material unless it is subject to getting wet from rain or snow. I would also run a saw kerf down the center of the bottom side about 1/3 to 1/2 half the thickness of the board. This will allow the bottom side to move in conjunction with the top side. The top side will move faster due to changes in humidity simply because it is exposed. This should greatly reduce cupping.
I did that several years ago to a walkway built with 2x8s as planks. Have yet to see a cupped board.