Please help: Sunroom Flooring Recommendation, weird situation

Hi, newbie here with a perplexing situation. Have an enclosed (walls/windows) sunroom built over former screened porch. This sunroom has a wall a/c unit and a space heater, but is not temp-controlled when not in use. The sunroom rests on a vented concrete foundation, about a 1 - 1.5 feet off the ground.
The builder, however, replaced the wooden deck planks that composed the screen porch floor. Those planks are the subfloor in the sunroom, and the planks go under the walls, so I'm not sure about the wisdom/feasibility of removing them and putting plywood back in their place.
We only have 1.25 inches from the top of these planks to the bottom of the swing-in french doors.
Porcelain tile seems to require a little more vertical space than we have - once you take 0.5" - 0.75" plywood, 0.5" backerboard, then thinset and tile (5/16") , you're over your 1.25". And a number of our tile folks seemed rather skeptical and hesistant to even do it with less thick plywood and backerboard. They seemed suspicious that the planks underneath are just going to swell with summer humidity and crack the mortar.
For the 0.5" thick carpet we prefer, it is also too close once you get the plywood and good rubber padding down. And there'd be no vertical room under the door for a throw rug for foot-wiping.
Now we're looking at the wood flooring options. My understanding is that solid wood is unwise, even with a moisture barrier. The choices are then engineered wood (for its "dimensional stability") and laminates (prob. with moisture barrier), correct?
HD person says to lay down our leftover roofing felt, then plywood - screw it down well to the decking planks, then nail down engineered wood. Does that seem reasonable?
We could also go with a laminate on top of wallboard foam - we have some of that leftover. In the stores, laminates look OK but feel all wrong. I understand they're very abrasion resistant and low maintenance, which I like. I just am not sure they'll look decent. The engineered wood floors look good but seem unreasonably costly, and they probably have one refinishing in them at best.
I'd appreciate any recommendations, esp. w/r/t the thickness of plywood needed over these planks to assure a solid subfloor. Maybe we'd be better off with two 0.25" thin layers! How about moisture barriers? Is roofing felt good for that? Seems like that foamboard would be great, as it insulates as well. Obviously, it is thicker (probably 0.125" or so).
Lastly, what kind of top-flooring would you recommend for this situation - where the floor should not encounter any water, but will at times experience temperature extremes and, to the extent the moisture barrier isn't perfect, some summertime humidity and wintertime dry air as well?
Thanks very much for any advice you can offer, Russell.
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Most of your conclusions seem right to me. Tile requires a very stiff subfloor to prevent cracking. I would worry about the humidity with any of the wood floors; they formica type laminates would probably be ok, but I agree, they just don't seem quite right somehow.
What about vinyl? (sheet) If you go with the solid type it's pretty near indestructable, and will work with the space you've got. Humidity won't be a problem if you put a vapor barrier under the underlayment. Pick a nice pattern, scatter a few rugs around and you're set.
Or...use vinyl only in a section near the doors, and transition to your material of choice for the rest of the room.
All else fails, get a carpenter to trim the doors and reset the threshold.
HTH,
Paul
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Paul,
Found your reply on Google; my newsreader - well, it didn't get it. I only download 500 at a time and must have missed it. Sorry.
Thanks for your response. Sheet vinyl isn't under consideration, for reasons of aesthetics. We don't quite want the mud-room look, even if it is practical.
Seems like we might be OK wood-swelling-wise with a good vapor barrier like 15# or 30# felt. The current planking subfloor may have some flex, but it is really well screwed down (2 screws per plank per spanned joist) and isn't going to move. We may put down plywood just to smooth it out, but even that may not be entirely necessary.
For better or worse, doors are steel and non-trimmable. We could raise them but it I think we'll get by.
The laminates - yes, I agree, they are lower maintenance but they just don't have the look & feel (or value-added). We're going to decide tonight, so time to consult the spousal unit.
Thanks, Russell.
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