Thanks Dana. Am glad you replied as I've been following this forum for some time and you seem quite knowledgeable when it comes to insulation and vapor barriers, etc. Just a bit of clarity if you don't mind. From your above, I should put the 2" XPS directly on the lattice, then a 6-mil vapor retarder (sealed), then the plywood through screwed to the vertical structural members, correct? I like the "rat-slab" idea too. Should this be done before or after the walls? I could embed the XPS into the perimenter of the rat-slab couldn't I ? At present the "floor" under the room is dug out about 3 or 4 inches, then geo-textile fabric and then 3 or 4 inches of gravel. From your advice I assume I should remove the gravel and geo-tex, then put down the new 6 mil poly vapor retarder and then the 2" concrete rat-slab. Can I re-use some of the gravel with the concrete or would it be over-kill? BTW the floor space is 12'x 14', the size of the floor of the Florida room. Looks like a few days of hard work over my summer home leave. Again thanks for your advice and I look forward to anything further. Cheers and best regards, Rughead.
The XPS is best applied to something with structure to it- the lattice itself may not be rigid enough- it may need at least a header & footer board to glue & screw it to, as well as whatever posts are holding up the lattice. Dual or triple layers of 1" 4x8' or 4x9' XPS sheathing willl be better able to span between structural elements than the 2" 2x8' or 2x10' t&g stuff, so you may find a way to make it reasonably easy to span between structural elements on the exterior layer. Lapping the layers & bonding them with -sized blobs of foam board cement on an 18" grid between the layers, will make the foam much less flexy for when you put the plywood up. Then, walnut-sized blobs on a similar grid between the plywood & foam gives the final assembly substantial structural rigidity. The plywood still needs to be through-screwed to the structural framing to make it code-legal as a thermal barrier though.
You don't need a poly vapor retarder on the XPS, but DO seal the vapor retarder on the ground to the XPS: Apply a bead of foam-board adhesive or caulk applied to the XPS, then staple the poly to the XPS with 1/2" or longer staples, then apply 1/8-1/4" of duct mastic (paintbrush or putty knife over that to seal the edge completely. (You don't want ground moisture finding it's way up to rot the plywood at the transition line.)
Yes, pull out the gravel, but you can put the poly on top of the geo-tex, then seal it to the XPS before putting up the plywood. The rat slab can poured after the plywood goes in, and should come up to at least an inch below the plywood unless you're planning to put up wooden kick board after the rat slab is set.
If you're mixing it yourself you can re-use the gravel as aggretate in the concrete. If it's crushed rock rather than rifer-rock/screenings it has some potential for penetrating the poly during installation, but the risk isn't high. A higher ratio of aggregate to portland cement also weakens the concrete, but it's a non-strucural slab- it's OK for it not to meet any floor-loading standard- it just has to hang together enough to keep rats, squirrels raccoons, skunks, possums, etc from digging their way in.