Okay... after reading all the post on backerboard, the rights, the
wrongs... I need some reassurance.

We have an 12'x16' room/add-on that was built off the back of our
house (not part of the original construction), over a well ventilated
dirt floor crawl space. The floor construction of this room is 1"
plywood on 2x8 joist, 16" centers. My wife wants me to ceramic tile
the area.

There is currently a threshold transition into the room (step down.)
To make the new tile area level with the adjacent room I need to raise
the project area about 2". My plan was to screw down exterior grade
plywood, followed by cement backerboard (screwed and glued to the
plywood 6" to 8" on center), then a waterproof membrane and finally a
layer of thinset in which I will set the tile.

I've read enough posts that seem to verify... no matter how solid I
make the subfloor, grout on an elevated subfloor will eventually
crack. One piece of advice was to screw down the plywood but only
"glue" the backerboard as a way to prevent grout cracking, however,
the current plan was suggested by several tile guys that said by
laying a membrane (tarpaper?) over the backerboard, the tile will
float as an entire unit and be less subject to cracking.

NET: I guess I'm looking for a few more "thumbs up" before I jump on
this approach. Is my plan sound or is there a better way to tackle
this job? Thanks
Don