Stick on Vinyl tile Q's
 

My wife is remodeling her office.
It is in a 100 year old building on pier & beam.
We are located in the South Texas Coastal Plains. High humidity, high
summer heat, mild winters.
The original floor was long leafed pine with no sub floor.
At some point the prior owners "upgraded" to 9" mineral fiber tile.
It is cracked and broken in several places.
My wife wants to put down stick on vinyl tile.

Questions:
1. Should I lay a sub floor (1/4" or 1/2" ply) over the existing tile.
Or not?
2. If I need to put a vapor barrier between new floor and old tile?
Remember it's over wood on pier & beam if that even makes a difference.
some have suggested it might.
3. If I do put down a vapor barrier, what would a suitable material be
to use?
Thanks

Randy