tile subfloor question
I have 2x10 joists @ 16" OC. My original subflooring is 1x12 planks laid perpendicular to the joists.
I'm tiling two different spaces--
Bathroom, approx. 4'x8' of tile, all of the subfloor except for about 5-6 sqft at the doorway has been replaced by 3/4" 'dryply' because of a moisture leak. I'm using 6"x6" porcelain tiles.
Entryway, approx. 4'x10' including a small closet, the subfloor is the original 1x12 plank, the orientation of the planks/joists is such that the planks run the "length" (10') of the space and the joists run the "width" (4') of the space. I'm using 12"x12" porcelain tiles.
I'm trying to match up (height) as best I can to strip hardwood laid directly onto the subfloor planks in adjacent hallways
My questions are regarding the best setup to put on top of the existing sub-floors--
1) 3/8" ply + 1/2" hardi?
2) 1/2" ply + 1/4" hardi?
I figure both of those will end up at approx. 7/8" thickness when including some space for thinset (and the fact that 1/2" hardi isn't actually 1/2")
Also, what type of ply is best? Since I have 1/2" hardi already, I decided to buy some 3/8" ply, I went to the orange big box, explained it is going under cement board for tile, and was pointed to some Georgia Pacific product marked simply '3/8" Rated Sheathing', at the time I was in a bit of a rush because I was approaching the cut off for renting their van for the day, when I got home I noticed that the sheets seem to "crackle" and flex an awful lot, they are also somewhat warped, but do lay flat with weight on them. I couldn't find the stuff on their website to verify the grade, however when I went to the blue big box site I found something that appears to be basically the same and it's rated "D-D". Is there some way to figure it out by looking at the sheets? I didn't see any obvious marks that would lead me to the grade. Also, I assume D-D grade is not OK? I would want A-C or something similar?
My plan is to screw the ply I put down to the existing subfloor (not the joists), then lay down thinset and hardi on top (with backer board screws where indicated).
My GC for the kitchen "glued and screwed" 1/2" hardi directly to the subfloor planks... at the time I had an inkling it might not be the best, but I assumed he knew what to do... since then what I've heard and read leads me to believe what he did was wrong.
I'm behind on this project which is what lead to me buying the materials when I had time and access to the van, but I'd rather be late then do it wrong... I don't mind eating the costs for any material I can't return, I just want to do things as correctly as possible :)