denshield ok if Ditra's not available?
I am not sure how your joists and the entire subfloor system is now built. Did you say your 2x6 joists span about 12'? If so, that is way too much.
If you can't find Ditra, I recommend either Hardie or Durock. You would use a modified thinset for any of the three CBU's, both to install it and to install tiles over it. Mapei is a world-class company with great products. You shouldn't need any waterproofing for a bathroom floor. But if you were to use one, it would go over the CBU, not under it.
Tile 4 You LLC Troy, MI
KERDI Shower Specialist ... DITRA Installs ... Product-Method suitability consultation. I have NEVER made a mistake, I thought I did once...........but I was wrong! - PRODUCTS ADVERTISED WITHIN MY POST ARE NOT ENDORSED BY ME!