I am planning to use 1/4" HardieBacker with 13"x13" porcelain tiles on the plywood sub-floor of a bathroom and a contiguous toilet on the 2nd floor of a house. This floor is right next to bathtub and shower, so the environment will regularly be highly humid. Plus toilets are known to overflow once in awhile.

Therefore I am concerned about moisture potentially penetrating to the wood and to the ceiling below.

My question is: do I set the HardieBacker directly to the plywood sub-floor or do I need to use some additional form of moisture barrier? If so, which?

Thank you.

My question is: do I set the HardieBacker directly to the plywood sub-floor or do I need to use some additional form of moisture barrier? If so, which?

Hardie and all other CBU's are set into thinset mortar spread with a 1/4x1/4x1/4 notched trowel, then, fastened, taped etc. Floors are not normally waterproofed, it's almost impossible to waterproof a floor unless you build a curb in the doorway and also seal every hole including around the toilet flange. Putting a membrane under would be almost useless since it would be compromised when fastened.

If you want to add a level of waterproofing anyway, you can apply one on top of any CBU, but you'll also need to figure out a way to waterproof up the wall at least a few inches. The best way to do this is to use Ditra instead of a CBU, then flash up the walls with Kerdi Band. Of course it's still not waterproof because of the doorways and toilet flange, but it's somewhat better.

Ask for more details depending on which way you decide to go.

Jaz