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In a typical bathroom floor tile installation you have the following
sequence:

Plywood subfloor, Thin-Set, Backerboard, Thin-Set, Tile/Grout.

If the grout is not perfectly sealed and the bathroom is heavily used
(with a lot of water on the floor), where does the water stop? I know
Backerboard is water-damage-resistant, but I hear it can wick and
absorb water. What are we relying on to keep the water from soaking
the plywood? Is it just the thin-set?


Grout is not waterproof. Some water will get through it. My understanding is
that backerboard absorbs some water, which then evaporates into the wall space
and through the other side. I was told NOT to use a vapor barrier on the back of
the backerboard to avoid mildew problems. It traps the moisture in the
backerboard. I assume the same would be true for floors.

There are products like Redgard which can be applied to the backerboard to
waterproof it before applying the tile. I assume that any moisture absorbed
would then have to evaporate back through the grout.

Epoxy grout is probably a lot closer to waterproof, but more trouble to put on,
and more expensive.