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.


So what about tiled walls in the shower.. the water impacts directly
on the tile and grout...

Mark