Hi:

I am in the midst of building a steamroom and have used Durock Cementboard for the walls, ceiling. The floor is mud. Would it be wide to use Kerdi over the cementboard or is this unnecessary. The cementboard is 1/2 inch and will be taped and mudded at the seams. Any advice would be appreciated.

You can use it if you want, but Schluter recommends sheetrock under Kerdi. It's cheaper and much easier to work with as well.

Either way, you don't have to treat the seams.

It sounds to me like the Durock is already installed. I'm gona guess you did not install a vapor barrier behind it, so yes you need to install a surface water and vapor barrier on all surfaces.

There's more to it than that however, so do more research.

Jaz