Can Kerdi be installed over plywood?
Also, can you put Kerdi over cement board?
First Kerdi can be installed on drywall or cement board. However if installed on drywall it is only protecting one side. Kerdi really offers very little advantage over hardiboard plus membrane.
Furthermore, can Kerdi be installed over drywall? Yes. Once you install KERDI over the drywall panels, they will be completely protected from the water and vapor in your tiled shower.
Besides, can you lay tile on plywood?
While tile can be laid on plywood, do not install tile directly onto the plywood subfloor itself. Use intervening layers of two sheets of plywood.
Do I need a vapor barrier behind Kerdi?
Kerdi is the vapor barrier, there is no need for anything behind it. Kerdi can even be installed over drywall. Cement board and grout are not water proof, if there's Kerdi no moisture is ever going to be able to get behind it. The weak points in the system are always in the corners where one wall meets another.