I'm getting started on a bathroom remodel, going to make a "wet room" with Redguard waterproofing membrane painted up the walls and floor to "tank" the entire room. I've also settled on using a Urethane-based grout, a 1-part premixed grout that is similar to Epoxy in that it is water-impermeable and needs little to no maintenance.
However, I'm concerned that by using an impermeable grout and a waterproof membrane behind it, I'm creating an environment where moisture could remain trapped indefinitely between the two with no way to escape. Is this a valid concern? If so, is there a way to mitigate the issue without sacrificing the benefits that a urethane or epoxy grout provides?