Schluter® KERDI-DS WATERPROOF MEMBRANE 3'3" X 98'5" = 323 SFA regular waterproofing membrane is designed to stop liquid water. A steam room operates at sustained high humidity and elevated temperature and that's a fundamentally different challenge. Standard KERDI handles it fine for most showers, but continuous use steam rooms need KERDI DS: the same bonded membrane construction with significantly lower water vapor permeance, specifically rated for steam exposure. KERDI DS is a modified polyethylene core