I have an unused sheet of Wedi Vapor 85 board (8' x 3') that is specifically for steam showers.
wedi Vapor 85 is a special variant of the wedi Building Panel and is designed for installation in steam showers and steam rooms. It will serve as a strong vapor retarder, by far exceeding the minimum requirements set by the Tile Council of North America (TCNA) for vapor retarders used in continuous use steam rooms. Additionally, it offers all the benefits of the traditional, time tested wedi Building Panel. It is light, yet strong. It adds insulation, and provides great bond for tile and other suitable surface finishes. It is a component of a fully functioning system which was developed to also vapor-proof seams, and fastener penetrations. The wedi Vapor 85 makes best use of wedi system technology taken from the wedi Building Panel, wedi Subliner Dry, wedi Subliner Dry Sealing Tape, and wedi joint sealants. Additionally, it works great with wedi Fundo or Subliner Dry on steam room floors.