Steam packs a lot of energy that penetrates a lot of surfaces. After losing the energy as its temperature is cooled down, it condenses back to its normal liquid form and can be absorbed by non-moisture resistant materials. The result is usually gradual deterioration which makes it a liability. Internal infrastructure and fixtures of steam showers must be steam-proofed in order to prolong its lifespan. To protect the wood framing, cover with 6mm plastic and staple over studs and joints. All edges must be overlapped by at least 1 foot. Seams for concrete panels must be sealed with mesh tape and thinset. All areas of backer boards must be covered with waterproof membrane. You can apply two coats of liquid polymers which dries into a rubber skin. Use tiles and grout as much as possible. If you choose to have stone walling, you need to seal the stone with an impregnator and apply every two years. All fixtures must be sealed with gaskets and o-rings like steam heads and valves. Remember to slope the ceiling so as condensed steam will not drip on your skin.