First things first... Because steam is a highly penetrating form of moisture, any room used for steam bathing must be made to prevent moisture damage.
If steam escapes the enclosure it can cause serious structural damage - and a serious headache.
If you're building a new house or plan to, you better let your builders know to use the proper materials to steam proof your bathroom.
If you are building your own steam room in an existing house, you must make sure that the bathroom can withstand steam.
The basic principals in safe steam bath installation are to...
1) prevent the steam from escaping, 2) collect the condensed water without dripping on the bathers, and 3) allow the water to drain.
To achieve these three goals, the steam room must be completely sealed and insulated.
To prevent steam from escaping as you take a steam bath, the structural materials should be waterproof and have an underlying vapor barrier of thick plastic.
Sheetrock or plaster must be designed to withstand moisture or be covered with a waterproof finish.
The walls and ceilings of the enclosure must be covered in an impervious material like ceramic tile or glass.
All the joints and connections must be sealed with silicone to prevent any moisture from escaping.
Once the underlying structure has been prepared, the next thing to consider is the height of the ceilings.
In order to maximize steam build-up and prevent cold spots, the ceiling should be around 7 or 8 feet high.
It should also be slanted so condensation can slide down the ceiling rather than drip on the bathers. A slope of 2 inches per foot is appropriate for a steam bath.
Now, probably the most important part - installing the steam generator. Without that, where will the steam come from?