http://localmile.org/trumpkins-notes-on-building-a-sauna/

A Sauna, be it Finnish, Bio or Hay, is distinguished by a bather experience with the following characteristics:

  1. We Are Heated Very Evenly By A Hot Air Bath Of Soft Convective Heat That Caresses Our Entire Body; head to toes, front to back and minute to minute. Every inch of our body is enveloped evenly by this hot air – we are bathed in hot air.
  2. We Can Choose Any Temperature from 75-105°c (ideally ± 15°c) for the convective heat we feel on our bodies.
  3. We Feel No Radiant Heat from the stove. Nor any heat that is point-source, harsh, or uneven.
  4. We Have Continuous Fresh Air To Breath without high levels of CO2, VOC’s or odors.
  5. Hygienic – We are not exposed to unhealthy levels of bacteria, mold or other harmful elements.
  6. We Can Create Steam by throwing water on the stones. This steam evenly envelopes our entire body head to toes.
  7. There Is A Very Slight Bit Of Comforting Air Movement over our body descending down from the ceiling. Not too much though and almost unnoticeable.
  8. We Feel No Cool Drafts, even when someone opens the door.
  9. Humidity Is Returned To Normal after creating steam in a Finnish Sauna (so that we can do it again!).

That’s our goal, how do we achieve it?  The realities of physics limit our options somewhat – key design elements found to be most critical then are:

  1. Feet ABOVE The Stones – Finns call this ‘the first law of lóyly’ and is still one of the most critical elements of sauna design. And for good reasons. Yes, this does require about an 8.5’ high ceiling which, though common outside of the U.S., is not so common in the U.S. Ignore information that says a  7’ or 7.5’ ceiling is best. It’s not. That low of ceiling results in the dreaded ‘cold toes’ that people in Finland avoid at all costs. This generally requires three benches (typical in Finland, Sweden, and elsewhere.) or a high platform with benches. ‘Above’ is ideally by at least 20cm (8”) above. 
  2. Feet ABOVE The Cold Zone – The foot bench should be above the lower third of the volume of space (height in a typical cabin sauna). This even if the top of the stones is a lot lower. Our entire body should be in the upper 2/3’s of the space.
  3. Ventilation – 20-25 CFM per person to remove exhaled CO2 and other contaminants. Downdraft is usually best and Mechanical Downdraft for electrically heated saunas.
  4. Heater Some Distance From Bathers – Bathers should not feel any radiant heat from the stove nor any heat or steam rising in to their face. There should be sufficient heater wall to bench wall distance for a good convective loop. A minimum of 180cm (6’) is recommended though more is better.
  5. Convection Heater – The heater should produce significant convective heat and as little radiant heat as possible. Those made by companies such as Narvi, Iki, Helo and Misa are designed to do this. Harvia is a value brand that is more hit or miss but some models are good. Heavy steel stoves produce too much radiant heat, too little convective heat and do not make for good sauna stoves.
  6. A Larger Room – A typical 4-person family sauna should be about 250x250x260cm (8’w x 8’d x 8.5’h). This results in more even comfortable heat and steam, and fresher air to breath. Every cm less, even for just one person, reduces the sauna experience.  Smaller is sometimes necessary but for the best experience reduce as little as possible.
  7. Changing Area – This is critical, especially in cold climates, to provide an air-lock as well as a space to pre-warm our body before entering the hot sauna.
  8. Ceiling Slope Higher Over Benches – If the ceiling is sloped then it should be higher over the benches and lower over the stove.
  9. Bench Air Gaps – Benches should interfere with airflow as little as possible. You need sufficient air gaps in the benches, ideally a larger gap between the benches and wall, and little or no skirt.
  10. Wood – The walls, ceiling and benches should of soft wood for its hygroscopic and noise quieting properties. Spruce, pine and various other soft woods work well. Cedar and similar woods should generally be avoided.
  11. Shower – Having a shower or two adjacent to and easily accessible to the hot room is very beneficial. 
  12. Drain – A trough drain is easy to do and is very nice to have.