This is a way interesting sauna.  The only major issue that I see with the barrel sauna is that you step right out from your ‘hot room’ to the outdoors.  If you go with one of these, and you live in a colder climate, be sure to position the barrel sauna so the door opens away from the prevailing winds.  Nothing worse than being in a 180 degree sauna, and someone opens the door to say hi, and ice cold 5 degree winter wind comes blowing on your sweaty face!  A cool look, but I prefer the conventional free standing stick frame 8×12 ’shed’ type sauna.  8×12 is an awesome dimension as:

1. 8×12 = 96 square feet, and under any building code restrictions i’ve ever heard of.  (Some municipalities require permits for structures larger than 100 or 120 square feet).
2. 8×12 allows for an ample size sauna room, say 6′x8′, as well as a 6′x8′ nice sized changing room.
3. An outbuilding can be customized to match your house: same siding, pitch, roof, even a cool out-patio for chilling out.
4. The 6′x8′ changing room can double as a home office, party space, or doghouse (figuratively) away from your primary residence.