Does the fresh air intake for the sauna and the intake under the stove both draw air from the change room? Also the Nippa heater? I assume it draws air from the change room to feed the fire? Fire can consume a lot of air also. From strictly HVAC point of view that room (change room) may get cold in the winter. If you keep drawing air from that room. Where will the air come from to replace it? Just a thought. I see your putting a lot of effort into your build, I cannot see everything, but I live in Wisconsin, It reaches -15/-20 deg F in winter. Cold air penetration always worries me. Look forward to seeing the sauna completed. I Just started My project.

I have that book😁

Propane or natural gas is not very common, but you are getting the flame like a wood burner but you don't have the cleanup which is nice. I was thinking about going that route but wanted to avoid the chimney going out my sauna wall/ceiling. Personally I think chopping wood is over rated because I'm lazy. With a capital L Running electrical is doable for many people but gas line can be tricky and I thing people are intimidated by gas. I do commercial HVAC so I had both options available to me. You could also do propane at a remote sauna with no electrical, but you would be swapping out propane tanks when they run out.

I like your layout and you put the bench on the high side. I see so many saunas where the benches are on the downward slope area of the ceiling. (the short side) Everything looks good. The only small input I would make is if your ceiling is 8 feet tall? Can you raise all of your benches a little more? Maybe when you come in the door have a step up to get onto the first bench. ( I have 2 steps) If the back wall is 96" tall, I would add a step. Say 10 inches. (I have 2, 8" steps) Then your feet would be very close to the height of the sauna heater. about 28 inches off the floor. With that high ceiling you will have the perfect steam pocket. Might ruin your view out the window though? I Like the large change room also, Very nice setup in all

This is just an example of the sauna foundation that I built. I dug a hole 1-1/2 feet deep and added 1 foot of crushed rock. Compacted the rock and laid 6 x 6 structural beams on top of the rock and leveled everything out. The sauna structure itself will be placed on top of these beams. Drainage and electrical will be run between the beams and once construction is complete rock will be filled in all Open spaces. This is Just an example of what I am doing. The structure I am building is 7‘ x 11‘

@Robert Haas I think that’s one of the best ways to do it. Concrete is too expensive and I’m getting too old to work concrete myself.

I hate to be the bearer of bad news. I have been waiting for 6 weeks. I'm no help.😧 My shipment is coming out of Canada