Do you happen to know a good link explaining the correct use of vapor barriers?
No, sorry. But the basic idea is very simple: when you have a surface (especially vapor barriers) cooler than the ambient air, you need to have an air gap for the surface consensation to be ventilated away. If there is no air gap on the warmer side of the vapor barrier, you will get mold on it, because moisture will condense to the cooler-than-ambient-air barrier surface; this is what the inspector didn't believe then. ("If it was so, don't you think the experts would have told us?")
I mean who could understand vapor better than a Finn?
We are quite fond of our saunas, yeah.. and when the sauna is heated to +80°C at least once per week, with indoors around +20°C, and outdoors temperatures swing between +25°C in late summer down to -45°C in some winters (although -20°C to -30°C is more typical winter temp), you do need to build the walls properly.
For what it is worth, we had a peek in the 15cm thick wall between the sauna and the bathroom, the wall also having a shower mounted on the bathroom side, roughly about thirty years after it was built. The structure was made from 2×2 pine, and it looked like new inside. The paneling on the sauna side has a ~ 1.5cm air gap, then some kind of sauna sheeting material (includes vapor barrier), then studding and rock wool, then another vapor barrier sheeting material (intended for bathroom walls that can get wet), then an air gap, and then sheeting and tiles. The two air gaps ensure that when moisture condenses on the vapor barrier, either due to temperature difference (on the sauna side) or through the tiles (bathroom side), they are soon ventilated dry.
Also, 35-year old abachi (Triplochiton scleroxylon), after at least 3000 cycles between +20°C and +80°C, is
amazing to work with. Extremely light for a hardwood, a bit soft and perhaps a little bit splintery when split, but it stays utterly stable, and cutting cross-grain it feels more like MDF than wood. (I deconstructed the sauna stepstool, and reused the parts into a handrail and as the step surfaces for a much better three-step stepstool; the benches in that sauna are a bit higher than traditional.) Anyone who wants to do some hand-tool woodworking should definitely look into heat-treated hardwoods like European aspen; they're lovely to work with, IMO.