Make sure that builder knows about ventilations. This is quite common mistake recently, and we know so many examples when they were not installed. To have two ventilation s absolutely essential — one at low level that brings fresh air in (usually under the low bench), and another, under the ceiling or on the ceiling — that releases the hot air after you complete your sauna session
Without ventilation your sauna is not breathing, and as well as you.
It is not good for your sauna as after some time cladding might start bending, the sauna will not dry out effectively, mold starts grooving, etc