Wood cultivation - cold bridge floor - damp floor during sauna operation

Hello,
we have had a wooden extension built on the terraces at the rear of the building.
For this, point foundations for the wooden posts were concreted, side walls insulated and paneled, beams attached to the house facade for roof pitch. The terrace slabs are laid on frost protection and gravel. We have integrated a sauna, and during the first heating, we have now noticed that moisture collects on the outer terrace slabs at the exterior wall outside in the interior space, probably due to the lack of insulation outward through the floor slabs. The floor slabs run from the interior space through to the outside. When airing, the slabs do dry again. The entire wooden wall has about 1 - 1.5 cm clearance from the floor; underneath, it was sprayed with foam. Basically, the question is whether this is harmful to the timber structure and whether the terrace slabs should be removed and insulation installed or if this issue is harmless.

I would appreciate feedback,
Thank you.

Well observed: missing insulation and probably inadequate airtightness with convection of the very warm/humid air into the wooden structure. This can lead to moisture accumulation, mold growth, and deterioration of the wooden components in the long term, e.g., between the exterior wall and [Brunnenschaum]. Wooden components can get wet but should be constructed in such a way that they can dry out easily. Moisture buildup must be avoided in any case. Allow sufficient air circulation on all sides.

Remove the terrace slabs along the exterior wall, excavate, create a concrete foundation under the wooden wall, and seal it from the outside with Styrodur boards and bitumen. Can I largely get the problem under control this way? What do you think?

A sauna always has a temperature just above 20° at the bottom. Under the given circumstances, it must have exhaust air openings.

If it is used infrequently (private), it may be sufficient to simply not connect the floor flush; it must be able to draw air. So no foam, no silicone, no styrofoam.

If it is used more frequently, it must be ventilated forcibly with a small duct fan. The slope of the duct must necessarily decline toward the outside, and the duct should be insulated. Otherwise, the condensation water will flow toward the wood. Where there is exhaust air, there must also be intake air. This does not require a motor.

Connecting it firmly to the floor is not a good idea for saunas; this is only done (for liability reasons) if it must be secured, for example.

The uninsulated terrace slabs as a base remain cold bridges where condensation must occur. However, the wood, especially slow-grown Nordic wood, can handle this if the sauna is allowed to breathe and the bottom is not somehow "glued" on.

Enjoy your sauna experience!

fach1werk