Arrangement of moisture protection when warming the floor with ecowool

Hello! Unfortunately, your question is not entirely clear to us and we would be happy to help you in more detail if you would explain its essence in more detail in the comments. For example, it is not clear what kind of “moisture-proof fabric” do you mean? How do you want to attach it to the bottom of the lags and how to still go around them? What kind of "water" is your floor in the sauna or are you talking about condensate?

In general, we are waiting for details from you, but for now I can only give advice on how to do this better.

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On the old wooden floor:

A waterproofing layer is best used in the form of bituminous mastics with polymer additives. The film can create the very condensate that you probably worried about. Logs themselves, if wood is chosen (and now they are actively using the plastic version), it is also advisable to treat them with protection from decay and hydrophobicity. In places where the lag is connected to the supporting structures, it is best to still use waterproofing material, usually no one fusses - roll roofing material is sold in every hardware store. Since you chose mineral wool, be sure to install a vapor-permeable membrane before arranging the subfloor, leaving a ventilation gap with a finish of 2-3 cm.

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By ground:

1. A layer of gravel mixed with bituminous mastic is poured onto the compacted surface.
2. Spread columns under the logs
3. To the lags they knock down the whetstones under the reel or net from the bottom
4. The heater is laid
5. A vapor passage membrane is laid on top, again, with a ventilation gap. The joints of the vapor barrier should be sealed with tape.

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On a concrete base:

Logs are installed on special fasteners or brick columns. But you can also install them directly. In principle, the technology differs precisely in the initial stage of work; a waterproofing membrane with an allowance of 15-20 cm on the walls is set apart. Of course, the joints are glued with adhesive tape.

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Good luck, we are waiting for clarifications from you.