Install underfloor insulation at high humidity

Hello everyone,

we are currently installing the under-insulation for our underfloor heating made of EPS. The tack board will then be laid on top. Around the edges, edge insulation strips and then flowing screed (anhydrite).
Right now it is quite damp and cold, so we have a relatively high indoor humidity (up to 90%) and cannot ventilate because it is just as humid outside.

Now the question arises for me. When laying the tack boards, or at the latest after the screed, I have sealed the system from below. At the same time, I have trapped the moist ambient air (which is also between the EPS boards). According to my logic, this would later create the ideal climate for mold growth.

Therefore, I wanted to clarify this now, while I can still do something about it, before the underfloor heating is installed or the screed is laid on top.

Therefore the request, if anyone here can help me:
- am I thinking about this wrongly and it is not a problem after all
- or should I now use a fan heater and a construction dryer to tackle the problem promptly

Many thanks in advance!!!

You want to pour thousands of liters of water with the screed into your house, but you worry about the few liters in the air?

Keep going!

That's true. But the thousands of liters of water are afterwards in or above the screed and can then be gotten out again by ventilation or the ventilation system. The "few" liters in the air under the screed are what scare me. Because I just can't get rid of those anymore...

What alternative do you have? Do you have time to wait a few months?
In my opinion, your concern is unfounded.

I am honestly sorry, but your "fear" is based on such a twisted way of thinking that I don't even know where to start...

The air under/between your insulation boards is not hermetically sealed off from the rest of the room. Even if you now create a desert climate inside the house to lay the insulation boards, at the latest when the screed is poured, you will have a high relative humidity in your place again, and this affects the air under, in, next to, and above the screed, because the construction is not airtight, contrary to your belief.

But even if a few hydrogen guerrillas were to escape with a submarine to operate Cocainemold plantations under your screed in the future, the dew point would first have to be undershot for water to form there. Since your building becomes warmer only through the setting process of the screed (exothermic reaction) and subsequently by heating (and thus can bind more rather than less moisture in the air), this will not happen at all. And even if it did, there would simply be no nutrient base for mold under your screed, because it always needs organic material to grow. The water would dry up and that’s it. Your worries are therefore completely unfounded.

Thank you very much for your answers!

It may be that I am a bit "paranoid" by now, but I have decided to get the dehumidifier. It can't do any harm ...

I do have one question though:

- do you put a "Schrenzlage" or foil sealing between the insulation and the staple boards?