ᐅ Installing Vinyl Flooring Over Anhydrite Screed at 50-70 Digits?

There are empirical values available, which are reflected in the applicable DIN standards.
Anhydrite screed used as a heated screed can be installed at residual moisture levels of 0.3% or 0.5% measured by the CM method.
This is also stated in the instructions for your floor covering, where the manufacturer refers to the BEB guideline “Assessing and Preparing Substrates, Installing Elastic and Textile Coverings.”
Digital moisture meters are only suitable for preparing a CM measurement.
Hopefully, @KlaRa will see this post and add some input.

T_im_Norden schrieb:

Hopefully @KlaRa will see this post and add some comments.

(... they already see it ....)
Absolutely correct, what "T_im_Norden" wrote!
If a calcium sulfate screed (CaF) used as a heated floor screed is not dry after several weeks, the problem lies elsewhere, not with the screed itself!
However, if I consider the measured relative indoor humidity of 15%, well, then I wonder what value one would still need to reach to realize that it cannot get any drier?
Screeds release their mixing water into the indoor air.
And if the indoor air is dry, it is a reasonable assumption that this is due to components that no longer release moisture into the air.
Whether we install a "full PVC" (this term does not exist) or a synthetic rubber covering on a too moist substrate: sooner or later, bubbles will form in the covering layer. The thickness or composition of an elastic covering has no influence on whether the covering will stay in place or lift due to excessive residual moisture in the substrate!
If measuring points in the heated screed are documented, a calcium carbide (CM) residual moisture measurement must be done shortly before the covering is installed.
If no measuring points exist, it gets interesting. Because then the installer raises concerns, and the building owner must decide whether to accept the risk.
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Awaiting the upcoming results with interest:
KlaRa

Hi @KlaRa,

so, a measurement of the wooden walls showed that they are very dry, partly below 15 digits.

The strange thing is, under the stairs for example, where we couldn’t sand, there is no sunlight, no drafts, etc., we have 40 digits... so much for that. In the hallway, directly adjacent to the living room, there are partly only 30 digits... someone needs to explain that to me. And in the living room, where sun constantly comes in and there are drafts from quick ventilation, we are stuck at 50–70 digits...

Yesterday I performed a foil test, covering 1 sqm (10.8 sq ft) for 24 hours in the living room area where we had about 50–60 digits, and no moisture was found under the foil.

In the front area, where we have about 70 digits, there were about 4 small water droplets visible...

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Thanks for your opinion, but I’m not any wiser now. A CM measurement can no longer be performed since the surface has already been skim coated (see picture).

And as I said, the room air is very dry, so I don’t understand why the screed still shows such values and why they don’t go away... my heating program has been running continuously since around February 24 (worried about the electricity bill 😱 – LWZ 5S Plus by Stiebel Eltron). Since then, we’ve had a constant room temperature of about 29°C (84°F).

What do you personally expect in this situation I described? Should I expect something (mold developing, discoloration of the floor, or something else)?

Sorry, I’m not an expert and just trying to make sense of it...

I’d just like to know if I can sleep peacefully over the next few nights without having to worry that, say, in three months’ time I’ll have damage in the living room and in the worst case have to replace the floor.

Thanks again!

@ "Arzok90":
Can you tell us from which "digit size" something is damp, less damp, dry, or completely wet?
No, there are no exact values that can be used for that.
The "digits" only indicate anomalies in a building.
And if 70 digits appear in a corner of a room, that would be a reason for me to open it up and check.
With a high probability, I would then recognize that the screed thickness is greater there than in other areas (which show lower values).
The foil test was already a good testing method!
Do it again in the middle of the room. Place a hygrometer under the foil, which is sealed at the edges with tape (stuck to the screed). Well, there are no generally accepted limit values for this method either, but if the relative humidity is around 70% RH, everything should be "in the green zone".
Before this test, however:
Turn off the heating!
Wait until the screed surface has cooled, open windows wide for extended ventilation.
Wait 3 days, heat up the floor again, immediately to "full power," and keep it for 2 days.
Cool down again and install the floor at room temperature.
PS: Who says you can't perform a CM moisture content measurement after the surface has already been leveled???
Everything is possible!
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Good luck: KlaRa

During the drying process, according to the construction management’s assumption, there was a phase separation. On the surface, nothing could be detected anymore (plastic sheet test), but the calcium carbide measurement was inaccurate. Only patience and the approach described by Klara helped.
Ultimately, due to time pressure, we proceeded with installation at a slightly higher reading. The tile installer used a special adhesive approved for this purpose. The parquet installer sealed the screed. So far, everything has gone well.