ᐅ Floor installer refuses to proceed due to residual moisture in the subfloor.

KlaRa schrieb:

Hello "pim1985".
In this case, the answer is not straightforward.
Objectively, the current state of the art is as "i_b_n_a_n" described. According to the rules for floor layers, this describes a residual moisture value of 0.5 CM-% listed in the current DIN 18365 for underfloor heating and calcium sulfate screeds.
However, this is opposed by the opinion of parquet installers (and professional associations). At the moment, there are actually two opposing groups in dispute, with "the power" seemingly influenced by industry lobbyists who are part of the DIN committees.
In fact, the value of 0.5 CM-% has been considered the state of the art without issues in Switzerland for years.
Back in Germany, the parquet installers’ "fear" of being held liable for moisture damage is, in my personal opinion, quite understandable.
Moreover, the sample extraction from the screed cross-section is supposed to be taken only from the upper third by floor layers, whereas parquet installers prefer to take their samples from the entire cross-section to determine the residual moisture content.
So, it is not easy to clearly answer your question based solely on technical standards.
And the DIN?
Well, it is assumed that a DIN standard accurately describes the state of the art at the time of its publication.
However, if you deal professionally with construction defects and standards, this is a misconception.
Also, the suggestion in one of the follow-up posts to your question, "pim1985", to apply a moisture barrier based on synthetic resin is a recommended approach I strongly advise against!
This method, which may or may not work in practice with cement screeds depending on the manufacturer, must absolutely not be used with calcium sulfate screeds! Entrapped moisture, whether as elevated residual moisture or moisture migrating later, is "fatal" to the moisture-sensitive calcium sulfate screed because elevated moisture leads to decalcification. The result: the screed becomes at least soft at the interface of the barrier.
So, what to do in this described situation?
On the one hand, there is the builder’s pressure to move in, on the other hand, the parquet installer’s understandable concern about flooring damage. Personally, I would take three samples across the entire screed cross-section at different sampling points, immediately seal them in double-layer, resealable polyethylene bags, and dry them as soon as possible using the gravimetric method at 45°C (113°F) until there is no change in weight.
This method is admittedly uncommon in practice. But it is used by flooring technology experts when clear results are needed.
Since both methods, the gravimetric and the CM method, provide values with different units (mass-% and CM-%), but with almost no difference up to 0.5, the result can be accepted as ready for installation if the sample from the entire cross-section is below 0.5. Because the gravimetric method in the drying oven yields slightly higher (correct) values, providing a safety margin.
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I further suggest that you print out this article and hand it to the site manager, who is certainly correct in his view, as well as to the parquet installer, who holds an understandable though conflicting opinion.
After all, it is not important who held which opinion when, but that no moisture-related flooring damage occurs later!!
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Good luck: KlaRa