Basement tiles / Decoupling mat yes or no?

Hello dear builders and experts,

my name is Fabian and I am a trained roofer myself.

Since I now want to take care of some things in my home that have nothing to do with my roof, I need your advice or the right approach from experts.

It’s about my basement. The floor slab is WU concrete with a layer of gravel underneath. On top of the floor slab there is 3.5 cm of insulation and 4 cm of screed. This has been like this since I was born, so 30 years. Construction moisture is therefore not an issue. I now plan to tile my basement. Recently, thanks to a clogged drain on the terrace, some water leaked through an electrical outlet in the basement into a basement room. The said wall is located directly below the terrace. This has made me think. In order to protect the screed and the insulation from such accidents in the future, I wanted to create a waterproof layer between the screed and the tile covering using the Schlüter uncoupling mats. Now, however, I am wondering if this is even advisable. WU concrete is water-resistant but not vapor-tight. This means in theory that water vapor can penetrate through the WU concrete to the insulation and into the screed. If sealing is now applied on the screed, the moisture would be trapped there and all of this could "drown" me. This is at least my conclusion. Since I no longer know what to do, I turn to you and hope for helpful advice.

So, seal and protect screed etc. from water from above or rather not because otherwise trapped moisture from below threatens?

Regards Fabian

Hello Fabian.
30 years ago, the topic of "Calcium sulfate screed" (CA) was actually not as well known as it is today. Back then, it was more of a rarity.
The screed in the basement floor (and CA screeds were never used there) will therefore certainly be a cement screed.
This is completely moisture-resistant. So if smaller amounts of water did indeed seep into the screed along with the insulation, well, then that would just be the case. It does not have any effect.
A DITRA uncoupling mat, however, has completely different tasks to perform. It would simply be nonsense to install one in the situation you described; here the phrase "using a sledgehammer to crack a nut" would apply.
The cement screed actually benefits from the water, as it also serves an "old gentleman" (like the 30-year-old cement screed) for subsequent hardening. That is indeed the case.
So don’t panic: if it was only one or two liters that penetrated the screed sideways, then that is little and no reason to worry or to invest money in the wrong place.
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Regards: KlaRa

Good morning KlaRa, first of all, thank you for your reply!

It is correct that the screed does not mind water. However, the insulation or the insulating effect does. Of course not with a few liters. My luck was that I noticed the incoming water quickly and was able to fix the problem quickly. But if this had happened during a vacation trip, I would have had much more water between the base slab and the screed. And without insulation drying, it would have stayed there longer. That is also why I am considering the Ditra. I am aware that it also has other functions. However, it can also be made waterproof.

So you are saying I should forgo that and lay the tiles directly on the screed? That would be about half the cost. But if a lot of water really were to get in, professional drying would have to be carried out. I am torn back and forth.

Best regards Fabiano01

Hello Fabian. You have to decide: a) are you building a house for normal living conditions or b) a building in which you want to consider all possible future damage situations that definitely do not belong to usual living? Including mudslides, earthquakes, and lightning strikes. That a domestic water damage event can occur is – as the name already suggests – an event that one does not have to expect. It thus deviates from usual usage behavior and constitutes a special situation. If water should penetrate or run under the insulation layer for whatever reason, what is the DITRA mat supposed to do in this case? Nothing. Right! The DITRA mat is an excellent system for uncoupling in an unstable screed situation. If you have concerns that the thermal insulation layer can no longer perform its function due to slight water intrusion, then forced drying with great effort must be carried out. But neither you nor anyone else can prevent, by means of DITRA, that the entire construction becomes damp or flooded in a special case of damage occurrence, which is also covered by appropriate insurance! Suggestion: Close the windows and doors in the affected room for 48 hours. Place a hygrometer on the screed surface there from the beginning. If the hygrometer shows a relative humidity above 70% after 48 hours, then think about the further drying process. If the value is below that, then lay the tiles after appropriate subfloor preparation measures and leave it to God’s will. Mnemonic: Problems are either present, then you must react accordingly, or you artificially conjure them up and thus make your life harder than necessary! ------------------- Final regards: KlaRa

Good morning KlaRa,

that sounds conclusive to me so far. I’m probably overthinking it. When I’m back from vacation, I will take another look at the issue regarding moisture. And then probably just and above all finally lay the tiles.

Thank you!

Regards Fabiano01