Subsequent basement floor slab with insulation

Hello everyone,

I am not very familiar with the topic and hope someone can help me.

I bought my grandparents' house, which was built around 1950.

The floor as well as the lower 60 cm of the walls have become wet due to all the rain, and the basement is starting to mold.

The basement walls rest on strip foundations, and the basement floor is simple screed on bare ground.

My plan now is to remove the screed, gravel the floor, lay a sheet against moisture, pour concrete with steel mesh mats, and then apply screed.

However, I have seen that sometimes polystyrene is laid between the concrete and screed; does that make sense?
Also, I would like to know what thickness I should get for the steel mesh mats.

Thank you in advance.

Best regards.

Why do you want both concrete and screed? Unless a combination of high loads and a perfect floor for laying complicated coverings is required (probably not in the basement), you can achieve a good surface with (trowel)smoothed concrete.

It would be conceivable to also place Styrodur / XPS insulation under the concrete, but whether this makes sense depends on whether and how the basement walls can/should be insulated.

But I doubt whether your project fundamentally solves the cause of the moisture. Normally, you would also have to include the walls in your waterproofing.

Why do you want both concrete and screed? Unless a combination of high loads as well as a perfect floor for laying complicated coverings is required (probably not in the basement), you can achieve a good surface with (trowel-)smoothed concrete.

It would be conceivable to lay Styrodur/XPS insulation also under the concrete, but whether this makes sense depends on whether and how the basement walls can/should be insulated.

But I doubt whether your project fundamentally solves the cause of the moisture. Normally, you would also have to include the walls in your waterproofing.

Thank you very much in advance for the information, I am a layman in this field and had only read that there are people who apply screed on top of the concrete, but for me it would not matter as I only want to use the basement rooms for storage.

Does this mean Styrodur/XPS insulation first on the gravel and then the concrete on top? Does the concrete need to be reinforced at the strip foundations? Since it actually doesn't carry any load? I also read that a foil is sometimes laid between the concrete and the wall, but I have not yet understood the purpose of this. Does the concrete necessarily need steel mesh mats? Since the floor slab is theoretically not directly connected to the house/basement.

After the floor, the walls are to be sealed on the outside and a drainage still needs to be installed. I had looked into sealing from the inside using a low-pressure method, but that exceeds my budget.

Screed on concrete (usually with insulation & underfloor heating in between) is of course now standard, but probably unnecessary for you since the basement is presumably unheated (?). If you skip the screed, you could possibly also gain ceiling height (depending). Screed would be recommended if you want/need to lay a waterproofing membrane on your concrete slab – then you could lead this waterproofing membrane up the walls (ideally 30 cm +, if no connection to a cavity wall membrane is possible) and cover it with the screed. Regarding waterproofing, I would definitely recommend consulting specialists; a lot can go wrong in old buildings.

Whether you need to reinforce the concrete can only be reasonably answered by a structural engineer. What loads do you plan to place on it? Are you planning to erect interior walls on it? And so on – I would, even if it costs a few hundred euros, maybe have a professional take a look here first before you start. And keep in mind that any reinforcement must be executed carefully, especially regarding ensuring proper concrete coverage. The concrete slab must definitely be decoupled from the strip foundation. You can of course also use something like fiber concrete, but someone has to plan it and it can of course only be delivered from the concrete plant (how do you actually want to apply the concrete down there?).

Insulation yes/no/where is a question of the overall concept – for example, is the basement ceiling insulated and everything below unheated? Then additional insulation brings rather few benefits. Or do you want to insulate the basement walls on the inside? Then it makes more sense to have the insulation above the slab and connect it to the interior insulation (then screed on top). Or do you plan, when waterproofing the basement walls (exposing them?), to also insulate them from the outside? Then insulation under the slab would be highly advisable. Again: if you want to work sensibly, you should possibly have everything planned first. There is also plenty of funding available for this, maybe you can combine everything?

Important: In any case, speak with a structural engineer beforehand. The word "schottern" sounds like you want to dig out additional material after removing the screed to add gravel. If you dig incorrectly, the house will literally fall on your head, so clarify the procedure exactly beforehand. If you do not include the walls in your concept, the whole thing will be quite useless in terms of moisture and mold; however, renovating the walls can become very costly.