Renovating vaulted cellar - experiences

Hello dear home builders,

this is about an old building from 1909, clad half-timbered with a natural stone vaulted cellar, which we bought to renovate. The experts say that the cellar is okay, but I have a different gut feeling. Therefore, I would like to ask for your opinions:

The cellar (about 30m²) and the cellar air now seem relatively dry. I am currently measuring around ~6°C and ~47% humidity. The moisture meter did not register anything on the walls either. However, according to the neighbor, it was already wet there and something was repaired. Specifically, a 0.5m high "sealing basin" made of cement wall was built on the inside of one wall. However, there are still two spots on the wall facing the slope where black mold growth can be seen. (The mold samples are already in the lab) The joints there are particularly damp/crumbling and can be easily hammered out with a hammer. Obviously, something is still seeping in there. At least enough to make the wall wet/damp. The cellar has a slightly earthy/damp smell, which is not pungent but, in my opinion, should be eliminated.

The intended use of the cellar is for storage only. However, I definitely want to avoid other cellar rooms/upper floors from getting the musty smell from the vaulted cellar. In addition, I am concerned that the load-bearing capacity of the wall will be affected in the long term.

There are unfortunately lots of approaches on whether and how to tackle such a thing. Some solutions (like from Isotech) apparently rely on insulating the cellar from the inside so that moisture cannot penetrate inward. But my gut feeling tells me that this cellar (at least this side) must be sealed from the outside. So dig, seal, lay drainage. Then the mold can be removed and plastered inside, after which the hope is that it will stay dry.

[*]What is the right approach for such vaulted cellars? Is interior sealing suitable or is exterior sealing essential?
[*]Does drainage only make sense on the slope side or does it definitely have to be installed on all sides (Other walls seem to have been spared for 100 years)
[*]Is a floor seal also necessary?
[*]What costs should one realistically expect here?

Thank you very much for the inputs!

Hm, well, I can tell you that the spots shown don’t look "good" to me either. What does the lab result say?

We have a house from 1921 with a vaulted cellar, a barrel vault. Our cellar is also damp, but within the normal range for the building’s age. For example, we’ve had boxes with decorative stuff in the cellar for 3 winters, and everything’s fine. We also don’t have any mold or anything like that; our house is also situated on a slope. In front, the cellar base is only about 50cm high, and in the back it’s as high as to where a door is inside. Where the cellar walls are mostly underground, the walls are also the dampest. In recent months we have knocked off old plaster, will renew the joints soon, and then apply renovation plaster, which will be enough for us. We also have renovation plaster on the outside, which will be renewed in the next few years. In your case, you first have to clarify where the damage spots come from. Are there water pipes anywhere on the walls inside or outside? Sewage? Rainwater?
We decided against drying out the cellar because our cellar has stood for 100 years and is still fine. So we leave the cellar uninsulated inside and outside. Is your house currently heated? Ours is inhabited and therefore heated, and in the cellar we also have a radiator for severe winters like this one. We have a very constant temperature down there, about 14° year-round. I find your 6° a bit puzzling right now.
So we will not insulate the cellar because you would have to excavate around the whole house, and even then I suspect you couldn’t prevent moisture from below. We will rebuild our ground floor ceiling and insulate it, so that the living area is then insulated downwards (and in all other directions) as one unit, and the cellar remains its own thing.

I can’t say anything about the costs, but I think you realize that it won’t be cheap if you want to completely dry out and insulate. Personally, however, I am of the opinion: all or nothing. Either you go the whole way and seal, dry, and insulate everything, or you follow the original principle of the typical damp cellar for the building’s era, look only for where the damage spots come from, and then insulate only the cellar ceiling or the ground floor ceiling.

Hm, well I can tell you that the spots shown don’t look "good" to me either.

Didn't you write to me in the other thread that I shouldn't worry about the basement? :)

The lab result is still pending. Well, I think the basement is already dry in terms of air. I could treat it from the inside, meaning mechanically/chemically remove the mold, new joints, new renovation plaster. The mold seems to be superficial. It can be scraped off. But I wouldn’t feel good if I didn’t find and combat the causes of these mold spots, but only remove the symptoms. And fighting the causes sounds like a lot of money here. I talked to a few companies today. Some said that vault basements are difficult to impossible to make completely watertight. One has a special sealing technique. Let’s see what they say on site. I mean, I’m not aiming to make the room usable. But I also don’t want to constantly fight mold infestation in the basement.

We have about 14°C very consistently down there all year round. Your 6°C surprises me a bit.

Yes, ventilation windows in the basement are always open. And I measured it on a day when it was -10°C outside.

I also didn't say that you need to worry about it because of that. If the rest of the basement apparently looks fine, it can be fixed. You don't always have to paint the devil on the wall right away. I would first wait for the lab result, treat the area accordingly, and otherwise leave the basement as it is and rather insulate the living unit. I wouldn't invest money in it and risk disturbing a functioning system in the end. Never change a running system, so to speak. If the problem only exists in one corner, there is a cause for it and not everything is always expensive and certainly not everything is always necessary. I can only advise you once again to approach the matter more calmly. 95% of all homeowners would not have sent a sample, but first removed mold, re-grouted, treated, and checked whether it even comes back. I also wouldn’t ventilate the basement constantly.

Remove plaster
Get rid of mold
Observe if it returns

According to our expert, plaster is deadly

Repairing joints is relatively easy. Clean the joints, apply new mortar

Plaster yes, renovation plaster no. It is completely diffusion-open and draws moisture from the masonry, so it helps with desalination, etc. It is really good, but expensive and always needs to be renewed sometimes. That's why it's also called sacrificial plaster. It's actually not plaster in the classical sense. For small superficial problems like that (at least that's how it looks to me), it is a good option.