Floor leveling in an old barn

Hello,
I would appreciate a few tips for the following task:

I can set up a workshop in an old former horse stable.
Problem: The floor is extremely uneven. It was apparently concreted in stages many decades ago. Everything looks dry, but I have height differences of up to 11 cm in a base area of 4.5x9m.

What would be the best option to create a floor for a woodworking shop with the appropriate equipment?

At the moment, I can most easily imagine, if it works in the unheated area, a dry screed with filling. Alternatively, a plank floor on supporting beams with height compensation. In both cases, there should probably be a moisture barrier on the old concrete (which one?). None of this has to last for centuries, but it shouldn't be rotted next year either. Mice and the like are to be expected.

The stable is brick-built. Windows and doors are and will remain drafty. Next to the workshop there is a large central heating system, so there is some waste heat and it does not freeze inside. I do not want to install heating there; I can just dress warmly if necessary. The installation height is not a problem, as no doors open inward.

Thanks for all suggestions!

CH
[QUOTE]

I would keep it simple and sturdy. On the old floor, a foil or roofing felt or similar, joists, and on top 22mm OSB or thicker floorboards. It's not exactly small at just under 50sqm and I would rather buy myself a nice machine than make the floor too expensive. You can also color OSB if you want. I wouldn't put anything between the joists, not even granulate or similar, also because of the price and I wouldn't know what that should achieve there. Indoors, heat radiators or something that I can use selectively and purposefully for heating (above the workbench or similar), everything else would be wasted money. You can still nicely decorate it afterwards according to your mood and taste. I might even take floorboards/OSB without tongue and groove, so that I can simply lift or replace them if needed (mice and other critters).

.....referring to the simple tip of my highly esteemed former house builder, I would possibly just fill sand between the beams. This can absorb/release moisture, insulate against impact sound, and does not cost unnecessary money.

....thank you for the response, that is probably the easiest to implement. I would rather use floorboards than OSB. What would be recommended as a leveling support for the beams to keep the foil intact? With 10 cm deep hollows, it would otherwise become unstable, right?

Maybe you could post a picture so that people can better imagine it; at least that's how it is for me. Without seeing it exactly, I could imagine small sandbags or sand packed in bags. Someone once did that on our terrace instead of expensive pedestals. For this purpose, that could be sufficient, and as I said, you could always remove some of it again if needed. Depending on the ground, I could also imagine something like a gravel bed for leveling, so rather like what you do outdoors, since the whole thing is happening rustic in the barn and moisture or something like that could always be an issue.

Hello questioner. There is little to add to the answer from "Auraki11". I would definitely use a thicker PE foil (0.15mm) as a vapor barrier underneath, overlapping the sheet edges by 30cm and securing them with adhesive tape. For the support of the storage wood, I would use 3-liter freezer bags filled with a sand-cement-water mixture due to the quite large unevenness of the substrate. After two bags are well mixed by hand and secured with a rubber band against unintentional spillage, the first support beam is placed into the still plastic mass and leveled with the adjacent storage wood in two directions, longitudinally and transversely. That’s how it works. You can put sand into the compartments, but I advise against it. Sand retains moisture, from wherever it comes, and if you don’t use fire-dried sand, you already have moisture in the delivered state of the sand. This is not good for the wood construction. Better use slag or an insulation material that you have left over from a construction site (de facto a "burial" of the insulation surplus :-)) Not necessary to clarify that! Good luck: KlaRa