I live on the first floor, it's cold from below, especially in winter. The floor is constantly freezing, even warm socks don't help. I want to insulate it, but I don't know what would work better - mineral wool, styrofoam, or maybe polyurethane? And do I really need to tear up the floors, or is there a cheaper way?
The main thing is to sort out the moisture situation before you insulate anything. The previous answer correctly pointed out basement ventilation, but there's another thing to check: make sure water isn't leaking in from outside and condensation isn't building up on the pipes. If the crawl space is damp - that's a real problem, and no insulation will help, it'll only make things worse. But if it's dry and has decent ventilation, then you can start.
As for the materials themselves: mineral wool in floors is risky - it absorbs moisture quickly and loses its properties. Foam or extruded polystyrene (XPS) is way more practical for a ground floor - they don't mind moisture and keep heat stable. Polyurethane is also a good option, but pricier. If you don't want to tear up the floors, there's a workaround with removable wooden floors that have insulation - you lift them up, put the material underneath, and put them back. It costs less than a full overhaul, though it's more labor-intensive.
But honestly, if the floors are really freezing - a half-measure might not cut it. Then you'll have to remove the old flooring and do it properly: vapor barrier, insulation, moisture barrier, new floor. Sounds expensive, but you end up with comfort for years to come.
First thing, check if there are any ventilation problems in the basement or crawl space - if it's damp and the air is stagnant, no insulation will help, you'll just trap moisture under the floor. If the ventilation is fine, the easiest and cheapest way is to lay a thick underlay with a heat-reflective layer under carpets or parquet - something like foil-backed polyethylene foam - and it'll already be warmer, without ripping up the floors. If you're ready for serious renovation, mineral wool is good but needs moisture protection; foam plastic is cheaper and easier to install, polyurethane is kind of pricey for an apartment, that's more for industrial spaces. But honestly, often what really helps is just raising the floors by 5 - 10 cm on joists with insulation between them - you get good results and the work isn't as destructive as a full overhaul.
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