Moisture smell in the basement, up to what percentage is normal?

Hello,

we have a house that is currently rented, and during a visit, we noticed that a room in the basement smelled of moisture.

The wall shows no signs of mold, and the walls do not appear wet.

It may be that the tenant never opens the window and therefore there is some moisture present.

The walls of the rooms are insulated with polystyrene.
Do you think it makes sense to remove the insulation to leave the walls exposed and avoid moisture buildup between the wall and the insulation?

I think that the sealing of the exterior walls (KSL 150 / 1.6) on the outside is no longer that good (the house is from 1976).

The property is flat and has no well or other similar installations.
There is also a 100 mm drainage around the house (possibly it needs to be cleaned).
The house has no mechanical ventilation.

What, in your opinion, would be the best way to find out where the smell is coming from?

I think that after all these years, the insulation of the exterior wall is no longer entirely in good condition. And through sufficient ventilation and removal of the internal insulation, the walls can better transport/ breathe out the moisture?

I would like to avoid major earthworks (excavating the basement at the exterior wall, etc.) around the house if possible.

Thank you

I would set up a hygrometer to measure the actual humidity (inexpensive at the hardware store) and require regular airing > In summer only at night, - or when it is warmer outside than inside the house, do not air to avoid condensation

If necessary, a small fan could also be installed to take over the ventilation. In the basement, this is probably rather unproblematic. Is there any reason to immediately paint the devil on the wall? Just because it sometimes smells damp doesn't mean the worst has to be expected—