ᐅ Musty smell in the basement: up to what percentage humidity is considered normal?

Hello,

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

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

It is possible that the tenant never opens the window, which could lead to some moisture buildup.

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 prevent moisture buildup between the wall and the insulation?

I believe the waterproofing of the exterior walls (KSL 150 / 1.6) on the outside is no longer in good condition (the house was built in 1976).

The property is flat and does not have a well or similar installations.

There is also a 100 mm (4 inches) drainage system around the house (which may need cleaning).

The house does not have mechanical ventilation.

In your opinion, what would be the best way to identify the source of the odor?

I think the insulation of the exterior wall may no longer be in good condition after all these years. And through sufficient ventilation and removal of the internal insulation, the walls might be able to manage moisture better / “breathe” more effectively?

I would like to avoid major excavation work (digging around the basement exterior walls, etc.) if possible.

Thank you

I would set up a hygrometer to measure the actual humidity levels (inexpensive at a hardware store) and require regular ventilation > in summer, only at night, or whenever it is warmer outside than inside the house, avoid ventilation to prevent condensation.

If necessary, a small fan could also be installed to handle the ventilation.
In the basement, this is usually less problematic.

Is there any reason to expect the worst right away? Just because there is a slight damp smell doesn’t mean something serious is necessarily happening.