Humidity in basement apartment

In addition to insufficient ventilation or heating, the cause of such moisture damage is, for example, the lack of building insulation, which leads to the condensation of warm humid indoor air on cold components (thermal bridges).

Or rising ground moisture due to a missing horizontal barrier in the masonry or missing or defective structural waterproofing of the floor slab. Or the missing or defective vertical structural waterproofing of the exterior masonry in the case of pressing or occasionally pressing water. Or leaking/defective downpipes or drainage pipes, which can lead to apparently inexplicable localized moisture exposure at individual points.

Before deploying the entire equipment of an engineering office for moisture measurement, the building physics and constructional relationships of the likely cause of moisture should be clarified through an on-site inspection. Building documents (construction planning details, building descriptions, contractor invoices) that describe the shell construction and the materials used are also helpful for this.

Never tilt the windows. At least 3 times a day open all windows fully to create a draft.

The tenant wants to know why not to tilt the windows. I have to explain it to him. Opening everything 3 times a day is not possible since he is usually out of the house from 7 until about 7 pm.

The room cools down at the spot without sufficient air exchange taking place.

The one picture where you can see the ls cable was also standing too close to the wall.

Furniture should generally not be placed too close to the wall.
And have you measured anything yet?

And have you measured anything yet?

No, that will only happen next week. The expert has no time before that.

Opening everything three times a day isn't possible, as he is usually out of the house by 7 and doesn't come back until around 7 p.m.

Then he also keeps everything nicely cold and doesn't heat when absent?

Speak gently and on equal footing with him. Not everyone knows that you can ventilate or heat incorrectly. At an on-site appointment, as advises, the professional can explain it to him from an expert perspective. He probably doesn't believe you and thinks he should now create dry air. At least, that could be the case.

As a tenant, he is in the better position, so hands off, otherwise he will reduce the rent and won't be pleased with you. You certainly can't use that. It is also important that he now has more time for appointments, cooperates, and participates.

I just spoke with him again.

He does not heat in the summer. He estimates the temperature at 20°. If he is asked to heat, he will want to have a very good reason for it; he actually doesn’t see why he should turn on the heating in the summer.

He keeps the windows tilted, even over a longer period. He rarely or almost never air shocks.

He also said just now that the couch stands right against the wall. However, the moisture forms to the left and right beside it.

He dries laundry in the bathroom. He also has no other place for it.....

A professional really has to discuss this. Then make a sample instruction on how to behave in this situation. And then I have to make an addendum to the lease - at least for the next tenant, if they do not want to sign it.

Really annoying; then I pay 250 euros because presumably there is improper ventilation....
But as you already wrote.... the tenant is in a better position there. I actually never wanted to rent out..... until I saw this supposed bargain.... I should have stuck to my original refusal to rent out......