Temperatures of the exterior wall, dew point, etc.

Hello everyone, I hope I am allowed to ask my question even though it is not about a new building but an existing building (mid-60s)! The situation is as follows: I have been living in a new apartment for a few weeks and have moisture problems on the exterior wall in bad weather. The landlord sent me an expert after that. The expert came to the following conclusion: - current room temperature 15.5°C (I had just aired the room shortly before) - current relative humidity 40% - temperature of the interior wall 18°C - temperature of the exterior wall 9.7-9.9°C He interpreted the result orally as follows: The temperature of the interior wall indicates that the room was consistently heated too little. The temperature is not sufficient for the legal minimum requirement of 20°C. As a result, the exterior wall would be cooled down too much and the humidity would now condense on it. According to the laws of nature, this is improper ventilation/heating behavior. If he now sends this in writing to the landlord, I am already expecting the expert’s bill and possibly a warning for improper use of the apartment... Now I have researched a little: 1. Why do I regularly find information about dangerous temperatures below 12°C on the internet, when the dew point depends on relative humidity and air temperature? According to an online calculator, my dew point is 1.5°C – so 9.7°C should be absolutely uncritical, right? 2. Can the room really be legally too cold? I cannot find any information about what tenants are required to do – except frost safety. 3. Is a temperature difference of more than 8°C between the interior and exterior wall really permissible and must tenants necessarily heat against it, or is this already a construction defect regarding insufficient insulation or damp walls? Currently, I only have the expert’s oral result. However, he excludes a construction defect based on the measurement results. How do you interpret the results? Thank you in advance!!

Hello,

... The landlord then sent me an expert. ..... If he now sends this in writing to the landlord, I’m already looking forward to the expert’s invoice and possibly a warning due to improper apartment use...

You do not have to accept such expert opinions. A one-time, momentary point measurement is completely unsuitable to describe or clarify the overall context. I use data loggers for such cases.

... Now I have done a little research:....

There is a so-called mold criterion according to DIN 4108-2 or DIN ISO 10211-2, a temperature factor fRSI >= 0.7 which must be met. Compliance can be determined for deviating boundary conditions.
Here, the component structure, indoor humidity, room temperature, etc., play a role. The component is firmly predefined. fRSI is therefore mostly determined by user-side boundary conditions. Exception: the component is dampened from the outside due to poor building waterproofing.

... Why can I not find ... on the internet ...

As a layman, you should be cautious about self-interpretations from the internet!

... and must definitely be counter-heated by the tenant ...

Yes, heated and ventilated. The worse the insulation of the exterior wall, the more so. If you move into an apartment or property with poor exterior wall insulation, you must also accept the necessary additional heating energy. You can inform yourself beforehand!

... Currently I only have the oral result of the expert. However, he excludes a construction defect based on the measurement results. How do you interpret the results?

It cannot be assessed from a distance at all.

Best regards

According to the Internet calculator, I have a dew point of 1.5°C - so 9.7°C is absolutely uncritical, right?

It depends... Even with 40% indoor humidity and 15 degrees room temperature, you can have a damp wall if it is cold enough. And since you measured the mentioned values after airing out, they are probably not very realistic. A long-term measurement would actually be necessary here.

But let's base it on your values: Do you know what happens in the centimeter range in front of the outside wall? Measure the air temperature one centimeter away from the outside wall. Here you will no longer have 15.5 degrees, but significantly less. If you measure a temperature of around 12 degrees here, then you already have a relative humidity of 90% in this small air pocket, which permanently lies against your cold outside wall. The wall will therefore feel damp even though the dew point has not yet been reached. You can also read that in the tables. But you also need the necessary background knowledge...

That's how it is with personal interpretations...

Here I can only agree with @Euro. You need to ensure higher room temperatures.

Hopegro