ᐅ Temperatures of Exterior Walls, Dew Point, and Related Concepts

Hello everyone, I hope it’s okay to ask my question even though it concerns an existing building (from the mid-1960s) rather than a new build!

Here is the situation: I have been living in a new apartment for a few weeks, and during bad weather, I have noticed moisture problems on the exterior wall. The landlord sent an expert to inspect the situation.

The expert came to the following conclusions:
- Current room temperature: 15.5°C (I had ventilated shortly before)
- Current relative humidity: 40%
- Interior wall temperature: 18°C
- Exterior wall temperature: 9.7–9.9°C

He verbally interpreted the results as follows: The interior wall temperature indicates that the room was consistently underheated. The temperature does not meet the legal minimum requirement of 20°C. As a result, the exterior wall has cooled down too much, causing moisture to condense on it. According to physical laws, this points to improper ventilation and heating behavior.

If he now sends this report in writing to the landlord, I am already expecting to receive the expert’s invoice and possibly a warning for incorrect apartment use...

I have done some research:
1. Why do I regularly find information online stating that temperatures below 12°C are dangerous when the dew point depends on relative humidity and air temperature? According to online calculators, my dew point is 1.5°C, so 9.7°C should not be critical, right?
2. Can a room legally be considered too cold? I can’t find any information about tenant responsibilities other than ensuring frost protection.
3. Is a temperature difference of more than 8°C between the interior and exterior wall really acceptable, and must tenants provide additional heating to compensate? Or is this already a building defect related to insufficient insulation or damp walls?

Currently, I only have the expert’s verbal report. He excludes a building defect based on the measurements.

How do you interpret these results?

Thank you all in advance!

Hello,

Maik87 schrieb:
... The landlord then sent an expert to inspect. ..... If he now sends this report in writing to the landlord, I’m already expecting the expert’s invoice and possibly a warning for improper use of the apartment...

Such reports do not have to be accepted. A single, momentary spot measurement is completely unsuitable for describing or clarifying the overall situation. I use data loggers for cases like these.

Maik87 schrieb:
... I did some research:....

There is a so-called mold criterion according to DIN 4108-2 or DIN ISO 10211-2, a temperature factor fRSI ≥ 0.7 that must be met. Compliance can be determined for varying boundary conditions.
Here, the component structure, indoor humidity, room temperature, etc., play a role. The component itself is fixed and predefined. The fRSI is therefore mainly influenced by user-related boundary conditions. The exception is if the component is damp from the outside due to poor building waterproofing.

Maik87 schrieb:
... Why do I find on the internet ...

As a non-expert, you should be cautious about drawing your own conclusions from the internet!

Maik87 schrieb:
... and tenants absolutely must use additional heating...

Yes, heating and ventilation are necessary. The worse the insulation of the external wall, the more is required. If you move into an apartment or building with poor external wall insulation, you must accept the resulting higher heating energy demand. You can always inform yourself beforehand!

Maik87 schrieb:
... At the moment, I only have the expert’s verbal findings. However, they rule out a construction defect based on the measurements. How do you interpret these results?

This cannot be assessed remotely at all.

Best regards

According to an online calculator, I have a dew point of 1.5°C (35°F) – so 9.7°C (49°F) should be absolutely uncritical, right?

It depends... Even with 40% indoor humidity and a room temperature of 15°C (59°F), you can have a damp wall if it’s cold enough. And since you measured these values after ventilation, they probably aren’t very realistic. A long-term measurement would actually be necessary here.

But let’s assume your values are correct: Do you know what happens within a few centimeters in front of the exterior wall? Try measuring the air temperature one centimeter (0.4 inches) away from the exterior wall. You won’t have 15.5°C (60°F) there anymore, but significantly less. If you measure around 12°C (54°F) in this small air pocket, then you already have a relative humidity of 90% constantly in contact with the cold exterior wall. The wall will therefore feel damp even though the dew point hasn’t actually been reached yet. You can also see this in the tables, but you need the necessary background knowledge...

That’s how it goes with self-interpretations...

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

Hopegro