Insulate condominium with moldy corners afterward

Hello everyone,

Actually, I registered here to gather valuable information for our new construction project and to exchange ideas with other builders.

Today, however, I have a different kind of question concerning our owner-occupied condominium.

We moved in April 2018 and are now spending the second winter in this apartment.

Already last winter, we struggled with a mold problem at the outer corners of the exterior walls (where the two exterior walls meet) in the bedroom (typical structural thermal bridge). We also had a building expert here who specializes in mold damage. He could not detect any structural defects or elevated or damp values.

However, the location of the bedroom in the apartment is problematic. Adjacent and each connected by a door to the bedroom are: the kitchen and the main bathroom (see floor plan).

The main bathroom also has a window that is too small (because the bathroom was enlarged afterwards). Therefore, some of the moisture from the warmest room (bathroom) tends to move into the typically coldest room (bedroom).



Ergo: More ventilation and even more attention to humidity. Said and done. We made sure that the humidity never permanently exceeded 55%.

The mold from 2019 was then professionally removed.

In addition, in October 2020, we had a bathroom fan installed to transport the excess humidity outside.

We thought we had solved the problem with the measures explained above.

But today we rediscovered mold in this corner (see pictures).

Outer corner bottom:


Outer corner top:


And now to my question: Do you perhaps have an idea how to fix the cause with as little structural work as possible and keep the corner permanently mold-free?

We suspect it may also be related to the wall design: Before moving in, we removed the textured wallpaper, applied a primer and a mineral coating plaster. This was then (unfortunately) painted with regular paint (Alpine White).

Our idea is therefore:
1. Replaster all walls and the ceiling with interior thermal insulation plaster
2. Wallpaper with painter’s fleece
3. Paint with lime, silicate, or mold protection paint

We would like to avoid using silicate insulation boards if possible, considering effort and cost!

Thanks for reading and for your help!

According to the floor plan, the bedroom is located next to the rooms with the highest humidity in the apartment. You write that the bedroom is cold? Then the exterior walls are cold as well. Try ventilating quickly in the morning and then the doors to the other rooms must be closed. And turn on the heating! Possibly ventilate quickly again in between. 55% humidity seems quite high to me.

What is the temperature of the wall? How much room temperature? It is possible to calculate whether a wall will mold.

According to the floor plan, the bedroom is located next to the rooms with the highest humidity in the apartment.

That is correct, but unfortunately it cannot be used differently. I think that will also be the main problem.

You say the bedroom is cold? Then the exterior walls are cold too.

Cold in relation to the other rooms. In the bedroom we always have about 20-21 degrees. The bathroom is 22-23 degrees and the other rooms in the apartment are usually 22 degrees.

Try this: ventilate intensively in the morning and then the doors to the other rooms must be closed. And turn the heating on! Possibly ventilate intensively again in between.
I find 55% humidity quite a lot.

We do that. In the morning after getting up we ventilate crosswise and again in the evening after work.

The door to the bathroom is actually always closed. Of course, it is still used, so a certain moisture transfer, for example after showering into the bedroom, is unavoidable.

It should be noted that we do not have a new building. Also, it is never continuously 55%. In summer, we logically do not have a problem here. But especially in the transitional seasons late autumn and spring, we really struggle here.

Mold only forms at a permanent humidity of over 60%. We have not had that at any time - especially not this year.

What temperature does the wall have? How much room temperature? It is possible to calculate if a wall will mold.

According to the expert, the exterior walls at the described problem spots unfortunately have a relatively high dew point, so moist air condenses here quickly. However, this is not unusual for exterior wall corners, as there are classically thermal bridges here.

The bedroom typically has around 20 degrees (20-21 degrees).

I once had a similar situation where the bedroom was on the exterior wall facing north. And it also had mold. Besides airing out and heating a little, I managed it by bringing more fabric into the room. I discovered by chance (disposed of laminated wardrobe and replaced it with untreated solid wood shelves with curtains) that wood and/or fabric helped regulate the moisture. They apparently absorbed enough moisture to deprive the mold of its basis. It may not be a substantial solution, but at least something.

Does the picture deceive me or does the wall already look really soaked? You say that it is a typical thermal bridge, but the expert could not find anything?

At 20 degrees and airing twice, this will rather not be manageable. More heating and ventilation are necessary here.