Does mold go away by insulation or not?

Hello,

we bought a bungalow from 1974 last year.
Before the purchase, we viewed the house four times. At that time, it had been unoccupied for about six months. During the inspections, we searched thoroughly for mold and even removed wallpaper in several places to make sure that no mold was present.
At the time of purchase, the house was equipped with double-glazed wooden windows and had no significant insulation. Only the flat roof was insulated with 10 cm of Styrofoam, and according to the previous owner, this insulation was installed in the 2000s.
After the purchase, we took the following measures:

[*]The roof was renewed by adding another layer of 10 cm Styrofoam insulation on top of the existing flat roof construction.
[*]Triple-glazed plastic windows were installed.
[*]Underfloor heating was installed, and we inserted 5 cm thick Styrofoam insulation beneath the screed.
[*]The exterior walls in the ground area (from the concrete slab up to about 0.5 m above ground level) were insulated.

Now to my problem and possibly potential solutions:
For about two months, we have noticed mold growth in the corners of the exterior walls. The walls are painted, no wallpaper.
The strange thing is that before the purchase, there was no mold in these areas.
My questions:

[*]Can this problem be eliminated by external insulation (about 20 cm Styrofoam)?
[*]What about ventilation then? There are ventilation systems that can be installed. Does anyone have experience with them? Are such ventilation systems absolutely necessary?

I am very grateful for any kind of answer.
Best regards
Julia

I assume that your mold is explicitly caused by your renovations (tighter windows + insulation) combined with insufficient ventilation. Additional insulation will only worsen this. Mold most often occurs due to insufficient ventilation...

I assume that your mold is explicitly caused by your renovations (tighter windows + insulation) combined with insufficient ventilation.

Very obvious, yes.

Additional insulation will only worsen this.

Nonsense!

Buy a hygrometer, calculate the dew point from the data. Then determine the surface temperature of the moldy walls. The result will be that the surface temperature is lower than the dew point temperature and therefore condensation settles on the wall and forms a breeding ground for mold. Currently, the dew point lies on the interior wall surface because the windows have a better insulation value than the wall. Therefore, it makes sense to insulate the wall accordingly to match the new windows, preferably with ETICS. Correspondingly, more ventilation is necessary than you have been doing so far. The old windows were less airtight than the new ones (and thus acted as forced ventilation). Alternatively, you can install a ventilation system that takes care of this for you.

Can this problem be eliminated by external insulation (approx. 20 cm of Styrofoam)?

Yes. Why?
In the past, it was a well-ventilated house due to the old windows. The humidity was either "vented out" in winter or it condensed on the old panes. The latter is definitely eliminated by the new windows.
Then the most vulnerable spots in the house remain. These are – just believe me – the outside corners (this has to do with geometry, that the corners are the most vulnerable). These are the coldest spots in the house. If a) heating is insufficient and b) there is no constant ventilation (I recommend, if possible, to install decentralized fans), water can condense there.
Can you measure the temperature of the masonry in the corner? Then you could verify everything. You can calculate online where the dew point lies.
If you now, to give an example, have 20 degrees inside the house at 60% humidity (which often happens at night in bedrooms), the dew point is "only" 12 degrees. When it is cold outside, the wall reaches this temperature quickly in the corner and the wall gets really wet ==> mold

Additional insulation will only worsen this.

Mold occurs in most cases due to too little ventilation...

The first statement is simply nonsense. The insulated corner then has 17-18 degrees and mold is impossible due to lack of moisture. It’s physics combined with biology.

Mold occurs due to moisture or condensation of moisture on cold spots in the house ==> see above.
Often after a window replacement, because then too little air exchange takes place.

Therefore, dear OP, the question is what the window installer put in his ventilation concept for you? I bet there is nothing about it...
According to DIN 1946, such a concept is mandatory, or at least the window installer must instruct you to take care of it yourself, otherwise moisture problems can occur. Just like with you.

The former is simply nonsense. The insulated corner then has 17-18 degrees and mold is impossible due to lack of moisture. It is physics combined with biology.

Mold arises from moisture or condensation of moisture on cold spots in the house ==> see above. Often after a window replacement, because then there is too little air exchange.

Note to myself: always READ EXACTLY what is written in 2025!

Of course you are absolutely right - I am currently renovating a newly acquired (but already old) property within the family circle, in which the previous owner applied 20 MM Styrofoam INSIDE. I really do not want to post what that looks like here.

20 CM and also outside is of course something else - but I would still (I am also still not a particular fan of "just slapping something on the outside") in any case first ventilate properly over a longer period...

The strange thing is that there was no mold in these places before the purchase.

Much has already been said about this. I have also heard it repeatedly: the new windows keep the moisture in the room and mold forms because of the heating. Windows no longer let anything through. And on average, a person loses half a liter of water overnight. Not to mention the water vapor from showering and cooking. What is water vapor and hangs in the air cannot get out and settles where there is no air circulation.

Does anyone have experience with this? Are such ventilation systems absolutely necessary?

They are not absolutely necessary, but very comfortable. Because as you can see, your ventilation behavior is not sufficient. You have to see how much time you can dedicate to it. Personally, that would be a waste of time to me even in a bungalow and also annoying if you have to plan a quarter of an hour three times a day for it. We had a ventilation system installed right away in the new building and are thus relieved from ventilating. In an existing house, you probably retrofit with a decentralized solution.