ᐅ Does Insulation Eliminate Mold or Not?

Hello,

Last year, we bought a bungalow built in 1974.
Before the purchase, we viewed the house four times. At that time, it had been unoccupied for about six months. During the inspections, we thoroughly checked for mold and even removed wallpaper in several places to make sure there was no mold present.
At the time of purchase, the house had double-glazed wooden windows and no significant insulation. Only the flat roof was insulated with 10 cm (4 inches) of polystyrene, which, according to the previous owner, was added in the 2000s.
After buying the house, we took the following measures:

• The roof was renovated by adding another layer of 10 cm (4 inches) polystyrene insulation on top of the existing flat roof structure.
• Triple-glazed PVC windows were installed.
• Underfloor heating was installed, and 5 cm (2 inches) of polystyrene insulation was placed under the screed.
• The exterior walls in the ground area (from the concrete slab up to about 0.5 m (1.5 feet) above ground level) were insulated.

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

• Can this problem be solved by adding external insulation (about 20 cm (8 inches) of polystyrene)?
• What about ventilation? 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 response.
Best regards
Julia

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

Adding more insulation will only make this worse.

In most cases, mold develops due to inadequate ventilation...

Jesse Custer schrieb:

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

That seems very likely, yes.

Adding extra insulation will only make this worse.

That’s nonsense!

Buy a hygrometer and calculate the dew point from the readings. Then measure the surface temperature of the moldy walls. The result will be that the surface temperature is lower than the dew point temperature, causing condensation to form on the wall and creating a breeding ground for mold.
Right now, the dew point is at the interior wall surface because the windows have a better insulation value than the wall.
Therefore, it makes sense to insulate the wall appropriately to match the new windows, preferably with an external thermal insulation composite system (ETICS / EIFS).
Along with this, you need to increase ventilation compared to what you have done so far. The old windows were less airtight than the new ones and thus acted as a form of forced ventilation. Alternatively, you could install a ventilation system that handles this for you.

julia123 schrieb:

Can this problem be solved by adding external insulation (about 20cm (8 inches) of Styrofoam)?

Yes. Why?
In the past, the house was well ventilated due to the old windows. In winter, the humidity was either “aired out” or it condensed on the old glass. The latter is definitely no longer an issue with the new windows.
What remains are the most vulnerable spots in the house. Believe it or not, these are the exterior corners (this has to do with the geometry, as corners are the most exposed). These are the coldest points in the house. If a) heating is insufficient and b) continuous ventilation is lacking (I recommend, if possible, installing decentralized ventilation systems), water can condense there.
Can you measure the temperature of the masonry at the corner? That way, you can verify everything. You can also calculate the dew point online.
For example, if the indoor temperature is 20°C (68°F) at 60% humidity (which often happens in bedrooms at night), the dew point is only 12°C (54°F). If it is cold outside, the wall corner will quickly reach this temperature, causing the wall to become damp ==> mold.

Jesse Custer schrieb:

Adding insulation will only make this worse.

Mold usually develops due to insufficient ventilation...

The first statement is simply incorrect. The insulated corner will then be at 17–18°C (63–64°F), making mold impossible due to lack of moisture. It’s physics combined with biology.

Mold arises from moisture or condensation at cold spots inside the building ==> see above.
This often happens after window replacements, as there is then insufficient air exchange.

So, to you, dear original poster, the question is: what has the window installer specified in their ventilation plan for you? I bet there is nothing.
According to DIN 1946, such a plan is mandatory, or at least the window installer must inform you that you are responsible for ventilation, otherwise moisture problems may occur—just as in your case.

nordanney schrieb:

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

Mold develops because of moisture or condensation forming on cold spots in the house ==> see above.
Often after window replacement, because then there is insufficient air exchange.

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

You are of course completely right—I am currently involved in renovating a recently acquired (but already quite old) property within the family, where the previous owner installed 20 mm (0.8 inches) of Styrofoam INSIDE. I really don’t want to post how that looks here.

20 cm (8 inches), and on the outside, is of course a different matter—I would still, however (and I remain no great fan of “just slapping something on the outside”), in any case, first ventilate properly over an extended period...

julia123 schrieb:

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

This has already been discussed extensively. I have also heard repeatedly that the new windows trap moisture inside the room and that heating causes mold.
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.
The water vapor that remains in the air cannot escape and settles where there is no air circulation.

julia123 schrieb:

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

They are not absolutely necessary, but very convenient. As you can see, your current ventilation habits are not sufficient. You need to consider how much time you can dedicate to this. Personally, for me—even in a bungalow—it would be a waste of time and also frustrating to have to plan three sessions of 15 minutes each day. We had a ventilation system installed right from the start in our new build and are therefore relieved from manual airing.
In an existing building, a decentralized solution is probably retrofitted.