ᐅ Interior insulation with expanded polystyrene and mold growth

Hello,
our house, built in 1965, has interior insulation on all exterior walls using polystyrene foam (exterior walls made of 30cm (12 inches) hollow clay bricks – polystyrene foam – 2-3cm (1 inch) plaster – wallpaper). Unfortunately, I do not know the exact thickness of the polystyrene. So far, the house is not insulated on the outside. We were not aware of the polystyrene insulation before (discovered by chance).

After the previous owner moved out, we found mold in some corners. This affects only rooms where single-glazed windows have been replaced with double-glazed ones. In the rooms with the original windows, there is no mold.

At first, we did not want to add insulation, but we believe there is a connection between the interior insulation and the mold growth. Has anyone had experience with this? Should we remove the polystyrene insulation and instead insulate from the outside?

Best regards

JessicaRe schrieb:

At first, we didn’t want to insulate the house, but we assume there is a connection between internal insulation and mold growth. Does anyone have experience with this? Removing polystyrene insulation and then insulating from the outside instead?

Internal insulation is (almost) always problematic. The new windows have made the rooms more airtight. In rooms with old windows, you probably have constant ventilation through them – which is not really good for heating costs...
The simplest solution is proper ventilation and heating. Always keep the rooms warm and ventilate by quickly airing out (shock ventilation).

Benutzer200 schrieb:

Internal insulation is (almost) always an issue. With the new windows, the rooms have become more airtight. In rooms with old windows, you probably have continuous ventilation through them – not ideal for heating costs...
The simplest solution is proper ventilation and heating. Always keep the rooms warm and ventilate with quick, intensive airing.

Also, the dew point with single glazing usually only occurs at the window. With double or triple glazing and poorly installed internal insulation, the dew point shifts into the insulation or onto the wall surface.

This is basically the problem with interior insulation. The dew point always shifts at most to the inner surface of the actual exterior wall. The result is condensation between a layer with very low vapor permeability and the thick masonry, which then remains permanently damp.
This can only lead to mold.
Exterior insulation ensures that the dew point is located outside the masonry!

Tolentino schrieb:

This is basically the problem with interior insulation. The dew point always shifts at most to the inside surface of the actual exterior wall. The result is condensation between a very low vapor-permeable layer and thick masonry, which then remains constantly damp.
This almost inevitably leads to mold.
Exterior insulation ensures the dew point is outside the masonry!

No.
Interior insulation works, just not with polystyrene foam. So removing old polystyrene / EPS is already a good start.
Then use capillary-active, vapor-permeable insulation materials for the interior insulation (such as hemp, jute, flax, wood fiber, calcium silicate), don’t seal the interior surface, apply lime or clay plaster with lime/clay/silicate paint, and that’s it. It works the same with 2cm (1 inch) or 20cm (8 inches) insulation thickness.
The only “risk” is open joints on the weather-exposed side, because a lot of moisture can enter there and, thanks to the insulation, no longer dry out. This applies equally to exterior insulation. So check the plaster or the brick joints accordingly.
In older buildings, install an exhaust ventilation system in the bathroom for permanent slight negative pressure, and the place stays completely dry.
By the way, the Glaser method for determining the dew point was not developed for interior insulation and should not be applied there. Someone just started using it, and since then it has been considered very dangerous — similar to the comma error with iron in spinach, but worse.

Whether a window has one or three panes doesn’t matter for the indoor air. Only with single panes or older double glazing from before 1993 does humidity tend to condense on the window itself rather than the old masonry wall. In all cases, you need to ventilate the same amount, though with old windows you are just warned more often.

Thank you for the addition. Yes, I assumed EPS.