ᐅ Humidity permanently too high, mold growth, possible causes?

Hello everyone,

I have the following problem that has been bothering me:

We have a house of about 100sqm (1076 sq ft), all on one level, built in 2001 from clay blocks (probably similar to aerated concrete blocks?). The attic is unfinished; the ceiling consists of chipboard on top, then insulation, and paneling underneath.

The house meets the building and insulation standards from 2001, is plastered on the outside, and was repainted last year with silicate paint.

We have repeatedly had problems with clothing in the bedroom’s wardrobe developing mold or mildew.

Also, the wallpaper behind the bed (on an exterior wall) showed mold.

We hardly heated the bedroom, only occasionally during very severe frost, and never overnight. The bedroom is ventilated in the morning and evening with the window fully open.

Because of the mold issue, I recently bought three hygrometers and placed them in the bedroom, office, and living room.

In the living room (where the stove is located and in use from afternoon/evening), relative humidity (RH) is currently between 60 and 65%. This is at the upper limit but still acceptable, I think.

In the office (12sqm (129 sq ft)) the RH quickly rises to 70% when I am there. Only by heating a lot (normally I keep it at 19°C (66°F), now around 20°C (68°F)) and frequent short ventilation can I lower it to about 50%, but it rises again quickly. So I have to ventilate every 1–2 hours.

In the bedroom, the level is always 70% or higher. I heated it well for two days and ventilated crosswise several times a day, which lowered it to 60%, but it rises quickly again. In the mornings it can even be 80%.

Is all this normal? Am I doing something wrong? Could moisture be entering the building somewhere? I mean, I can’t heat all rooms constantly at 22°C (72°F) and ventilate all the time. I don’t want it that warm in the office or especially the bedroom. About 19–20°C (66–68°F) is enough for me in the office, and 16°C (61°F) is plenty in the bedroom.

In our previous apartment, the bedroom heater was never on, yet we never had mold.

What can I do to get the mold problem under control and permanently lower the relative humidity? Would renting a dehumidifier and drying out the building thoroughly be advisable? Or is that just addressing the symptoms?

I also checked for any damp spots on the ceiling or walls but found none.

DrKingschultz schrieb:

Is all this normal?

At these temperatures and with your usage? Yes, it is normal. If you don’t heat the bedroom and two people tend to sweat out 2 liters or more during the night, mold will simply develop. It’s self-inflicted.

That’s why people always say: heat and ventilate.

Tip: Maybe install a decentralized ventilation unit.

What do I need to do to bring the relative humidity to acceptable levels?

Should I heat all rooms continuously? To what temperature? And then ventilate frequently?

Does it make sense to get a dehumidifier first to lower the relative humidity to normal levels?

A dehumidifier is only effective as long as it is running. Maybe it continues to work for a few days afterward, but then it would need to be turned on again. The only permanent solution would probably be to install fans. Most likely in the bathroom and kitchen, where the most moisture is generated.

Otherwise, as already mentioned: heat and ventilate. There are no fixed temperature settings for this. It depends on the humidity and temperature inside and outside.

DrKingschultz schrieb:

We have repeatedly experienced problems with clothes in the bedroom wardrobe developing mildew stains or mold.

The wallpaper behind the bed (on the exterior wall) also had mold.

Place wardrobes and beds preferably against interior walls, and never against exterior walls without ventilation behind them.
Do you sleep with the heating off and windows closed? Do the windows get fogged up in the morning?
Is the heating system underfloor heating or radiators?
We also have a bungalow with expanded clay aggregate, without any mold issues.

DrKingschultz schrieb:

What can I do to manage the mold problem and permanently reduce the humidity levels?

If the problem occurs only in the bedroom, you can consider installing a decentralized ventilation system.
And yes, moisture is released into the room while sleeping. As far as I know, an adult emits about 1.5 liters (3.2 pints) of moisture during one sleeping period. The way to counteract this is by manual ventilation (daily airing through window ventilation) or automatic ventilation.