if you really want to reliably check for the usage of the shower, you could also install a water flow sensor in the warm water line leading to the shower, or install occupancy sensors inside the shower.
You could for instance connect this sensor to a Fibaro smart implant:
This is the flow i use.
Using the humiditysensor of my Itho box and depending on tempature it starts on middle or high modus after a sudden increase of humidity.
After a sudden increase the humidity value of 2 minutes ago is written to a variable, when the RH comes close to this value it goes to low mode.
Well, the script still runs fine since I posted it (over 2 years ago). It works for me.
There used to be light moulding on the ceiling of my bathroom, but no more. The RH settles down to somewhere between 75 - 80% at 17 degrees. It’s absolutely fine.
The rest of the house is another thing. You should indeed not have too much humidity in the room where you stay for longer, but in the bathroom one typically stays for maybe half an hour tops. Usually shorter. As long as no mould is forming (ceiling, sealants, paint), that is perfectly fine.
