So, it turns out that there are a lot of people around the world who want to know if it's making sense to air their rooms. To fulfill their ongoing demands I'll just try to translate my popular airing entry. (Thanks to Rune and Christian for their help!)
There was an older post, which was viewed pretty often. Unfortunately my DDNS provider at the time - who once told me they will never terminate their free service - finally decided to shut down all free accounts. In further consequence all of my sites and blog posts disappeared from the indexes of the search engines within days.
I had to deal with it and found a new free DDNS provider: dnshome. Since then I had no problems with them and bit by bit the number of visitors on my blog rose again. The number is still lower than before, but hey - who am I? Other people do very good jobs, too 
The intention of my post originally was to estimate with minor error if it's making sense to air our house by measuring the air temperature and air humidity inside and outside. Often people tend to air even when it's counter-producitve causing delapidations. Afterwards, they are complaining about wet and mold-infested walls and suchlike. They don't realize that they are responsible for the damage themselves.
Though there are some devices who can advise you wether to air or not (e.g. TFA 'Bel Air'), they all lack the calculated numbers.
Thanks to a friend you can have this now. The following script is expecting the values for air temperature and humidity inside and outside and calculates the dew points and absolute air humidity values. Additionally it shows a hint if you should air your rooms or not.
Of course it depends on the quality of the measured values, so keep half an eye on your hygrometers.
Please do not expect a killer app here. It's simple and will stay simple. It's very usable as it is. I wrote a BASIC version many years ago. That version running on a very old SHARP PC 1401 helped us to dry out our old house.
