@DeanXeL , this is a really wonderful advice, thanks a lot! There is always a way, one only has to find it! B)
And the other thing: As far as I understand, any room, that has no ceiling, is "exploded". It will count towards the outside of a building, even if there is some floor and a roof above, and even if the game counts this "exploded" room as interior in terms of weather...
I call rooms that way, as soon, as they show the outside wallpaper or frieses or trims on the inside. But I have to admit, that I still don´t know exactly, what causes this to happen, if it´s not the missing ceiling. For example, if you build a porch, and you use the room tool first, and this porch has a ceiling, you can´t build a frieze under it at the facade of the house. Do you destroy the ceiling, and you then have a roof on this opened/exploded porch, the frieze will follow the facade of the house, and not run around the porch anymore.
Those things, really confuse me, and I think, I´ll still need some time to learn, how this works.