Divide space for basement levels

[Giov]

Hello,

I'm trying to change a model geometry and construction after interesting feedbacks I received from a recent training session.
The situation is this: the basement level of my building has perimeter masonries half below the ground level and half not. According to what was explained during the course I should divide the basement level with the option "divide space"; and setting to the part of the wall below the ground at constant temperature of 10°C.
I did it, the space has been successfully divided "without partition"; But what I got is a space divided in 2 with new ceiling. The "virtual" horizontal surface I created in order to divide the external masonries has been translated in an intermediate ceiling. That is wrong since the basement level has not have intermediate ceiling.
If I decompose the surface, it is possible to see that the surface is considered both as a ceiling and as a hole. The hole is reasonable, but the ceiling?

thanks