Thanks for advices.

I have finally got it working.

It seems the problem was that my flat cover had only 2 faces, the front and the back.

So I have created the same room at a higher level, because a higher level room have a configurable thickness. I set the thickness to 0.1 cm. And exported the room to an .obj file.

It worked. The greenhouse seems to be rendered correctly.

But I divided it into 4 parts, in order I can build greenhouses of different lenght: 2 parts for the metal skeleton, and 2 parts for the plastic cover.