Thanks for the informations.

My d value is high because it is a plastic cover.

I started to design myself the greenhouse with curved cover, using only SweetHome3D and a text editor, exporting some objects, like a curved wall, to .obj, and importing it as a standard object.

The 3D model I have designed is correctly rendered with a Ni value of 1.0.

So it seems the problem comes from the 3D model I have downloaded. It doesn't render correctly even after a lot of experiments in the .mtl file.