Many thanks for your help, Keet.
Indeed, applies to my wall, the semi-transparency is effective.
I don't knw that it was essential to active just 1 step of transparancy in the 3D view, I just tried to apply this in opaque mode.
I de-activate 3D transpaency, because (I don't know why ...), when I ...

Sorry but I've somme difficulties.
I've tried to generate many PNG with transparency, but each time it appears black or grey, but never white. And the transparency is not effective.
Which parameters should I use on the 2 sides ?
Angle 0 ? Scale 1% ?
You may have a correct PNG for me ?

Thanks for ...

WOOOH KEET
Thanks a lot for those tips.
i will choose the second solution and try to apply a semi transparent PNG.
Have a nice day !

Bonjour le forum !

Je souhaiterai rendre mes murs extérieurs semi-transparents, sans pour autant utiliser l'option de "Modifier la vue 3D" où je veux rester en opaque.
Est-ce possible svp ?

Merci par avance pour votre aide.