[gmenzel]

two tests

Your scene doesn't really show the effect very well. Try a darker fabric against a bright background.

[seghier]

two tests

Your scene doesn't really show the effect very well. Try a darker fabric against a bright background.

it's better if you share your scene with your settings

[rusteberg]

[luis.hijarrubia]

Could you elaborate on that, please? You mean a 2-sided material? How would the back-face material influence the opacity of the front-face material?
Did I misunderstand?

If I'm not wrong (i'm still learning a lot about maxwell), backface material can affect when using production engine, but not on draft (FIRE). That backface material can affect light rays coming from behind.