Using 2D-backdrop with sun from opposite direction ?

dream-weaver

I have a room with windows on two opposite walls.
I can perfectly use a 2D-backdrop on the side where the light is coming from. But when rendering to the other side of the room, the backdrop is almost completely white.
I am using only HDRi as the single light source and iray.

Is there any way to prevent the backdrop from getting bleached and overexposed by the light?

dream-weaver

I just found a workaround.
Placing a big upright plane onto the roof works for blocking the light being cast on the backdrop.