Solid looking wall is semi-transparent in Lumen system

  • Main window, bottom: a solid looking wall
  • Small windows, top: Lumen, reflection and surface cache are seeing an open wall.

I have a problem in which a solid looking wall is considered somewhat transparent by the Lumen system.
My hypothesis is that this leads to performance loss (because of overdraw of geometry behind the wall). Is that correct?

The solid wall consists of individual blocks, which is probably the reason for this. How would I make sure the blocks become solid?