Modeling light-tight walls

With my gas station's exterior coming together nicely, I'm going to look at…some interior walls.…The reason for that--and I'll pick this window and hide it to show--is that I've…modeled this single-sided, and it'll come into Unity this way.…So if I put transparency on the window, which I would like to, smudged, dirty,…and maybe broken but still transparent,…I don't want to see through the back of the walls and see that there's nothing there.…That's important one.…So what I'll do then is to really quickly model out light-tight interior walls.…

I'll work this over a couple of walls at a time so I can see what I'm doing clearly.…I'll start with his big wall and make sure I pick the piece underneath it.…I'll choose Display > Hide > Hide Unselected.…I'm going to keep my interior walls as clean as possible, assuming that they…will be in the dark. We may actually get in and explore the building at some…point, but at least I need to block the light.…I'll go into my front view and press 4 for a wireframe.…