Prevent inner edges from being at front

Hi,

I am working on game where I will need simple walls with holes in it. I've modeled all the objectes in Blender and exported them to Unity.

But when I am using my models in game I am getting a strange problem with inner walls of my blocks. By some reason inner walls of my blocks are visible above the top block parts. Here the sample:

Is there any way to prevent it?

Thank you!