Seeing through downward mesh faces

Development Rendering
1

Hi! I can see through mesh faces that normals points downward. I am using starter content assets.
How can I fix it? Thank You.

2

That’s normal, polygons have a direction they face and by default they aren’t visible from the opposite side. You can make them two-sided in the material

3

I changed the material Property, but it brought in another problem. Now I can enter inside the mesh and only the upper face is blocking the player.

The material properties:

set.png

Here is the view inside the mesh:

set2.png
set2.png936×232 180 KB