not displaying all sides of thin plane mesh

[77I77I]

Hello. I created mesh items using blender.
I always have this problem when I make items like a thin plane.

For example, I made that using the blender,

image1
But when i upload that in the game, it's not same

image2

image3

It's okay on the blender like this, but In the game, the side of the object is invisible in all directions

it's very trouble. Does anyone know about this problem? please Tell me how to fix it

Thank you

[Wulfie Reanimator]

Faces are always single-sided in SL. You will need to duplicate the walls and "flip normals" so they face the other way.

You can enable "backface culling" in Blender to see the same effect.