I have made a model of my Office in Blender, but when i imported that in UE 14.4, and when i play, from inside of my office i am able to see outside of the walls, i dont know how is that happening, can anyone help me in this ?
Can you make a screenshot of it? It’s hard to understand what you mean without seeing it.
Usually when a wall becomes invisible, it’s because the Face is facing the wrong direction.
You can fix this by going to the UV tools, at the category normals you can flip the direction.
As you can see all my files, Screenshot 1 from blender, I imported it to fbx, but after importing it in unreal, i get such transparent view from inside. From Outside I am able to see exact thing, but from inside, i am not able to see walls.
Try to use the Modifier: Solidify and make all the walls about 5cm thick.
Thank You Very Much That Helped alot 
Can you help me in 2 more things, I have made that model of exact size of my office using measureIt tool, but scaling in Unreal seems to be different, how do I match that? also should I do collision to all interior items in blender or unreal ?