Help on Mesh From Blender

I’m currently designing a game it is the entire area of Manhattan. I recently finished this cathedral in blender. However when I import the mesh into unreal some of the walls are missing and you can see into the inside. When you walk tworads it you can go right through the building but when you go inside, you can’t go back out. Please help me as this is really starting to aggregate the hell out of me

You’ll need to make sure that your polygons are facing the right direction, as for collision, you’ll need to probably make your own collision mesh.