Texture partly invisible problem

Well, I have an House which i made in Blender, Smart UV Mapped it. The texture looks fine in Blender but now I export it in Unity, and drag it on the same house, i get the problem that is can’t see a part of the house but when i drag my mouse i don’t see the other part but i see the part i couldn’t see??? So what is making this invisible?

Thanks for reading and help!!

this is caused by normal are in incorrect direction.

3d software draws texture on both sides of polygon, but unity and other 3d real-time engines - only on front face. so fix faces in 3d software