" I am at home" generally refers only to the concrete object the "house", thus saying "I am AT home" almost always refers to your place of "dwelling" expressing that you are currently, physically at your "place of dwelling". Such as when you friend calls you to ask where you are at. You should say, "I am AT home."
"I am home" can take on a more flexible meaning. It can be a more abstract saying such when returning to your country after a long trip, you could say, "I am home" Or visiting the parents after many years, you could also say, "I am home" even though you are not referring to your actual place of dwelling. But you normally would not say, "I am at home" in these two cases.