Is it batt or bat?

Whether it is batt or bat depends on meaning and context. Bat is correct for flying mammal that is nocturnal and uses echolocation, for baseball bat which is wooden or metal club used to hit ball, for verb to bat meaning to hit, and for phrases like bat around meaning to discuss. Batt is correct for batt insulation which is fiberglass insulation manufactured in pre-cut rolls or batts that fit between studs, for quilt batt which is layer of batting inside quilt, for batt as short for battalion in military abbreviation, and for batt as in to batt down or batten. So bat with one t is animal and sports equipment, batt with double t is insulation and quilting material. Many people confuse because insulation is often mispronounced as bat insulation but correct spelling is batt insulation. In construction, you buy batts of insulation. In baseball, you buy bat. So if you mean animal or baseball, it is bat, if you mean insulation rolls, it is batt.