Talked to two guys who install class A chimneys. One guy says, "the ceiling support box has to extend 18" below the ceiling." The other guy says, "nah, since you're only going through 1' of attic, just use a 24" ceiling support, put it 3" below ceiling, let the top of it out through roof, screw it in to framing, then cut the excess above roof in several places and nail down the flaps." Second guy says that eliminates need for attic insulation shield. Who's righter? Second guy makes me nervous, but if he's right, I like his approach.