"Asbestos," Lionel quietly clarifies, seeing as nobody else seems like they're going to. "It's an insulator. And," he glances around, "this place could be old enough to still have it. If things are cold, they stay cold. If they're hot, they stay hot." Which, could be useful, given the weaknesses and preferences of the vines. "It's also toxic," he adds. "Though it's probably not our main concern."

Mary Grace just mentioned 'sacrifice' after all.