Is a hygrometer a plumbing tool? In truth I do not know. Am just a landlord here in the Pacific Northwest (think rain) challenged by a tenant who is nothing if not cavalier about keeping the heat up to where I don't have to worry about MOLD. He cleans anything obvious before I arrive to inspect so I wonder if there isn't some tool out there that, placed against drywall, would tell me if there's moisture collecting behind the scene, so to speak. I'd be most grateful for counsel.