Smart Vapor Barrier For Ceiling in Cold Climate?

A homeowner in northern Michigan is renovating an old house with a wing that has an open, plank ceiling made of thick oak boards laid right on top of the exposed rafters. The attic above it was never insulated, and the owner wants to blow in cellulose or mineral wool to a high insulation level. The problem is that the boards have gaps between them, so loose insulation and dust could sift down into the room below. The owner asked what to install over the planks, from the attic side, to block that dust without trapping moisture inside the wood ceiling.

People experienced in building science who answered the question explained that the real job of whatever material goes down is to stop air movement, not just moisture. Blocking air leaks also blocks most of the moisture that rides along with that air. One option is a specialty "smart" membrane that changes how much moisture it lets through depending on the season, which helps if the room will be air conditioned for long stretches in summer. But in this case, since the attic is vented and the plank ceiling itself breathes on both sides, plain plastic sheeting can work just as well, and is far cheaper. Another option is spraying a couple inches of foam directly onto the planks, which also seals gaps and adds insulation, though gaps wider than a hairline could let a bit of foam ooze through. Either way, sealing the edges of the barrier tightly to the surrounding framing matters most for keeping it effective.