Cut & Cobble above existing ceiling?

A homeowner in northern Michigan is renovating a 125-year-old house that was once used only in warmer months but is now a full-time residence. Above a bathroom, old fiberglass batt insulation in the attic rafters is being pulled out and replaced with a thicker layer of blown-in cellulose. The problem: there's no vapor barrier (a layer that blocks moisture from passing through) between the drywall ceiling and the joists above it. In a cold climate, that gap can let warm, moist bathroom air push up into the new insulation during winter, which risks condensation and mold inside the ceiling assembly.

The homeowner asked about a technique called "cut and cobble" — cutting rigid foam boards to fit snugly between the joists on top of the existing drywall, then sealing the edges with spray foam to create a vapor-resistant layer. This avoids tearing out the drywall or adding insulation above the joists, which isn't possible here because mechanical equipment runs through that space.

One expert on the building-science forum where this question was posted suggested a simpler fix: skip the cut-and-cobble approach and instead paint the bathroom ceiling with vapor-barrier paint from below. Combined with careful air sealing (blocking gaps and cracks) from above in the attic, this would address the moisture concern with far less labor than fitting and foaming rigid boards between every joist bay.