Would it make sense to blow in extra cellulose insulation over 18 in over just the shower in the attic?

A homeowner asked whether it makes sense to add extra cellulose insulation in the attic, piling it up over 18 inches, but only in the spot right above the shower. The idea was that the shower gives off more heat and moisture than other parts of the house, so maybe that area needed extra insulation for comfort. The home already had 15 inches of attic insulation and had been fully air sealed (blocking gaps where indoor air leaks into the attic).

Building science experts who answered pushed back on the idea. One pointed out there's no real connection between a shower producing heat and moisture and a need for more insulation above it, and that adding insulation there wouldn't make a noticeable comfort difference. Another explained that during a shower, the stall itself loses far more heat to the surrounding bathroom air than escapes upward through the drywall and 15 inches of insulation already in place. So thickening the insulation just over the shower wouldn't do much.

As for the moisture concern, the experts said that's better handled elsewhere: through air sealing (which was already done), vapor retarders if needed, and a bathroom exhaust fan that's properly sealed and vented outside. For anyone with a similar setup, the takeaway is that spot-thickening attic insulation over a shower isn't likely to change comfort or humidity levels in the bathroom. Consistent insulation depth across the attic, good air sealing, and a working, well-vented bath fan matter more than adding extra insulation in one small area.