I’m working on a bathroom remodel in Portland OR (Climate Zone 4 Marine) and could use some guidance on how to handle a tricky insulation and air sealing situation.

The bathroom extends a few feet out from the main body of the house, and the shower is mostly located in this bump-out. Above it is a small, enclosed attic space with a roof above – the area is about 28″ deep, 18″ tall on the house side, sloping down to 6″ on the outer wall side. This space isn’t connected to the main attic at all.

Right now, it has bird block vents to the exterior, but once the shower is completed, I won’t have good access to this area again. I’ll be installing Kerdi Board on the shower walls and ceiling, so heat and humidity will be getting into the space to some degree.

What’s the best way to air seal and insulate this space before it’s closed off?

• Should I treat it like an unvented roof?

• Closed-cell spray foam isn’t my favorite but would it be the best solution in such a tight cavity?

• Is there any benefit or risk in leaving it vented, given the size and isolation of the space?

• Should I be worried about heat and humidity from the bathroom causing condensation or moisture issues?

Any advice or insight would be much appreciated, thank you!