I am looking for advice on how to air seal my bonus room in zone 4a.  The main house uses the taped and caulked sheathing as the air barrier and will have fiberglass batt insulation but aside from the gable wall, the bonus room does not have sheathing.  I have knee walls, a shed dormer with a 3:12 pitch roof / ceiling, the ceiling above the knee wall follows the 8:12 pitch roof and finally the ceiling in the middle is level and a 9’ high.

The only thing I can think of is to use the drywall as the air barrier but how effective will that be?  What would best practice be for this type of room configuration?

thx.