I am in zone 4a (central NC) and have decided to have the bonus room above the garage finished off during the initial construction of the house and so I need to make some decisions quickly regarding air-sealing.

The main house will have an insulated and sealed crawlspace, R21 fiberglass in the walls, R49 in the ceilings and uses the taped and sealed sheathing for the primary air barrier.  In my bonus room, I have one long knee wall and two shorter ones, with a shed dormer in the middle, so there are three spaces that will need something to hold in the insulation and provide air sealing.

I have come up with the following options but am not sure which one is more effective at air-sealing and allowing drying to the inside and outside (this space will have a mini-split for heat and AC).

1. install sheathing on the attic side of these walls; tape and seal like the rest of the house exterior
2. install Tyvek on the attic side of these walls; tape and seal
3. install rigid foam on the attic side of these walls: tape and seal

Which construction would be best for my space and / or most cost effective?

Thanks!