i have a bonus room above my garage I'm in zone 7. its a truss system using 2x4 trusses for the cavities. along the bottom and top of the wall 4" xps was foamed in-between the joists to block the floor cavity and to meet up with the spray foam on the 2x12 top cord. the remaining part of the knee wall was filled with a rouxl 2x4 wall and then the wall covered with 6mil poly and drywalled i borrowed a flir gun to check out the house for fun and notice i loose some heat through the knee wall. when i started researching this style of attic the "cape cod" it says i need an air barrier on the backside of the knee wall. I have lots of sheets of left over 1.5" xps foam could I add that to the outside of the wall to act as an air barrier even though their is a 6mil vapour barrier on the inside of the wall on the warm side

the drywall is not taped i could also take down the drywall remove the 6 mil poly and fasten the 1.5" of foam to the inside which would probably be easier then trying to fit in the narrow space behind the knee wall

The back side of the Roxul would still need an air barrier, but it's better & easier to put the XPS on the interior side. Don't pull the 6 mil poly when you do this, instead, detail it as an air barrier.

On the cold side of the kneewall you can use vapor permeable broadsheet goods, such as house wrap for an air barrier. Where floor joists go under the kneewall, cut in air-dams out of the spare XPS, sealed with can-foam to block thermal by pass currents from chilling the floor. this is imporant even if the floor is insulate.

If you are putting rigid foam somewhere, I'd tape it as an effective and perhaps additional air barrier.