2x6 stud walls, and the header’s have an inside and outside vertical 2x8, with the 2.5” center is empty. The outside sheathing is 1” polyiso with a foil (no OSB or plywood). I want to fill the headers before the windows are installed this week. Drying is to the inside, so no inside VB, and permeability is important. I was thinking 1.5” R6 Roxul ComfortBoard IS or 2” R8 EPS foam. I tried window/door spray foam, but the 2.5” gap was too large, and the foam just falls out to the floor. Am I overthinking this, and I can just stuff it with pink fiberglass?