When nailing together built-up window and door headers, I use 1/2-in. foam insulation between the 2x stock instead of 1/2-in. plywood (see drawing). The sheathing — whether foam board or plywood — is acting only as a packing material to adjust the thickness of the header, so it makes sense to use something with a high R-value.
Todd S. Pettinger, Springwater, NY