Trying to figure how long to make my diy rafter battens (protected air space for eave vents into attic). Going to make them out of 1/2" foam board. Want them to extend high enough so that when installers blow insulation, it is unlikely that they will get the insulation into the "protected" air cavity, so I'd think it should be something like x inches above target insulation height. First time for me to use "blown", so no idea on how "controllable" it is. FWIW: no ceiling at this point - just framing; doing this pre-drywall.