I’m planning on using a construction method described in “Breaking the Thermal Bridge”, FH#250.  This involves adding a foam furring strip combination over 2×6 exterior walls to create a thermal break.  In my case, I’m having a 1″ layer of foam sprayed in the stud cavities as a starting point.  I then intend to use fiberglass batts.  My question has to do with the depth of the combination wall – 5 1/2 studs plus 1″ of foam plus 3/4″ of furring.  So the cavity to be filled is 7 1/4″ minus the 1″ of foam or 6 1/4″ of final depth.  If I use 5 1/2 fiberglass batts, I end up with 3/4″ of air space between the foam and the fiberglass.  Is this acceptable?  Any thoughts are appreciated.