where I'm from, Duluth, MN, the airspace is required by code. I think
it is important to use a chute the entire length, not only because it
ensures airflow, but it also reduces "windwash" which strips heat away
from your insulation. I like to use cardboard vent chutes (available
at good lumberyards). I also think adding a layer of foam to the
underside is a good idea. even a layer of 1/2" polyisocyanurate
(thermax is a good brand) would reduce thermal bridging and provide an
air/moisture barrier.