  The role of matter circulation between the disk and halo in establishing the
volume filling factors of the different ISM phases in the Galactic disk is
investigated, using a modified version of the three-dimensional
supernova-driven ISM model of Avillez (2000). We carried out adaptive mesh
refinement simulations of the ISM with five supernova rates (in units of the
Galactic value), $\sigma/\sigma_{Gal}=1$, 2, 4, 8 and 16 (corresponding to
starburst conditions) using three finer level resolutions of 2.5, 1.25 and
0.625 pc, allowing us to understand how resolution would affect the volumes of
gas phases in pressure equilibrium. We find that the volume filling factors of
the different ISM phases depend sensitively on the existence of a duty cycle
between the disk and halo acting as a pressure release valve for the hot (T>
10^5.5 K) phase in the disk.
