  We present a new (semi-)analytic model for feedback in galaxy formation. The
ISM is modeled as a two-phase medium in pressure equilibrium. The remnants of
exploding type II SNe percolate into super-bubbles (SBs) that sweep the ISM,
heating the hot phase (if the SB is adiabatic) or cooling it (in the snowplow
stage, when the interior gas of the SB has cooled). The resulting feedback
regimes occur in well-defined regions of the space defined by vertical
scale-length and surface density of the structure. When SBs blow out in the
adiabatic regime, the efficiency of SNe in heating the ISM is ~5 per cent, with
\~80 per cent of the energy budget injected into the external halo, and the
outcoming ISM is self-regulated to a state similar to that found in the Milky
Way. Feedback is most efficient when SBs are pressure-confined in the adiabatic
regime. In some significant regions of the parameter space confinement takes
place in the snowplow stage; then the hot phase has a lower temperature and
star formation is quicker. In some critical cases, the hot phase is strongly
depleted and the cold phase percolates the whole volume, giving rise to a
sudden burst of star formation. Strong galactic winds are predicted to happen
only in critical cases. This model provides a starting point for constructing a
realistic grid of feedback solutions to be used in galaxy formation codes. The
predictive power of this model extends to many properties of the ISM, so that
most parameters can be constrained by reproducing the main properties of the
Milky Way. (Abridged)
