  Type II supernovae (SNe), probably the most important contributors to stellar
feedback in galaxy formation, explode within the very dense star-forming
clouds, where the injected energy is most easily radiated away. The efficiency
of type II SNe in injecting energy into the interstellar medium (ISM) and in
re-heating a fraction of the original star-forming cloud is estimated with the
aid of a two-phase model for the ISM of the cloud. We argue that when SNe
explode the star-forming cloud has already been partially destroyed by ionizing
light and winds from massive stars. SN remnants (SNRs) will first cause the
collapse of most of the cloud gas into cold fragments, until the diffuse hot
phase has a low enough density to make further radiative losses negligible.
This is completed in ~3 Myr, with a modest energy loss of ~5 per cent of the
total budget. We compute that a fraction ranging from 5 to 30 per cent of the
cloud is reheated to a high temperature (from 10^5 to 10^7 K); these numbers
are very uncertain, due to the very complicated nature of the problem, and to
the uncertain role of thermal evaporation. Small star-forming clouds, less
massive than ~10^4 Msun, will be destroyed by a single SN. In all cases, a high
fraction of the energy from type II SNe (~80 per cent for large clouds, smaller
but still significant for small clouds) will be available for heating the ISM.
