  We study heat diffusion after an energy release in a deep spherical layer of
the outer neutron star crust (10^7 < \rho < 4 x 10^{11} g/cm^3). We demonstrate
that this layer possesses specific heat-accumulating properties, absorbing heat
and directing it mostly inside the star. It can absorb up to about
10^{43}-10^{44} erg due to its high heat capacity, until its temperature
exceeds T ~ 3 x 10^9 K and triggers a rapid neutrino cooling. A warm layer with
T ~ 10^8 - 3 x 10^9 K can serve as a good heat reservoir, which is thermally
decoupled from the inner crust and the stellar core for a few months. We
present a toy model to explore the heat diffusion within the heat-accumulating
layer, and we test this model using numerical simulations. We formulate some
generic features of the heat propagation which can be useful, for instance, for
the interpretation of superbursts in accreting neutron stars. We present a
self-similar analysis of late afterglow after such superbursts, which can be
helpful to estimate properties of bursting stars.
