  The effect of on-site electron-electron repulsion $U$ in a band insulator is
explored for a bilayer Hubbard Hamiltonian with opposite sign hopping in the
two sheets. The ground state phase diagram is determined at half-filling in the
plane of $U$ and the interplanar hybridization $V$ through a computation of the
antiferromagnetic (AF) structure factor, local moments, single particle and
spin wave spectra, and spin correlations. Unlike the case of the ionic Hubbard
model, no evidence is found for a metallic phase intervening between the Mott
and band insulators. Instead, upon increase of $U$ at large $V$, the behavior
of the local moments and of single-particle spectra give quantitative evidence
of a crossover to a Mott insulator state preceding the onset of magnetic order.
Our conclusions generalize those of single-site dynamical mean field theory,
and show that including interlayer correlations results in an increase of the
single particle gap with $U$.
