We present a theoretical analysis of the above-surface neutralization of highly charged ions in front of LiF. The study is based on the assumption that the dominant electron transfer occurs in the classically allowed region. Estimates of critical distances and corresponding quantum numbers for capture from an ionic crystal within the classical over-barrier (COB) model are presented, which differ considerably from corresponding results for metals. The role of the dielectric response of LiF is investigated. In addition, classical trajectory simulations are performed for the first time for a slow highly charged ion approaching an insulator. It is shown that capture effectively begins about 3 a.u. closer to the surface than estimated from the COB model. This correction can be incorporated into a modified COB model. The energy gain for grazing incidence ions is obtained using a staircase model which includes the deceleration due to charge-up of the surface.