  We use a functional approach to calculate the Casimir energy due to Dirac
fields in interaction with thin, flat, parallel walls, which implement
imperfect bag-like boundary conditions. These are simulated by the introduction
of delta-like interactions with the walls. We show that, with a proper choice
for the corresponding coupling constants, bag-model boundary condition are
properly implemented. We obtain explicit expressions for the energies in 1+1
and 3+1 dimensions, for massless and massive fields.
