A study of the efficiency of a planar leaky-wave antenna in half-width substrate integrated waveguide technology with low-cost and lossy material is presented. Theoretical results show the limitations in the performance of leaky-wave antennas using low-cost substrates in terms of radiation, dissipation, spillover and aperture efficiencies. It is shown that, for a given lossy substrate, the maximum achievable radiation efficiency is related to the dissipation rate. Also, a maximum useful antenna length is reported for a given substrate.