Assessing the reliability of low dielectric constant materials is an important problem for the scalability of integrated circuits to reduced dimensions. Here we report results of a study of the influence of moisture incursion on dielectric relaxation and reliability in porous low-k (2.55) dielectric constant BEOL capacitors. We apply impedance spectroscopy to measure the dielectric loss tangent as a function of temperature and frequency. From the results, we identify two relaxation modes of water in the porous dielectric, exhibiting thermal activation energies of 0.32 eV and 0.56 eV. Study of the lifetime against time-dependent dielectric breakdown (TDDB) as a function of the dielectric loss tangent measured at AC voltages below 0.5 V yields an inverse power-law relationship. The dielectric loss tangent thus has the potential to serve as a low-voltage test of the susceptibility to early failure and to provide a link between high voltage, accelerated TDDB testing and the underlying physical processes responsible for dielectric breakdown.