The ionic conductivity of the superionic conductor RbAg4I5 was studied at low temperature using a modified electrochemical impedance spectroscopy (EIS) as well as dielectric measurements (DM). Experiments using RbAg4I5 samples of constant crystal imperfection density were carried out in the temperature range 50 K less than or equal to T less than or equal to 110 K and in the frequency range 10(1) Hz less than or equal to f less than or equal to 10(7) Hz. The current data are in good agreement with previous low-frequency EIS measurements carried out in the frequency range 10(-3) Hz less than or equal to f less than or equal to 10(5) Hz and high-frequency wave-guide measurements in the frequency range 18 GHz less than or equal to f less than or equal to 40 GHz. The results confirm a nearly temperature-independent low-frequency ionic conductivity below a transition temperature T < T-t = (75+/-15) K which is caused by a change of the ion conduction mechanism. At T > T-t, the ion transport occurs through grains and subgrains crossing grain and subgrain boundaries, whereas at T < T,, the ion transport takes place only via crystal imperfections, i.e. along grain and subgrain boundaries. Finally, the new results presented feature a frequency dispersion of the ionic conductivity in the mid frequency range which follows a power law with an exponent of p = 1 +/- 0.1, thus confirming the so-called 'new universality' law.