Effect of bottom surface thermal energy loss for a packed bed solar air heater is investigated using both one-dimensional transient and steady-state models. While the former is solved numerically, closed-form solution is obtained for the latter. The effect of variation in base insulation thickness on the system output is studied. For a given bottom insulation thickness, the dependence of its effectiveness on various thermo-geometric parameters is also analyzed. It is observed that a collector with an uninsulated base loses about 60% of the available incident solar energy. In comparison, when the base insulation is as thick as the base wall, i.e., 50 mm here, the base loss fraction drops to nearly 6%, thus highlighting the importance of base insulation. Further, it is seen that the efficiency of a particular base insulation thickness lessens with larger length and width of collector, and rises with a larger mass flowrate of air flowing through it. This work presents a mathematical tool to calculate appropriate insulation thermal resistance to be applied at the base of packed bed solar air heaters that yields the best possible thermal performance alongside minimum insulation cost.