A new concept for use of phase change material (PCM) in building envelopes has been investigated. The concept is called a RCR system in analogy to an electrical circuit with a capacitor between two resistors. Here, the thermal capacitance of the PCM is sandwiched between the thermal resistance of conventional insulation. The PCM used was hydrated calcium chloride dispersed in perlite and contained in watertight test cells. One cell had a PCM/perlite ratio of 2:1 by weight; the other had a 6:1 mixture. Extruded polystyrene (XPS) was the insulation below and above the PCM. Heat-flux transducers on the top and bottom of each cell as well as thermocouples from the top to the bottom of each cell allowed us to follow closely the progression of freezing and melting in the PCM as we subjected the cells to both steady and diurnally varying simulated outside temperatures. Computer modeling with a transient heat conduction program was successful in proving that we understood the relevant energy transfer mechanisms and thermophysical properties. For the diurnal cycles, with twice the amount of XPS below as above the PCM, much of the energy stored during daytime by melting PCM flowed to the outside at night when it froze again. Comparisons were made to the behavior of conventional insulation. With PCM, the total daily energy flow into the conditioned space below the test cells was lower and the peak flow rate was delayed in time and decreased in magnitude.