RHP not maintaining cage temp

Hello everybody, I have a question about my RHP not heating the tank enough. I have a 80watt RHP hooked to a herpstat 4 in a AP T8. It had been fine but with a slight cold snap doesn't seem to keep the temps up (house is 64). The thermostat temp is 5 degrees under what I have it set at and its at 100% power most of the time. The basking spot is still at 90 but the rest of the cage is 80 to 73. Should I have purchased the 120 watt RHP or is something wrong with my system. I thought the 80 watt would work. Thanks