Recently, we've started using the AC less for cost and environmental reasons. When it gets below 82 degrees (F) at night, we open the windows. My understanding is that the cool air from outside should eventually replace the warm air in the house.
This generally has worked ok the past few months, but recently, it seems to be less effective. Last night, it got down to 76 degrees around 9 PM, 73 degrees when I woke up at 7 AM, but the temperature in the house didn't drop at all from 80 degrees according to our thermostat, which was set to circ with the AC completely turned off. All windows in the house were open. The thermostat is in the front part of the house, a 1,200 sq foot ranch. (This part of the house is an open kitchen/living area with eight windows, so there should be plenty of airflow.) We do not have an attic above that part of the house, but we do have an attic through the rest of the house.
We live in a suburb of Philadelphia, for climate information. It has been pretty hot lately. Is this due to hotter weather recently, a thermal mass problem, airflow problem, inaccurate thermostat, something else? I'm new to passive cooling, but I feel like with a 7 degree difference between outside and inside, the temp should have dropped at least a degree or two. Not sure what to make of this exactly. Any insight would be great.