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.
Best Answer
Here's what may be happening...
At night, heat stored in the roof and walls due to solar gain soaks into the home's interior. This is balanced to some extent by cooling from outside air flowing through the home. Three outcomes can result:
The home cools somewhat if the inside/outside air temperature differential is substantial, and if there's a breeze to move air at a significant rate.
The home stays at the same temperature if there's not enough breeze and/or the temperature differential is small. Heat soak and cooling are essentially balanced.
The home actually warms, especially on the upper levels of a multi-level home, due to heat soak. This occurs if there's little or no breeze and the temperature differential is small.
The change you've noticed is probably related to lighter overnight winds and slightly reduced differential. If you don't have a differential of at least 10 degrees, and the wind is calm, you'll probably want to bite the bullet and keep the a/c on. Run circulating fans and/or the furnace fan to keep all areas of the home comfortable.
If you do open the home up in questionable conditions, run box or dedicated window fans to exhaust the home overnight. Run fans inward in sleeping rooms for maximum comfort.