For energy efficiency, when should windows be opened for overnight cooling?
We have a 2 story home with air conditioning in the Chicago area. In summer, we’ve done several things to improve our energy efficiency. Our a/c doesn’t need to run during the day, even on the hottest days. The interior temp rises gently up to 76-80F and, if needed, the programmed thermostat causes the a/c to run overnight (to 76F) when electricity is cheaper. If we know tomorrow will be 85F+ we set the overnight set point to 74-75F temporarily. It’s always cooler at night but the temp varies. On hot days it may be a low of say 75-80F overnight. On warm days the low may be 65-70F. We rarely have 80+F days and 55-60F overnight lows. It’s usually humid in the summers. Naturally when the nights are cool and crisp, we’d like to get some free cooling. Trouble is the outside is usually more humid than the interior because the a/c dehumidifies when it runs. We are satisfied with the ventilation we get so for this discussion, the window settings are only for free cooling.
From experience, I look for an inside/outside temp difference of about 10F. I go outside to still air and assess the humidity. If the humidity is low by feel (we have no sling psychrometer) and the 10F is there, we turn off the a/c and open selected windows. If there’s no breeze, we get some stack effect cooling but a breeze is better. We have exhaust fans but they consume electricity so we prefer they not run unless there’s a ventilation need. Sometimes I regret the windows open because we wind up running the a/c later just to lower the humidity. By early morning we usually have the windows closed again unless the seasons have changed.
I’ve looked at the psychometric chart again and read the NOAA tables on relative humidity comfort index but they seem to recommend opening the windows when I already know from experience just boosts the humidity and later forces us to run the a/c. If someone has a proven rule of their own, I’d like to hear it. I know if we lived in a more arid climate, I’d have the windows open more often. I realize personal preference is a factor. Those indifferent to humidity effects already enjoy an energy efficiency advantage.
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