Right now is a great time to get a free boost in cooling your home. This is because, at night, the outside air temperature is typically much lower than the temperature you would like to cool your home. This time of year, there are many nights where the temperatures will get down to 55 - 60 degrees, along with a low relative humidity.
If you allow the night air in, on those dry cool nights, then you can reduce the retained heat in your house - both in the air in the house, but more imporantly, in the wall surfaces, the floors, the furnature...everthing in the house can have the energy reduce, for free, if you allow the cool night air to extract the heat from all of those elements.
Because hot air rises, and cool air sinks, it's my goal to vent the hot air up and out of our house when the weather permits. That means we open our attic door, allowing the hot air to rise up into the attic, where it is vented up into the atmosphere. This also helps reduce the retained heat in the attic, which means there's less retained heat in our 2nd floor ceilings that the air conditioning system would have to overcome.
To assist with this, we open downstairs, outside doors and/or windows while having the attic door open. This improves the amount of air that can be exchanged. Since hot air rises, in this case going up through our attic, we want to replace this air by drawing in outside cool air on the lower levels. This cooler air on the first floor displaces the warm air, assisting it in the hot air's movement up and out of our attic.
On those nights, like now, when the outside air temperature has a 55 - 65 dF temperature, then letting that cooler air into our home allows all the items in our home to swap out their retained heat, gradually lowering the retained heat in every item in our home. This lower retained energy in all of the objects in our home means our air-conditioning system has an easier time to maintane the lower temperatures we want when the outside air temperature starts climbing.
So, when do we close the attic door? Any time the outside air is warmer than the desired temperature in our home, we close down the draft by closing those attic doors and other open windows, or anytime we are leaving the house. Another consideration would be - once you've allowed the heat of objects in the home to be exchanged, then anytime the outside temperature is above the temperature of those objects, then you might want to close down the outside ventilation. An easy way to determine this? Your home thermostate, which is usually mounted to a wall, will also be influenced by the temperature of the wall. So by comparing the temperature of the thermostate (not the setpoint) to that of the outside air, anytime the outside air is at or above that temperature, you might want to close down the outside ventilation. So, for example, if my interior thermostat is showing 65 dF, then anytime the outside air gets at or above 65 dF, I want to close down the attic, windows and doors.
As a quick estimate, our typical temperature swings here in the Atlanta area (ignoring storms, fronts) are about 20 degrees. So, unless something unusual is going on (e.g., cloud cover, front, etc), then on average, I can expect on a day where the high temperature is 80 dF, to have a nighttime low temperature of 60 dF, or 20 degrees cooler. At high daytime temperatures of 90 dF, an average swing to low would be 70 dF, and the cooling from the outside air is less benificial. Above a daytime high of 90 and it probably would not be benificial to exchange night air from a temperature standpoint.