Using cool nights for a cooler home during the day at no cost
In cooler climates like here in Maine, even as we swelter through hot summer days we still typically enjoy cooler, more comfortable nights. Even when temperatures push above 90°F (32°C) nights can drop to around 70°F (21°C) at night. This temperature swing provides for a cheap and easy means of helping to keep a home cooler on hot days without the need for air conditioning.
The strategy could be summed up with four words: "which side is cooler." At night when the temperatures drop below indoor temperatures, open all your home's windows to allow in this cooler air. In the morning before work, and/or before the outside temperatures rise above indoor temperatures close all the windows and in unused rooms, close the blinds. Closing the windows will keep the hot air out and closing the blinds will reduce solar heat gain.
The "which side is cooler" tactic will be particularly effective if your home has been weatherized to reduce drafts, has plenty of insulation in the attic and has proper attic venting. The attic insulation and venting is particularly important as they will reduce heat transfer from attic to the living space and reduce attic heat gain respectively.
Over the past couple of weeks we have been routinely experiencing outdoor temperatures over 90°F (32°C) by mid morning. Despite this, but deploying the tactic of opening windows at night and closing them in the day, we've been able to keep our home's indoor temperatures below 80°F (27°C). It may still be warmer than we prefer inside, but it is not uncomfortably hot and we haven't felt a desperate desire to go buy an air conditioner.
Obviously this tactic won't work if brutally hot days are followed by sweltering hot nights. For those climates with cooler night time temperatures, however, this could tremendously reduce/eliminate cooling bills while providing for a comfortable home even on really hot days. Best of all, there is no cost to deploy this home cooling strategy.