Heat Follows Cold

If it’s 98 degrees outside and, thanks to central air conditioning, a comfy 72 degrees inside your house and you open a window, the heat from outside will jump right in through the window and keep jumping in until it’s just as hot inside as it is outside. Like water, heat constantly seeks equilibrium; heat moves to cold until everything is the same temperature.

Since you probably spend much of your summertime reminding the kids not to leave the doors open, you already know that opening a window when the AC is on is a dumb thing to do (unless you have an evaporative cooler, discussed below). But open windows and doors are just the largest and most obvious avenues for mingling indoor and outdoor temperatures. The smaller avenues, like gaps around light fixtures in your ceiling, are much less obvious and usually ignored, yet these are often the ones that matter the most.