Air Leaking Out Causes Air To Leak In
When you heat the air in your home it becomes lighter than colder air. This buoyant air rises to the top of your home and leaks out dozens of places. Since your attic is vented, think of the attic as outside.
Think of a cubic foot of air as roughly the size of a basketball if it were square. For every basketball of air that leaks out of your home in the attic, another basketball of air has to leak in at the bottom.
A Cycle That Never Stops
The air that leaks out take your heat, comfort, and money with it. The air that leaks in is cold in the winter and is hot and humid in the summer making you uncomfortable. During the winter you have to heat the new air so you can be comfortable, but once you do that, it rises and leaks out of the top causing a cycle that never stops.
The only reason outside air leaks in is because the inside air leaked out. If no air leaked out, there would not be any room for new air to leak in.
In this way we know that by sealing air leaks in the top of your home, or equally the other way around we can make rooms at the top/bottom of the home less drafty, making your home more energy efficient and comfortable through any season.