Basically air moves through the building as if the building itself were a chimney, exhibiting the ‘stack effect’. Picture a wood burning stove that has the draft control on the front at the bottom. If you shut the draft control completely, your fire will go out; it will suffocate. If you open it completely your fire might burn wildly. Air goes in through the stove’s draft control; air goes out through the chimney. That’s the stack effect.
Short of teaching a lot of physics in the form of building science, suffice it to say that if air moves out of a building an equal amount will move in. The outdoor air moving in is unconditioned (not heated or cooled mechanically) and contains pollutants and moisture. The air moving out was heated or cooled (depending on the season) with your hard-earned dollars. Any movement of air means that air must have a place to travel, a hole or pathway, and it must be a difference in air pressure to send it on its way.
Think of a house with 2 floors and a basement. Now, open a window in the basement (just like the draft control on the imagined stove), you’ve made a hole. Because warm air rises, you now have a difference in air pressure. The top portion of the house will be higher in pressure, causing air to leak out any opening and then look out, here comes the draft. So, to complete the picture, open a window on the second floor and you have the stack effect in action, moving outside air in the basement window, through the house and out the upstairs window taking your heated or cooled (conditioned) air out with it. When that unconditioned air is moving from outside to inside and up through the house, it is taking moisture with it and unwanted moisture in places it does not belong, like inside walls, can be very bad.
Now, knowing what you know, I hope this motivates you to take some simple action to stop that air leakage in your home. Caulk, foam backer rod (for large gaps) and minimal expanding spray foam insulation are your tools of choice. Just ask the folks at the local home improvement store for help in locating what you need.
Because of the stack effect, air-sealing should start on the lowest level of the house then move upward, air sealing on one floor at a time. Find the drafts using your hands, by noticing spider webs moving in the draft, or perhaps using a lit incense stick or fireworks punk to observe the smoke movement (please be careful with using anything flammable). Now, once you know where the leaks are, seal them using caulk for smaller holes and minimal expanding spray foam insulation for larger holes. If the gap is large, you might need foam backer rod to insert in the gap before using caulk or spray foam.