Air Sealing is incredibly important. Bugs, even rodents get in as your money leaks out. In short, every wire, pipe, light, window, door, skylight, bath fan and duct should be absolutely air tight. In addition to those smaller air leaks, there are many large air leaks. Uncapped chases and top-plates, poor air-barrier alignment at showers, tubs and window seats, and exposed attic wall insulation are very common. Here is a technician using gun-foam to seal the top-plates of an older home. The idea here, is to keep air from inside the wall from getting in the attic. You can’t seal this leak from below. These “continuous leaks” above every interior and exterior wall on the upper floor are the reason you see staining on carpets around the perimeter of rooms.

Crawlspaces also allow for air-loss but the total amount is much less due to the fact that on top of the floor system, a CONTINUOUS sub-flooring is installed before the walls are erected. As you can see in the above photo, the drywall is not continuous as it is installed after the ceiling structure, and ceiling drywall is pieced together leaving very little to stop air from inside your walls from communicating with the attic air. For crawlspaces, the most valuable air-sealing is to actually plug the foundation vents which is a “whole nother” story.