A very typical location for air leakage in and out of a home is in the basement at the top of the wall around the exterior perimeter. We here at Comfenergy call that the rim joist. As air leaks into the home at the lower levels like the basement it brings with it dust, pollen, and other outdoor pollutants with it. If you don't believe me, pull a piece of the fiberglass insulation out of the rim joists and turn it around and look at it.
Dirty, right??? That the fiberglass insulation doing two things:
1. Allowing air to travel into the home.
2. Filtering the outdoor pollutants as evidenced by the dirt on the back side.
Comfenergy uses a closed cell spray foam to seal up these leaks and offer a better insulation value in at the rim joists.