On top of letting cold air in during the winter months, and letting hot air in during the summer, unsealed rim joists are the easiest point of entry for spiders, insects, and even small rodents. A full seal can dramatically reduce their presence in the home.
The air entering around the base of the home, just above the foundation, is often damp and this can result in increased mold growth in the basement. Because basement air is drawn up into the living area, this can contribute to health problems. Seal out the moisture, and the basement can remain dry and free of mold.
A variety of openings, in addition to the gaps between abutting rim joists, make the area prone to major air leaks. Plumbing penetrations (for example, a pipe for an outdoor garden hose faucet), and electrical wiring (for an outdoor meter) are common examples.
It’s easy to see why fiberglass batts are not a good solution. Moist air and condensation on pipes and on the rim joists render it ineffective, and it doesn’t block air flow into the basement. Only foam products create a vapor barrier while also providing good thermal performance, impervious to moist air and condensation.