Rim joists, which are the areas above the foundation wall of your home, are major sources of energy loss. You can seal and insulate yours in just a couple of hours.
My preferred method is to use spray foam. This both insulates and seals the area.
You can get the spray foam kits, which include everything except for the mask and coverall from Home Depot.
You can find out how to do this here: https://youtu.be/pA_usBEXrqc