How to Insulate Basement Rim Joists ~~ Cut heat loss through the rim joists and tighten up your house with rigid foam insulation. In just a couple of hours, you can seal and insulate your rim joists, which are major sources of heat loss in many homes. This project will help lower your heating costs and save you money. Insulating the rim joists is one of the best things you can do to make your home more energy efficient. And it’s easy, too, so anyone can do it.