Why You Should Seal Your Rim Joists
If you are making some improvements to your basement, you may also want to air seal your rim joists. They are the outer joists which support the under-flooring. Because of their position, they are vulnerable to moisture, mold, and other atmospheric problems that can affect the standard of your basement. Adding an air seal to a basement is a good way to ensure that the wood remains solid.
If you are trying to air seal your rim joists, then what you actually want to do is to protect it from moisture. Air carries droplets of water wherever it travels, including into wooden joists. Preventing air from moving around the joists also prevents water from accumulating. Stopping air flow is also important if you want to heat the basement, as changes between hot and cold air can crack the joists.
Another problem with wooden joists is that they are vulnerable to plant and insect damage. Air sealing your rim joists keeps them from being attacked by mold, which could rot the wood. This type of protection can also limit the amount of damage from termites, wood beetles, and wasps.