Air Sealing
Rim Joist
A rim joist also known as band joist is a horizontal board that runs along the outer edge of a floor or deck frame, perpendicular to the main joists, providing lateral support and capping the ends of the joists. They are an often-overlooked connection when it comes to air sealing. While sealing a second-floor rim joist retroactively can be tricky the joist is often exposed in unfinished basements and thus a very low hanging fruit for tightening your envelope.
Attics
There are two types of attics that require different air sealing strategies in order to avoid condensation issues, vented and unvented.
A vented attic is unconditioned and often has vents at the soffits and ridge or at the gable ends. The goal of a vented attic is to match the outdoor temperature to avoid condensation issues at the roof. In order for a vented attic to work successfully the ceiling plane of the floor below needs to be airtight. Leaks in this plane create opportunities for heat and moisture to escape creating condensation risks. Sealing needs to happen prior to the installation of insulation. For new construction homes a smart membrane can be used to achieve an air barrier. However, simply using the drywall of the ceiling also works and is often easier for existing homes.
An unvented attic is conditioned and the air barrier and insulation shift to the underside of the roof in order to keep the exterior conditions outside. Unvented attics are less common in existing homes as the assembly is more complex and less forgiving. They are also more difficult to retrofit, especially if you want to avoid spray foam, and are therefore typically seen in new construction. However, they are useful if you want to use the attic to house mechanical equipment or as extra living space.