The rim joist space is the perimeter area where the floor framing meets the outside walls or foundation. In a basement, it is near the top of the foundation wall, around the ends of the floor joists. It may also be called the joist header space or foundation header space. Gaps in this area can allow outdoor air to enter the home.
This area deserves attention before a basement’s walls or ceiling are closed. During basement renovation planning, ask how the existing framing will be inspected, air sealed and insulated. Insulation and air sealing serve different purposes, so adding insulation alone does not necessarily close leakage paths. Any dampness or damage should be assessed before it is covered, and the proposed materials should suit the existing construction.