Rim Joist
A rim joist is the board that caps the ends of floor joists around the perimeter of a floor system, forming the outer edge of the floor frame. It sits on top of the sill plate or wall below and provides lateral support to prevent the joists from rolling or tipping. The rim joist is also called a band joist or band board.
In-Depth Explanation
The rim joist serves multiple structural and practical functions in floor framing. Structurally, it closes off the ends of the floor joists, preventing them from rotating under load, and it transfers vertical loads from the wall above to the wall or foundation below. The rim joist also provides a nailing surface for exterior sheathing and siding, and it defines the thickness of the floor system visible from outside the building. In energy-efficient construction, the rim joist area is a notorious source of air leakage and heat loss because it sits at the junction of the foundation and floor system where multiple materials meet. Insulating and air-sealing the rim joist with spray foam or rigid foam is one of the most cost-effective energy upgrades in existing homes. The rim joist is also where deck ledger boards are attached, making it a critical structural element for deck connections. Engineered I-joists require specially manufactured rim boards rather than standard dimensional lumber because they are deeper and need a matching rim to maintain alignment.
Usage Example
The energy auditor recommended spray-foaming the rim joist in the basement as the single most impactful air-sealing measure for the home.