A rim joist is a horizontal framing member running along the outer perimeter of a floor system, perpendicular to the ends of the floor joists, closing off the floor structure’s edge and providing a surface for attaching sheathing and the wall framing above.
This member also transfers loads from the wall above down through the floor system to the foundation below, making it a genuinely load-bearing component rather than simply a decorative edge piece.
Because the rim joist sits directly at the transition between floor levels or between the foundation and the first floor, it’s a common location for both structural connections and energy performance challenges.
What Does a Rim Joist Involve?
- Horizontal lumber running along the floor system’s perimeter
- Connection to the ends of the floor joists
- A bearing surface for the wall framing above
- Insulation and air sealing, particularly at foundation transitions
Why Are Rim Joists Important?
Serving as both a structural connection point and the closure for an entire floor system’s perimeter, the rim joist plays a role in transferring loads, providing lateral stability, and, particularly at a foundation transition, significantly affecting a home’s overall energy performance. This area is frequently under-insulated in older homes, making it a common and often cost-effective target for energy efficiency improvements.
Because the rim joist area typically has numerous small gaps and penetrations, thorough air sealing here, often using spray foam, addresses both insulation and air leakage simultaneously in one of the more consequential areas of a typical home’s envelope.
Rim Joist vs. End Joist
A rim joist runs along the perimeter, perpendicular to the floor joist ends. An end joist is the last floor joist within the system, running parallel to the others but positioned at the very edge of the floor’s span, distinct from the perpendicular rim joist enclosing that edge.