Framing
Rim Joist
The outermost joist running along the perimeter of a floor frame, perpendicular to the floor joists, that caps the joist ends and provides lateral stability.
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In the Field
The rim joist is a major energy loss location — insulate it well. In seismic zones, the rim joist connection to the mudsill (anchor bolts and hurricane ties) is critical for lateral resistance and is verified during framing inspection.
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Rule of Thumb
Insulate rim joists with rigid foam cut-and-cobble (2 inches of closed-cell foam) or spray foam for best performance. Batts alone don't seal the air gap at the rim.