HouseChalk
Rim joist
The rim joist, also called the band joist, is the board that sits on top of the foundation wall and caps the ends of the floor joists.
Why it matters
Rim joists are a major source of air leaks and thermal bridging. Air sealing and insulating the rim joist is one of the highest-impact energy improvements in many homes.
Where people get this wrong
Rim joists are often overlooked. Gaps around wires and pipes at the rim joist can leak significant air. Spray foam is a common solution here.
Real-world example
You feel a cold draft near the baseboard in a first-floor room. The rim joist behind that wall was never air sealed, and cold is transferring straight through.
Where this hits your build
This shows up during framing. Once walls are up and covered, you lose access. The pre-drywall walkthrough is your window to verify.
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