The humble rim joist—that critical transition zone where your home's foundation meets its framing—might be the most overlooked vulnerability in your entire building envelope. Michael Church dives deep into this crucial component in week seven of the Crawl Space Repair Challenge, revealing why building science experts consider proper rim joist insulation a non-negotiable aspect of home performance.
Moisture management stands front and center in this detailed exploration of rim joist insulation options. Michael walks through the key differences between open cell and closed cell spray foam, explaining why the density, R-value, and moisture-blocking properties of closed cell (R7 per inch) often make it the preferred choice for rim joist applications. He shares a compelling personal anecdote about how closed cell foam's structural integrity once saved him from putting his foot through a ceiling—demonstrating the unexpected benefits beyond just thermal performance.
The critical prerequisite of ensuring dry conditions before installation becomes clear as Michael reveals shocking examples of wood rot discovered behind improperly installed insulation. His preferred installation method—combining foam board with spray foam—offers the perfect balance of performance and practicality, particularly when future maintenance access might be needed. Whether you're tackling this project yourself or hiring professionals, this episode equips you with the knowledge to ensure your rim joists contribute to your home's durability and efficiency rather than becoming its hidden weakness. Ready to seal those critical gaps in your home's thermal and moisture barriers? This episode shows you exactly how to do it right.

Support the show

Check out Michael's book, "Crawl Space Repair Myths-Busted" now available on Amazon!

Schedule a consultation: https://buymeacoffee.com/csninja/extras
Need help: https://crawlspaceninja.com or https://supply.crawlspaceninja.com