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Should I close off bottom of wall cavities before sealing sill bays in crawlspace?

aklab | Posted in Energy Efficiency and Durability on

Hi everyone, I am working on sealing, insulating, and conditioning my 100-year-old house’s crawl space in South Carolina, zone 3. The vented crawl space has been a great place for pests, mold, and moisture over the years. Fortunately the foundation walls are over 3′ high so I have plenty of room to work with down there. 

My main question right now is about sealing and insulating the sill bays. In videos and demonstrations I see people pressing the foam board all the way back against what I guess is the rim joist. However, my house has a single large beam directly on the brick piers, which supports the joists and studs. No sheathing, just siding on the other side. First picture shows this. 

Because I figured I needed some air behind the siding to avoid condensation there, I’ve been putting my foam board closer to the interior edge of the “sill” like in the second picture. 

But, I’ve found the wall cavities are completely empty, except for a few places where the previous owners had batts put in while doing other repairs. Third pic shows one of the cavities. Do I need to be sealing the bottom of these before sealing each sill bay with the foam board? 

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