Looking for some advice on air sealing while I have the baseboards off. – GreenBuildingAdvisor

House was built in 1993 in zone 4A. Slab on grade construction.

I’ve currently got all of the baseboards removed in preparation to install new flooring and since this is the only time I’ll have the access to the bottom plate area, I was hoping to air seal wherever necessary. What I’ve found, though, is that there is surprisingly little guidance online showing the specifics to the lay person. The closest I’ve found is this Matt Risinger video: https://www.youtube.com/watch?v=w0DlMQg1Jb4

On the main level, I’ve found some pretty big gaps between the slab and the foundation wall. Near the edge of the slab, the concrete is thin, cracked and loose, and is sitting on top of what appears to be styrofoam. I was thinking I should remove the loose concrete and fill the gap with Sikaflex self leveling concrete sealant.

Upstairs, there is a bonus room with knee wall attics. I’m unsure if I should air seal from inside the attic, or from the bonus room. I’ve read Martin Holladay’s articles on the challenges of air sealing bonus rooms, but I think mine is slightly different from the ones he’s discussed because the subfloor seems continuous, and there is a bottom plate sitting on top of that continuous subfloor. 

So I guess here are my questions:

1. How should I deal with the big gaps and the loose concrete around the perimeter of the slab?

2. Where should I air seal in the bonus room/knee wall attic? If I seal from the bonus room, my understanding is that I place a bead between the bottom plate and the subfloor, but isn’t there also a leaky gap where the drywall touches the bottom plate?

3. Should I also air seal the bottom plate on the entirety of the 2nd floor? And again, do I just seal the gap where the bottom plate touches the subfloor, or should I also try to seal the drywall-to-plate junction too?

I am just a motivated lay person who is trying learn to do this the right way and I appreciate as much hand-holding as you all are willing to give.