If you're resting the fireblocking on top of the foundation instead of beside it as illustrated below, first install a capillary break, the simplest being closed-cell foam. If you're following the illustration below, it would make sense to use handheld spray foam to fill the gap shown.

Some renovators/builders insist on treated wood for all below-grade framing, regardless of local requirements. FWIW, I don't.

Fireblocking details per IRC 2012. (Fairfax County, Va.)

Thanks. I plan on doing the short walls like this, but for the ones with joists perpendicular, that's a ton of cuts to make. I'll have 1" of XPS foam behind the studs walls, so something needs to cap those in the joist bays. I may rip some 3/4" OSB and hold it flush against the plate and foam the remaining gap for air and water barrier, but the plate is surprisingly non-straight. Hopefully a couple irregular gaps in the back filled with fire foam won't set off a red flag.

Dan

From the same source above, here are some further diagrams illustrating fireblocking for offset or perpendicular framing.

The relevant part of IRC 2012 is 302.11.