Insulating crawlspace floor joists vs. walls
We have a home near Raleigh, NC. The home was built in the 1970s on a vented crawlspace on sandy soil. It has batt insulation between the floor joists. The rim joist and the sill plate are untreated lumber on block walls with 2 courses of brick at top of the block.
The crawl space has had moisture problems which have been largely addressed with plastic moisture barrier and 2 sump pumps with perimeter drain and a dehumidifier. There is indication of previous termite problems in rim joist and sill at front and back door but no apparent problems elsewhere.
We would like to improve the insulation and create an unvented crawlspace. We are weighing the options of insulating the crawlspace walls vs. insulating the floor joists. We are concerned about future monitoring for termites and what to do about the rim joist!?
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You can insulate the walls with 2-3" of rigid foam, stopping 3" from the top of the foundation as a termite inspection gap. Insulate the band joist with cut'n'cobbled rigid foam can-sealed into place. Sculpt some R15 high density batts for insulating over the inspection gap.
Here is a link to an article that should answer your questions and tell you everything you need to know on this topic: Building an Unvented Crawl Space.