Building Homes or Extensions/crawl space insulation
I plan to build a home in Gem County Idaho (climate zone 5, near Emmett.) Code adopted there is IRC 2009. I've not worked with that code before. (And I don't live in that area now.)
House will have ideal conditions for closed crawl space, dry climate, reasonably well drained soil, top of hill, no excessive water anticipated. This seems a natural application for rigid EPS on the interior of the concrete stemwalls. However the 900 page IRC (if I interpret correctly) prohibits that unless a cost-prohibitive fire protection is added. Thermax apparently meet code, but again is cost prohibitive. That seems to leave fiberglass as the only reasonable option, and that has a spotty history in crawl spaces. I'm finding lots of Internet advice recommending EPS or XPS (as one would expect) but cannot find anything that says "Section XXX of the code allows it." Am I misinterpreting the code or missing something?
My interpretation is that the foam plastic insulation must be fire-retardant or be enclosed in a fire retardant material (like drywall).
There are several products out there that meet this requirement, such as Thermax. I'm not sure how you are defining "cost prohibitive" but the added cost of fire-retardant crawl space insulation, when compared to the whole expense of a new house, is fairly insignificant.
If your budget is so tight that the code compliant insulation kills the deal, then maybe you should consider a slightly smaller house, or a few less expensive features. But if the extra expense is simply bothering you because it seems so much more costly than you perceive it should, then you need to simply get past this and move ahead with the construction.
You could also consider insulating the space from the outside, but this comes with another set of issues.
Whatever you do, please don't consider installing drywall in the crawl space, due to significant mold risk. There are other covering alternatives, such as lightweight cement board, bit these will likely be as costly as the Thermax, and more labor intensive.
Best of luck, and please let me know if you have any other questions.