Tom,
That's why we need more info. In my experience, most crawl spaces are below grade and the walls are not exposed to air, except for the top 8" or so. Some insulation for the top 2'-3' might be useful, but below that the soil stays pretty constant temp --depending on location.
Agree completely on the benefit of covering the soil with plastic to reduce moisture.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags