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I used this DoE calculator:

http://web.ornl.gov/sci/roofs+walls/insulation/ins_16.html

For Washington, DC, it recommended R25 for crawlspace walls and R11 for basement walls. Why would they be different? What is the recommended value for above-grade vs below-grade concrete block walls?

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The earth is a good insulator itself relative to cold air that may circulate outside a crawl space. The temp of the ground even few feet down is much warmer (or, in the summer, colder) and more stable than the outside air. If the air outside is in the 20s, the soil even a few feet down is much warmer (in climes as far south as yours). Ground source heat pumps take advantage of this fact.

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  • Ah, so by "crawlspace" it essentially means above-grade wall and "basement" means below-grade wall? – Scott Stafford Oct 16 '14 at 14:14
  • Yes, that is the important distinction... above vs below grade. After all, you can have a "basement" with with walls exposed to outside air. Conversely, my house is on a slope, and most of the "crawlspace" is below grade. So yes, where earth insulates on the outside, less insulation required inside. – bobfandango Oct 17 '14 at 15:25

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