A "Builders Guide to Frost Protected Shallow Foundations" by the National Assoc of Home Builders:
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http://www.toolbase.org/docs/SubsystemNav/Foundations/4495_RevisedFPSFGuide.pdf
EDIT UPDATE(Aug 2006): The above site has been so redesigned that I almost could not find the original info. Here it is:
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http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf
Great report! The best I've seen on the subject. Also has good construction details and charts of insulation values for building materials as well as foam insulation products.