:::: I still object to the nomenclature used here. We should be able to define R-value without first having to defining some other symbol... deltaT / (Q/A) seems much preferable to me. If can be generalised with 3-D vector operators etc in the text body, but this simple 1-D definition is appropriate for the first cut.    

 :::: On the matter of U-value, the first place most people see U-value is in the context of heat exchanger analysis. We need to generalise our treatment of U-value as "overall heat transfer coefficient".