The attached workbook provides instructions and calculations that will allow you to use a single area to enter the average characteristic of multiple areas. This calculator can be used for above grade walls, attics, ceilings, and framed floors (over crawl spaces, garages, etc).
Attaching V3 of spreadsheet which now supports combining floors over garage with floors over ambient (e.g. cantilever/overhang) by including the % Temper input.