Lets say I calc the R of the walls, the R of the ceiling, the R of the floor, and the R of the windows. A bunch of resistors in parallel. Do I need to calc 1/Rtot = 1/Rwall+1/Rceil+1/Rfloor+1/Rwindows? Or can I solve BTU=hrs*sqft*degF/R for each surface and add up the BTUs?