Answer
COMcheck computes an envelope index that is a reflection of the difference between a 'design' building energy performance factor (EPF) and a 'code' or budget building EPF. The total building load (made up of cooling, heating, lighting, and miscellaneous plug load) is determined by the location you are in, the specific assembly and building types you specify, and internal gains from lighting and miscellaneous plug loads, and of course the thermal properties and orientation of the envelope assemblies. The methodology used to determine EPF is provided in ASHRAE 90.1 Normative Appendix C: Methodology for Building Envelope Trade-off Option.