What would the "insulation rating" cover? E.g. the R-value of the stuff inside walls, which wouldn't cover windows or gaps or lots of other things, or do they somehow measure the R-value of the house as a whole?

Every house and rental I've encountered so far has given me the monthly heating/cooling cost up-front, which has so far been fairly accurate, assuming you hold at an area-normal temperature indoors. And they've all been able to tell me the R-value of the insulation in the walls and roof or attic. If ever it's more than like $100 for a single-family home, you pretty much know the insulation is bad or terrible.