Question : Is there a way to calculate energy savings for external, 2inch insulation on basement walls that does not extend above the soil surface, leaving the top couple feet of foundation wall exposed to air.
Background: I am having my basement foundation walls waterproofed externally. The contractor is offering to provide external insulation for what seems like a reasonable price, and according so some research, there are some advantages to doing this externally if you have the opportunity. Some sources also suggest that the payoff is rather quick, 2~5 years (but did not state what assumptions were behind that).
The hesitation comes from the fact that what is under discussion is cutting the foam board short of the top of the soil. Basically if we wanted it to go all the way up (as would be ideal) I'd need a different contractor to tie into the siding and protect the exposed insulation. Currently there is about 2.5ft of exposed block below the start of the siding, which is the norm for this neighborhood. I do know that a large part of the value of the insulation comes from the top few feet below the soil and the part exposed to the air.
I do not plan on finishing my basement in the near term or trying to keep it the same temperature as the rest of the house. House is small (850sq foot bungalo), in lower Michigan.