This is a question that comes up a lot especially when modeling apartment and condo complexes where there are interior and exterior units nearly identical. In some cases an interior unit with a common wall or two will have a higher e-ratio than an exterior unit identically designed but with the common walls replaced with exterior walls. This does not mean that the interior unit is less efficient and in fact if you look at the cooling numbers and not the e-ratio you will often see they are lower than the exterior unit, but so is the standard reference home.
So why do we see these numbers change?
While changing a given wall from exterior to common in a multifamily project removes the load for this wall, it also affects performance compliance results according to the Glazing section of Table R405.5.2(1) of the Florida Energy Conservation Code. Specifically in the 2017 Code footnote "h", and in the 2014 Code footnote "b", of the table reduces the reference house's glazing area based on common wall area. This in turn reduces the reference house's loads, raising the e-Ratio for the proposed house. The logic behind this code adjustment is that a house with less exterior wall area will typically have less glass area. Note that a similar reference house adjustment was included in the 2010 code as well. The software calculation follows the energy code. To make a change to this phenomena requires a change in the Florida building code.
The following results show two identical houses with one thing changed. In the first home on the left there are no neighbor walls out of five total walls. In the second home on the right there are two neighbor walls out of five. Otherwise they are geometrically identical. Notice how in the left home without neighbor walls the proposed heating and cooling loads are higher so the over all home is less efficient than the proposed one of the right with neighbor walls. However you can see that the standard reference is also high in this home. In the home with the neighbor walls the standard reference loads decreased significantly. Because the reference is now so much more efficient it drives the e-ratio up even though the proposed home is more efficient as well.
