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Brief update. There are two answers effectively. I contacted IES regarding the adjacent building and how best to handle them. The question I asked on how to best deal with adjacent buildings. The response I received was to create a construction with 'very low U values' for the demising walls; effectively making it adiabatic. Which, with an adjacency to a conditioned space this makes sense. These spaces are classified as 'exclude from square footage' within IES. From what I can tell, this appears to be an approach that works.

The second part is that I have a separate inquiry into IES concerning wall construction layers; part of which deals with material thicknesses. Currently, while building constructions, the material thickness can be adjusted, and IES displays an updated u-value. This is expected, as I'd be more concerned if a change in insulation thickness did not yield a u-value change. The issue discovered through many runs and examination of output files is that while it appears this u-value as an input, the actual simulated u-value is only for the original thickness of the material - which is based on the CBECC-Com approved list of materials. IES must be passing [or only allowed to pass] the material name/designation and not any of the modified parameters [re: thickness]. It is a communication breakdown. I have not seen any IES documentation that explicitly states that this is how materials are handled; but it also does not surprise me with the restrictive nature of the CBECC-Com engine. Even a static text warning that modifications dont matter would suffice, but I digress. The solution here that I have found is to either use a different material OR to use multiple layers of the material needed. So, rather than selecting a single layer of EPS, which comes in at 2", and then modifying the thickness to 4", which does update the displayed u-value; do two layers of the EPS, which add to 4". Note, this is predominately aimed at continuous materials [continuous insulation or thickness of built-up other materials; NOT framed constructions which are handled differently]. This initially impacted my demising walls and basement slab; causing the massive phantom heating load referenced in the question.

I will updated this based on what I hear back from IES on the second issue.