Spandrel panels are considered as opaque surfaces per the ASHRAE 90.1 definition. Kalwall and similar products count as fenestration since they allow in light. What about a "shadow box" in a curtain wall? I think it should be counted as opaque since the light it allows in doesn't -typically- reach an occupied area, which will create a significant penalty for that portion of the wall (~U-0.55 vs. ~U-0.064 baseline).
However, I have a variation that has 12+ inches between the glazing and the opaque surface, and technically the opaque surface is inside the thermal barrier. This was done in pursuit of a certain visual effect from the outside, but the assembly behaves like fenestration. We're making a recommendation to make the interior wall act like an exterior wall by making it airtight and adding insulation, and having the curtain wall hang outside of the thermal barrier, but that introduces a new level of complexity for the sliver of space in between the glazing and the interior opaque surface.
I understand this is not a typical situation, but wondering if anyone had previous experience with the issue.