I have a general question about how to model and prove compliance for a
window in an interior wall that separates conditioned space from
unconditioned space.

We have a project where there is a conditioned area in the parking level
with a full height storefront window system separating the room from the
unconditioned garage.

As far as I know, this "wall" is actually an interior wall per ASHRAE
90.1-2004. Or rather, it's not defined as an exterior wall (at least
clearly enough). For certain modeling and compliance programs, you
can't even assign a window to an interior wall. To further complicate
things, the entire wall is window, meaning there is no actual cavity
space for insulation (which proves difficult for things like Comcheck
compliance).

Does anyone know how ASHRAE 90.1 views this "wall". Is it really an
exterior walls? Are ALL walls that separate conditioned from
unconditioned space considered exterior walls? If so, this is not
properly defined in ASHRAE. And if not, there doesn't appear to be
anywhere that governs U-values for windows in INTERIOR walls that
separate conditioned from unconditioned area. In fact, from what I can
tell, ASHRAE advises you to IGNORE windows in interior walls. Which is
all fine, unless your entire wall is window. But at the same time,
interior walls appear to require a maximum U-value of 0.120. For a
window, this is nearly impossible.

Can anyone interpret 90.1 for me? Am I really allowed to ignore windows
in interior walls even if they separate conditioned space from
unconditioned space?

Thanks in advance!

James Hansen, PE, LEED AP