jay wrote:
I have a room in my house (I live in the Northeast) with two dual-pane
windows next to each other (separate individual windows within the
same frame).

Today sometime toward the end of the afternoon, one of the inner
panels of one of the windows developed a big crack. This particular
room has a problem in which, since it faces the south, the room tends
to get warmer than other rooms in the house on sunny days. This is
due to heat from the sun making its' way through the window.

Heat move by three vectors: Radiation, Conduction, or Convection.

The double-glass windows attempt to reduce "conduction" (i.e., the outer
pane gets cold/hot but does not transmit that condition to the inner glass.
Double-glass windows do nothing about radiation.

Reflective film is a radiation barrier. So, here's what happened.

It's cold outside. The outer pane gets cold. The cold is transferred, via
conduction, to the air between the panes. This air, in turn, cools the
outermost layer of the inner pane's glass.

Meanwhile, radiation from the sun is heating the reflective film. It, in
turn, heats the innermost part of the inner pane.

Inner pane is now hot on one side, cold on the other.

Bang.