I'm having a window replaced with a vinyl replacement window. So the
frame of the current window (steel) will be left and the new one has a
lip on the outside that goes beyond the window to the stucco.

I still have to confirm this, but it appears that the company does not
fill in the small gaps from the undersized replacement to the old
frame with any insulation. They caulk the outside fin and then the
inside trim.

Any opinions on if it is important to put some sort of insulation? One
thing I realized was that there would be the vinyl and then about 3
inches of space then the trim vinyl sealing the other side. It
occurred to me that the *glass* only has air in between it too, and a
much smaller gap.

Actually, I am more concerned about noise than anything, and I seem to
recall people in here saying fiberglass insulation does little if
anything for noise.

Is this an issue in the install that I should even be concerned about?

--
John