Thanks for all the informative replies!
For background: the house is a "bungalow," which is basically like a "cape cod," except with one fewer dormer, as I understand house styles. Or in other words, the space in question has, for walls, two gables and the roof. For insulation: I laid down a layer of the shiny reflective stuff, which didn't seem to help at all. But it was cheap, and really easy to install. Then I put in 2" extruded polystyrene foam. Then I put in a layer of R-19. I would have put in more, but I only had so much depth of roof joist to play with, and R-19 was really deeper than fit.
My take on this, then, is that (a) given the poor insulation in the rest of the space and the enormous heat load from the roof, it won't make much of a difference and (b) you really can't tell by looking at it anyway. Or in other words: make the wife happy and skip the low-e glass because it won't make much of a difference anyway.
Sound about right?
Thanks,
Dan