Somehow I think that might vary a bit. 20% don't have indoor plumbing, I'd be shocked if they had 3-glass windows in those homes. Of course, standard for new construction could be it ,as I guess there is also very little new construction in poorer areas.

People that don't have indoor plumbing certainly can't afford new windows, but their old (handmade) windows are usually double-glazed for warmth (and the part of the house that has single-glazed windows and uninsulated walls is abandoned for the winter.

Source: spent a few summers fetching water from a well and crapping into an outdoor toilet.