You might have a point about air currents especially since the cold is not so noticeable when sitting in the chair furthest from the windows.....We haven't tried a thermometer but maybe we'll do that once the cold weather begins (too soon).
I had a window person take a look yesterday, and he felt the windows were not worth replacing. They are only double glazed, with air in between the glass, but they are hung in a 'fancy' way so that they are cantilevered about 6inches beyond the insulated stud wall to form a wide windowsill inside. He thought triple glazing would add too much weight and possibly make some 'evil' stresses because of the cantilever, so even if it helped solve the problem, it could create new, worse issues.
He did suggest that the outer 3 or 4 inches beyond the stud wall to the glazing is probably uninsulated. So each window is like a box projecting from the house exterior wall, and that box itself is not insulated except whatever R value you get from 3 inches of wood. (hope I am explaining clearly). Which I suppose is very bad and maybe not repairable.
So I guess we should thicken the insulation in the crawl space exterior walls, and make sure it is on the inside of the concrete too and not just on the outside. Then insulate the heating ducts if possible so the air that comes out of the heat registers is a bit warmer. But still let some warm air heat the crawl space so the plumbing lines don't freeze.
As for radiant heat, is that the same as electric floor mats that would go underneath the new floor surface? That was my first idea to fix the heating issue but I didn't realize that the heat from the floor would only rise a few inches. But maybe we should do the crawlspace insulating plus the radiant heat since the windows might not be fixable. If you meant something else by 'radiant heat' please let me know...
Thank you all for your input....feel free to keep adding ideas, I really appreciate it.