Covering a window with drywall
Interesting exercise, but it did not stop the condensation and so I am back to the octagon window I hate. Left the drywaller installing the foamcore and went on the hunt for caulking. By the time I got back he'd taped the the foamcore as opposed to caulking it and had the finished drywall insert in place. Looked great but a few days later we had a real cold spell. All the windows in the house were sweating profusely---including the one we'd covered. You could actually see water droplets on the glass in front of the blind we left for aesthetics and when, at the end of the day, they had not dissipated and the window was completely fogged---I ripped out the drywall insert. I suspect that although the windows are double glazed they are pretty crappy. I think it may also be that the drywaller failed to properly weather seal the insulation. If had it to do over, I would fit the foamcore, seal it and then tack the drywall over the hole to see if it was going to work. This would allow tweaking of the insulation before the laborious process of fitting and mudding the drywall.
Thanks for your help, though.