I have found that even regular heavy curtains make a significant
difference. My livingroom was cold on the feet (leaky fireplace damper)
and cold on the back (couch backed up against large 1 pane window.) I
put a chimney balloon that I bought on Amazon in the fireplace chimney
to keep the cold air from comming in that way and we bought heavy light
blocking curtains for the big front window. Im not sure what the
R-value is of thsoe two items but I know they only cost me $40 for the
chimney balloon and $265 for the curtains and the livingroom is much
more comfortable especially at night. I close the curtains when the
direct sun is not comming through, but it is south facing. Im not sure
my wife would go for the foamboard on the windows and firepalce idea.

Jerry