Insulating weatherproofing brownstone doors

my north facing doors are super cold during the winter. I have weatherproofing around them and they seem to seal as well as they can (for 100 year old oak doors). the single door behind is also well weatherproofed but my foyer is still cold. I am looking for ideas on making the foyer warmer…

has anyone replaced the original single pane glass with a double pane sealed units? The old glass seems to be loose in its groove and I’m not sure how to weatherproof around it.

thanks

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