Noticed this myself years ago when I was helping a mate investigate an old mill in Salford. Every single cold spot we logged was either in a doorway or at the bottom of a staircase, never flat against a wall. At the time we put it down to draughts and poor insulation, which honestly is probably the explanation for 90% of cases.
But here's the thing that stuck with me. One room had no windows, no vents, no obvious airflow at all, and the doorway still dropped noticeably on the thermometer. Couldn't explain that one away as easily.
My working theory, for whatever its worth, is that doorways and thresholds have always held symbolic significance across cultures. Liminal spaces and all that. Whether thats physics or something stranger I genuinely don't know. Be curious to hear if others have documented this properly rather than just anecdotal like me. Has anyone actually done a systematic sweep comparing wall temps to doorway temps in the same location?