Here is the door to the basement:
The lower right quarter of the door has additional weather stripping and is warm, while the left side, and the upper right edge, allow cold drafts to seep through. I know from experience that these are very small drafts (at least an order of magnitude smaller than with the previous door), but can be easily reduced.
This window needs its top edge sealed - you can see the cold flowing down the rolled down shade. Also, even though the heating system has been off for more than 8 hours, the cast iron radiator is still a few degrees warmer than the rest of the room.
Upstairs bedroom. You can see the outline of the bricks and the slightly cooler cement between them. Notice how the rafters in the ceiling get really cold close to the wall. The hottest objects in this view are electronic devices (left - router, middle - stereo in standby, right - wall wart), busily turning electricity to heat, even though they are not needed at the moment.
Now, a cold spike down the middle of the house: One of the ventilation shafts appears to be sucking cold air through the chimney:
pulling it down, through the whole upper floor:
to the lower floor and abruptly stops halfway down:
The place it stops appears to be totally random - there is nothing visible at that level. In fact, the room's thermostat is on that wall (the small rectangle just under where the cold stops), and its cable uses that shaft to connect to the boiler in the basement. The bottom of the shaft ends in the basement/garage, and I've more or less closed it up down there. Either way, the cold should either go all the way down, or not enter at all. Strange. All the more reason to speed up planning of the new ventilation system with a heat exchanger, which would allow me to totally block all of the ventilation shafts from the cold.