As Robert Kahabka looks at the building under static conditions, the insulation of the sidewalls looks pretty uniform. But in the corner, the rim joist is clearly defined. At this point it's unclear if this temperature difference is due to missing insulation or air leaks, or both.
With the blower door running, the dark area in the corner grows as cooler outdoor air infiltrates the band joist (2). At least two floor joist bays are changing temperature as well. If he ran the blower door a little bit longer, we might see more change, or if we had more "delta T" (a greater difference in temperature between outdoors and indoors), we would likely see more change.
Kahabka likes to look at interior wall partitions, too. "They can throw you curve balls," he says. "The interior walls often don't appear to be part of the building envelope, but they can be connected." As he looks at a partition wall in the living room (3), he sees nothing in the flat area of the wall; he cannot see the frame. But clearly there is a temperature difference at the connection between the wall and the ceiling, and the first joist cavity in the ceiling is a shade darker, or cooler, as well. Though not shown in the photo, this discoloration carried all the way across the room. Under depressurization, Kahabka predicted, this area would change significantly.
And that is exactly what happened. When Kahabka looks at the same interior wall after the blower door has been running, he sees that the dark area along the top plate has grown as the blower door moves air down the interior wall system.
Recessed lights are not typically a problem when they are between floors of a building. But when they are in the pressure boundary of the building, they become a serious source of air leaks. The can lights in the home office are in a poorly insulated ceiling that's completely open to the garage, and therefore are in the pressure boundary of the building. This image was taken with the blower door running, and the incoming air has caused the surfaces around the light to drop about 3°F. This is air leaked directly into the building—you can feel it when you hold your hand up to the light with the blower door running.