We recently had the local electric company come do some thermal imaging in our house. They bring an infrared camera that looks like a video camera that senses heat.
These photos show a corner of Martin’s room. The visible photo is on the right and the infrared photo is on the left. The blue is cold and orange is warmer. When we moved in a decade or so ago and had blown-in insulation installed. Over time, it settles, and this photo shows a great example of visualizing the settling of the insulation between the wall studs. One of the next house projects is new siding and when we do that we can fix the insulation problem from the outside before putting the new siding up.