I got my hands on a thermal imaging camera for a few hours recently and took a look around the house to see what I could learn. The camera detects infrared radiation, which is proportional to surface temperature. When you're inside a house in the winter, the poorly insulated bits look blue, because they're colder. From the outside, the poorly insulated parts look red, because they're hotter than the surroundings. (This is assuming you have the camera set to adjust the spectrum to cover the temperatures in the field of view. The camera I was using, a Flir i60, could either adjust automatically or stay fixed so you could compare temperatures across multiple pictures.)
I learned some interesting stuff. The gables of our attic appear to be completely uninsulated. There is one stud bay missing insulation next to our front stairs. I hadn't noticed it, but once I knew to check, I could feel the temperature change with my hand.
Our windows are generally our most poorly insulating surface, but in the picture below, you can see the panels in the front door are a close second. (The large box of fire is a radiator.)
I was thinking I should insulate the garage roof, but the thermal camera revealed that the door was leaking more heat.
I had noticed that our cats preferred lounging in certains parts of our kitchen floor, but I hadn't noticed the large cold stripe down the middle.
Here, you can see that there are two rooms we aren't heating-- their windows are blue.