When I first started using thermal imagers in 1983, not only were they heavy as can be but they also were not very sensitive. As a result, we typically used them at night in the winter because we needed a large temperature difference. With today's modern systems we can get by with far less of a difference and still get good images. We also know a lot more about how to use (and not use!) these remarkable tools.
We've recently begun to say "IR 365 Everywhere!", a bold vision of being able to use your imager every single day of the year. Granted on some days, you may either have to carefully pick your time or manipulate the environment—and, honestly, there may be days where the best you get is a lesson as to why imaging didn't work so well—but that is the goal. More and more of the folks we work with are achieving it. The key is mastering the imager as a tool and understanding when and where to be with the building AND, importantly, using a blower door at the same time.
Home Energy Magazine published an article that I wrote in the March/April 2008 issue. If you don't have a copy, I'm happy to send one to any of you who want it. Just drop me a note at <jsnell"at"thesnellgroup.com>. Since I wrote that piece, we've gotten even better at hitting the mark and so have the imaging systems that are now available. Exciting!