We all know that exterior walls have air leaks. Its exterior, it is bound to happen Did you know that in an average and in a lose construction house you can get air leaks. Here is video proof of an air leak from interior walls. Yes there is a vacuum from the return, but if it will leak here and accumulate dust on the surface.

Do you have a dusty light switch or electrical outlet? you do, great now you know why and will have to seal up that air leak.