If we come to learn the insight, we see that, it is the space on the inside of the pane which tends to heat up and expand the air in turn. This expanded air then forces out through the sealing system of the window. On the other hand, when the inner space air content contracts and cools down, humid air is drawn back into the panes. It is this humid air that brings a lot of debris and dust particles and deposits it on the interiors of the glass windows. Hence turning it all cloudy-white in appearance. This is called fogging.