Ron, I had the same problem as you do with Anderson double glazed casement windows with a third pane attached to the outside. The third pane would fog or frost up when I tried to seal them too tight to the window frame. What I finally did was to space out the flange on the third pane by using small spacers (the end of a flat toothpick) under the flange at the bottom and top clips that hold the pane on. This small amount of air was enough to stop most of the condensation. I suspect that you can never seal several panes of glass tight enough to stop condensation. Hence the invention of sealed thermopane windows with desicant(sp) inside.