Venting may not be necessary
We're planning to double-glaze our existing picture window by adding panes of glass on the inside. Should we drill a hole from the inside sill at an angle which comes out just below the outside pane of glass? I recall reading somewhere that there should be a very small vent to the outside. Is there a better way? Richard C. Steffens, Framingham, Mass.
The vent is intended to eliminate moisture between the windows which may fog up as the condensation increases. However, in talking with a spokesman for Owens-Corning about your situation, he told me of one man who double-glazed six large-size windows on his home and did not add any vent holes to the frames at all.
"He had no condensation problem of any kind," the Owens-Corning man said.
If a moisture problem does develop, then you would have to correct it. Why don't you try it without the holes first.