They lose their seal and the argon gas (added between the two panes to minimize heat transfer) leaks out as the vinyl frames or ... loss through the glass. Window Inserts Fit Inside Existing ...

Replacing aluminum inserts with composites. When vinyl windows reach a certain size threshold, vinyl frame and sash profiles require reinforcement in order to meet wind-load deflection requirements.

This is a much bigger and more complicated job than installing a vinyl window insert and may require a pro. If the frames are ...