I have 12-year-old Kolbe double-hung windows. One set of windows in a 2nd floor bedroom has had exterior condensation on the inside upper-half of the storm window (not the actual double-hung itself). In cold weather this will ice/frost over.
What does this indicate? Is it a problem? Assuming it is an interior leak, how would I figure out where the source is?
Condensation on the inside of the storms is caused by warm, moist air leaking from the inside of the home around the interior, primary windows into the space between the primary window and storm. Once the moist air is there, the moisture condenses on the storms – the coldest surface.
This is a common problem and unless it is excessive, quite normal. There is no easy fix. You can try to seal the inner, primary window, reduce the humidity level in your home, or even open the storm a little to vent out the moisture.
You can find more information here:
Fogged Up? Clearing the Air About Window Condensation Problems