I may be wrong, but this thread
may have some bearing on the question?
Draught from glazing pane, uPVC French Doors
I have uPVC French Doors to 2nd floor Juliet balcony, which I’ve had never ending issues with and within my Housing Association property. After sealing all around the French Doors and the uPVC unit with adhesive vinyl tape, I’ve discovered a noticeable draught emitting from around the glazing...
www.diynot.com