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Air at the casing, but the sash seals seem tight
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What observations best distinguish air passing a sash or meeting rail from air entering between the window frame and rough opening, then coming through around the casing? A smoke pencil or a light tissue might help trace the path, but could air movement or pressure differences make those results misleading?
If the evidence points to the frame-to-opening joint, when is it reasonable to remove interior trim to inspect the gap, and when is a professional leak assessment the better next step? I’m interested in how others separate a trim leak from a failed weather seal without opening up the wall unnecessarily.
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Removing the casing is most useful when the suspected leak is localized to the frame perimeter and the trim can be taken off and reinstalled with little damage. If tracing suggests the path continues behind finished surfaces, or access would mean substantial demolition, a professional leak assessment can help narrow down the route before opening the wall. The key is whether removal gives access to a specific joint, rather than simply creating a larger opening.
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