Use location and timing to separate cold surfaces, condensation, particles and growth around glass, frame, seals, flashing, sill and surrounding wall.
What the timing tells you
The phrase during winter is diagnostic context. Compare the symptom before, during and after that trigger.
What is special about this location
Glass, frame, seals, flashing, sill and surrounding wall create pathways that are not visible from the finished room.
Start with competing explanations
Compare cold surfaces, condensation, particles and growth. Do not select a cause from appearance alone.
First useful check
Use the linked check to create a timeline or pattern, then follow the strongest cause family to its full dossier.
Repair decision
Avoid cosmetic work until the source or mechanism is controlled and the original trigger no longer reproduces the symptom.
Observe→Compare triggers→Repair source→Verify outcome
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