Short humidity peaks followed showers
The moisture source was intermittent rather than a constant roof leak.
↓House Autopsy · ventilation-and-airflow
The attic frost that began in a bathroom
A disconnected exhaust duct and ceiling leakage placed warm humid air against cold roof sheathing.
Trigger conditioncold weather and shower use
Four layers, one connected story.
The fan discharged into the attic
A loose duct connection delivered moisture beyond the ceiling.
↓Cold sheathing stored the vapor as frost
Water became visible only after freezing on cold surfaces.
↓Duct repair and air sealing ended the pattern
Later cold periods no longer produced localized frost above the bathroom.
Canonical connection
The symptom and cause are separate objects.
The autopsy preserves how evidence connected them without collapsing uncertainty into one magic answer.
A disconnected exhaust duct and ceiling leakage placed warm humid air against cold roof sheathing.
The visible symptom was the final chapter
The investigation followed the source, transport path, affected material and verification step separately.
What changed the direction
Timing and location evidence ruled out attractive but weaker explanations before repair work began.
The durable lesson
Repair the controlling source and then verify the outcome under the conditions that previously triggered the symptom.
Source→Path→Damage→Verify
