Use this atlas page to organize evidence, not to declare a diagnosis. Supply, exhaust, transfer paths and pressure differences that move moisture, odors and outdoor air through the house. The useful starting point is a repeatable pattern tied to location, timing and conditions that changed or stayed normal.
Read the visible pattern
Start by matching the symptom to the operating state of ventilation & airflow. Record the first location, direction, edge or cycle where the symptom becomes noticeable. Photograph from the same position, preserve original timestamps and note comparable areas that remain dry, quiet, cool or odor-free.
Trace the connected mechanism
Air follows pressure differences and available leakage paths, so the room where a symptom appears may not contain the source. Compare at least two plausible routes rather than choosing the first familiar explanation. Ask what this system predicts about location, delay, direction, recurrence and unaffected comparison points. A strong hypothesis predicts where the symptom should begin, what trigger should reproduce it and which nearby areas should remain unaffected.
Use safe, non-destructive checks
- Compare fan-on and fan-off conditions
- Record door and window position
- Map supply, return and exhaust behavior
Pair every reading with its instrument, location, time and environmental condition. One reading can support a pattern but cannot certify a hidden material condition or prove that a repair is complete.
Respect the stop boundary
Do not block required ventilation, enter ducts or alter combustion-air paths without competent review.
Prepare the next decision
Summarize the trigger, first visible change, rooms or systems involved, checks already completed and what would count as a verified outcome. A useful handoff gives the next person a timeline and competing explanations instead of only a symptom label.
Primary references for this topic family
These references ground the general building-science or safety context. They do not diagnose a specific house, replace local requirements or certify a repair.
