The comparison question
The practical question is not whether Duct leakage and distribution losses is possible. It is which of several routes best explains stale air, drafts, door movement, uneven temperature or odor transport at this location and under this trigger.
Candidate pathway A
Use the facet mechanism—How leakage on supply or return sides can change room pressure, comfort and contaminant transport.—as one candidate. Write its predicted timing, location, direction of spread and response to a safe control change.
Candidate pathway B and other lookalikes
Keep insufficient outdoor air, duct imbalance, envelope leakage, blocked transfer path and room-specific heat gain or loss visible. A competing cause deserves its own prediction rather than a vague note that it is possible.
Observations that actually separate the candidates
Prefer differences in lag time, event dependence, geometry, direction, operating state and recovery. A generic stain, odor, sound or crack often supports several causes at once and therefore has weak separating power.
False confidence to avoid
Do not count repeated observations of the same non-specific clue as independent confirmation. Do not let the easiest repair define the cause. Cosmetic improvement can reduce visibility without controlling the pathway.
Turn the comparison into a next decision
Choose the next safe observation that produces different expected results for the two leading explanations. If no non-destructive check can separate them, document the uncertainty for a qualified assessment. The source set for this guide includes 4 official records selected for this path. They provide general technical and safety context; they do not determine the condition of a particular house.
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.
