CHAPTER 2 — Why Duct Leakage Changes Moisture Behavior
Duct leakage doesn’t just waste energy — it changes how air and moisture move inside a home. Even small leaks in supply or return ducts can shift pressure patterns, pull humid air into unwanted places, or push conditioned air into spaces that were never meant to receive it.
None of this is dramatic in the moment, but over time, duct leakage influences comfort, indoor humidity, and the long-term condition of building materials.
Note: This page is general education, not a diagnostic checklist. SJ&H Roofing provides roofing services only and does not perform HVAC design, testing, or repairs.
Supply leaks vs. return leaks
Duct systems have two primary sides:
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Supply ducts deliver conditioned air to rooms.
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Return ducts pull air back to the air handler.
Leaks on each side affect the home differently.
Supply leaks
When conditioned air escapes into an attic, crawlspace, or wall, the house may experience:
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Lower indoor air volume
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Slight negative pressure
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More outdoor air being pulled into the home
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Increased humidity load
Because the system is pushing air out of the house and not all of it returns, the home often tries to “make up” the missing air through cracks and gaps.
Return leaks
A return leak pulls air from wherever the leak exists — attics, wall cavities, garages, crawlspaces — instead of pulling clean indoor air.
This can lead to:
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Dust, insulation fibers, or unfiltered air being drawn into the system
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Elevated humidity if the leak is in a warm attic
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Pressure imbalance as the system pulls in extra air from undesirable locations
How duct leakage shifts pressure inside the home
Duct leaks act like small, unintended fans. They push or pull air in ways the system was never designed for.
Examples:
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A supply leak in the attic can make the main living area slightly negative, encouraging more outdoor air infiltration.
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A return leak high in a wall can make the system pull warm attic air into the home, increasing moisture load.
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Rooms with closed doors and supply vents can become slightly over-pressurized, especially if the return path is blocked.
Pressure changes of just a few pascals can redirect moisture movement through ceilings, insulation, and wall cavities.
Why moisture follows these pressure changes
Air carries moisture.
Where air goes, water vapor follows.
When duct leakage shifts airflow patterns, the moisture distribution changes too:
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Negative pressure areas tend to pull humid outdoor air into the structure.
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Positive pressure areas tend to push indoor air into gaps where it may condense on cooler surfaces.
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Air leaks toward the attic can move water vapor into insulation or sheathing.
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Air leaks toward the interior of the home can bring in humid, unconditioned air from attics or crawlspaces.
Over time, these shifts influence:
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Surface condensation patterns
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Attic humidity levels
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Moisture in wall assemblies
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Comfort and “stickiness” indoors
Where duct leakage commonly occurs
Most homes experience leaks at:
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Boot connections
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Supply plenum seams
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Return plenum seams
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Flexible duct connections
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Unsealed take-offs
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Kinks, tears, or compression in flexible duct runs
Even well-designed systems can leak once age, vibration, thermal expansion, or poor installation practices take hold.
Why this matters for roofs and attics
Duct leakage affects attics and roof systems because:
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Leaky ducts can raise attic humidity by pulling indoor air upward.
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Moisture movement toward the attic can cause staining, discoloration, or frost on roof sheathing.
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Air leaks in ceilings behave differently when the home becomes slightly positive or negative due to duct imbalance.
While roofing does not cause duct leakage, duct leakage can influence the environment directly below the roof — which is why understanding the airflow patterns matters in the bigger picture.
Summary
Duct leakage changes more than airflow — it changes how moisture moves inside a home. Whether through supply leaks, return leaks, or pressure shifts, air movement will carry water vapor along with it. Over time, these changes can influence humidity, comfort, and how building materials respond.
This chapter sets the stage for Chapter 3, which explains how cooling cycles themselves affect indoor humidity.
ENCYCLOPEDIA FOOTER
All information in the SJ&H Home Services Encyclopedia is written with consideration for the climate, storm patterns, and construction practices common to the Mississippi Gulf Coast.
This encyclopedia is not sponsored by manufacturers, suppliers, or vendors. Content is intended to be unbiased and grounded in generally accepted industry standards rather than product marketing.
Content is reviewed and updated periodically as building codes, storm data, and industry best practices evolve.
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This page is part of the SJ&H Home Services Encyclopedia.
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