How Air Pressure Differences Form Inside Homes

Homes are full of small pressure differences that most people never notice directly. You feel the results as drafts, doors that move on their own, or a room that always seems harder to heat or cool—even though the thermostat looks normal.

This page explains, in simple terms, where those pressure differences come from and how they shape air and moisture movement inside a house over time.

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.

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What “air pressure difference” means

Air pressure is simply how hard air is pushing on surfaces. When two areas have different pressures:

• Air moves from higher pressure toward lower pressure.

• Air carries moisture and heat with it.

• Small gaps become pathways for that movement.

The actual pressure differences inside homes are small, but continuous airflow over time can move significant amounts of heat and moisture through walls, ceilings, and attics.

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Main causes of pressure differences inside homes

Most homes experience pressure changes from four main sources:

1. Temperature differences (stack effect)

2. Wind pushing on the building

3. Mechanical systems like fans and ducts

4. Interior room and door layout

These forces overlap, so a room can shift between positive and negative pressure throughout the day.

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1. Temperature differences and the stack effect

Warm air rises because it is lighter than cool air. When indoor air is warmer than outdoor air, it tends to rise and escape through leaks at the top of the house—attic hatches, recessed lights, framing gaps, and ceiling openings.

As warm air escapes upward, outdoor air is drawn in near the lower parts of the home to replace it.

This is called the stack effect, where:

• Upper areas of the home lean slightly positive (pushing air out)

• Lower areas lean slightly negative (pulling air in)

In hot climates or tall buildings, an opposite pattern can occur, but the principle is the same: temperature differences create gentle vertical airflow.

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2. Wind acting on the building

Wind creates higher pressure on the side it hits and lower pressure on the opposite side. Air will try to move through the structure from high-pressure zones to low-pressure ones, using:

• Window and door gaps

• Framing cracks

• Attic pathways

• Wall cavities

Wind direction and speed constantly shift, so airflow patterns inside the home shift too.

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3. Mechanical systems: fans, ducts, and appliances

Mechanical systems can unintentionally create pressure zones.

Exhaust fans (bath fans, range hoods, dryers):
Remove air from the home. If replacement air cannot enter easily, the home becomes slightly negative, pulling in outdoor air through cracks and gaps.

Ducted HVAC systems:
Supply ducts push air into rooms.
Return ducts pull air back to the air handler.

If:

• ducts leak,

• doors are closed, or

• rooms have supply but no return path,

then some rooms may develop slight positive pressure while others become slightly negative.

Combustion appliances:
Older models relying on indoor air can struggle with depressurization when fans or leaky ducts alter the airflow balance.

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4. Interior doors and room layout

Something as simple as a closed bedroom door can alter airflow.

Example:
If a room has a supply vent but no return, closing the door isolates the supply air. That room may build up slight positive pressure while the rest of the home becomes slightly negative as the system tries to balance airflow.

Over time, these patterns influence where moisture and air move through ceilings, walls, and attics.

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Why pressure differences matter

Pressure differences matter because airflow moves more than just air:

• Heat

• Moisture

• Odors

• Dust

• Pollutants

A few examples:

• Slight negative pressure can draw humid outdoor air into wall cavities or attics.

• Slight positive pressure can push indoor air into gaps at the ceiling plane, where moisture may accumulate on cooler surfaces.

• Air leakage near attic penetrations can create concentrated moisture paths that appear as staining or discoloration on sheathing.

These effects show up subtly over months or years, not all at once.

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How this connects to roofs and attics

Understanding pressure differences helps explain why:

• Some attics develop frost or staining even with proper roof coverings

• Ceiling air leaks can contribute to moisture problems in attics

• Moisture-related sheathing issues often begin on the interior side long before appearing on the exterior

Air does not stand still. Wherever it moves, it brings moisture and heat with it, which is why controlling airflow at the ceiling plane is often as important as ventilation at the roof level.

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Summary

Air pressure inside homes is influenced by:

• Temperature differences

• Wind

• Mechanical systems

• How rooms and doors are arranged

These small pushes and pulls direct heat and moisture movement throughout the structure.

This chapter is the foundation for the rest of Ring 2, which explores how ducts, cooling cycles, plumbing vents, electrical components, insulation, and air barriers all respond to these airflow patterns.

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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.

This page is part of the SJ&H Home Services Encyclopedia Index: https://sjhroofpros.com/encyclopedia/

This page is part of the SJ&H Home Services Encyclopedia.
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