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 chapter 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 only provides roofing services and does not perform HVAC design, testing, or repairs.
What “pressure difference” means
Air pressure is simply how hard air is pushing on surfaces. When two areas have different pressures:
– Air tends to move from higher pressure toward lower pressure.
– Air carries moisture and heat with it.
– Small gaps become active pathways instead of harmless cracks.
Inside a home, those differences are usually small, measured in tiny units. The numbers are small, but over time, constant gentle pressure in one direction can move a lot of air and moisture through walls, ceilings, and attics.
Main causes of pressure differences in homes
In most houses, four forces dominate:
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Temperature differences (stack effect)
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Wind pushing on the building
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Mechanical systems (fans, ducts, and blowers)
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Door and room layout inside the home
They often overlap, so the same room can shift between slightly positive and slightly negative pressure at different times of day.
Temperature differences and the stack effect
Warm air is lighter than cool air. When indoor air is warmer than outdoor air, it tends to rise and look for a way out near the top of the house.
In cool or cold weather, indoor air often moves upward and outward through leaks at the top of the home: attic hatches, recessed lights, gaps around framing, and other openings at the ceiling level. That escaping warm air has to be replaced, so outside air is pulled in near the lower parts of the building—around rim joists, sill plates, or lower-level leaks.
This pattern is often called the stack effect:
– Upper areas of the house tend to be at slightly higher pressure (pushing air out).
– Lower areas tend to be at slightly lower pressure (pulling air in).
Reverse conditions can happen in some climates during hot weather, especially in very tall or complex buildings, but the principle is the same: temperature differences create gentle vertical air movement.
Wind acting on the building
When wind hits one side of a house, the windward side tends to run at higher pressure (air pushes against that wall). The leeward side and sheltered areas can be at lower pressure.
Air will try to move from the high-pressure side to the low-pressure side through any available path:
– Gaps around windows and doors
– Cracks at framing joints
– Openings into attics or wall cavities
As wind direction and speed change, the pressure patterns around the home change with them. A room that feels calm on a still day can become noticeably drafty when wind hits the right wall.
Mechanical systems: fans, ducts, and appliances
Exhaust fans, ducted HVAC systems, and certain appliances can create intentional or unintentional pressure zones.
Common examples include:
– Exhaust fans (bath fans, range hoods, dryer exhaust). These remove air from the home. If replacement air cannot enter easily, the house can run at slightly negative pressure, drawing outdoor air in through leaks.
– Ducted heating and cooling systems. Supply ducts blow air into rooms, and return ducts pull air back to the air handler. If ducts leak, or if some doors are closed while the system runs, some rooms may end up slightly over-pressurized or under-pressurized compared to the rest of the house.
– Combustion appliances. Certain older appliances that rely on indoor air can be sensitive to pressure differences, especially when strong exhaust fans or leaky ducts are operating at the same time.
The key idea is simple: whenever a fan moves air out of or into a space, it is changing the pressure balance.
Interior doors and room layout
Even without wind or strong temperature differences, simply opening and closing doors can change the way air moves.
If a bedroom has a supply register but no dedicated return, closing the door tightly can let that room build up slightly higher pressure when the system runs. The rest of the house may see a small pressure drop as air is pushed into that closed room and struggles to find an easy path back.
Over time, these small patterns influence where air and moisture travel inside wall cavities, into attics, or across ceilings—especially around light fixtures and access panels.
Why pressure differences matter for air and moisture
Pressure differences alone are invisible, but they drive airflow, and airflow:
– Moves heat (warm air carries heat wherever it goes).
– Moves moisture (water vapor travels with the air stream).
– Moves contaminants (dust, odors, and other particles).
Some practical examples:
– Slight negative pressure in the house can pull humid outdoor air into wall cavities, attics, and floor assemblies.
– Slight positive pressure in a finished space can push indoor air into gaps at the ceiling, carrying moisture toward cooler surfaces.
– Around roof penetrations, openings between the attic and living area can become preferred pathways for this air movement.
None of these effects are dramatic in the moment, but over seasons and years, they can influence where condensation shows up, how materials age, and how often certain areas need attention.
How this connects to roofs and attics
From a roofing and attic perspective, air pressure differences help explain:
– Why some attics show more frost, staining, or discoloration than others even with similar roof coverings.
– Why air sealing at the ceiling plane (the line between the home and the attic) often matters as much as attic ventilation.
– Why moisture-related roof sheathing issues sometimes start on the interior side long before the exterior covering shows anything unusual.
This chapter is not a diagnostic guide. The goal is simply to show that air does not stand still inside a house, and wherever it goes, it takes heat and moisture with it.
Summary
Air pressure differences in homes come from temperature differences, wind acting on the building, mechanical systems like fans and ducts, and the way doors and rooms are arranged. Each source creates gentle pushes and pulls that guide air through gaps, cavities, and openings. Over time, those movements influence comfort, energy use, and where moisture tends to concentrate.
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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