The Stack Effect: Why Air Moves Through Your Home
Warm air rises and pulls outside air in down low, turning your whole house into a slow chimney that constantly draws in whatever is around it.
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The simple idea
Your house behaves like a chimney. Warm air is lighter than cold air, so indoor air that your furnace has heated naturally floats upward and escapes through gaps near the top of the house, around attic hatches, recessed lights, and upper windows. As that air leaves up high, it has to be replaced, so the house pulls new air in down low, through the basement, crawlspace, and gaps near the floor. This constant upward draft is called the stack effect.
Think of it exactly like a fireplace flue. Hot air going up the top creates suction at the bottom. Your home does the same thing all winter, quietly and continuously, whether you want it to or not.
How it plays out in your home
The stack effect decides where your indoor air comes from, and that is more important than it sounds. The air being sucked in at the bottom is not clean, filtered outdoor air. It is whatever is sitting in your basement, crawlspace, or attached garage.
- Musty basement smells that spread upstairs are the stack effect carrying damp air through the house.
- Cold drafts near the floor in winter are outside air being pulled in to replace rising warm air.
- Soil gases, crawlspace moisture, and garage fumes can all get drawn upward into living space.
The taller your home and the bigger the temperature difference between inside and outside, the stronger this draft becomes. A three-story house on a freezing night is a powerful pump.
The takeaway
The stack effect means your house is always inhaling from the bottom and exhaling from the top. If the bottom is a damp crawlspace or a fume-filled garage, that is what fills your lungs. Control the leaks at the top and clean up the air sources at the bottom, and you turn an uncontrolled chimney into a house you can actually manage.
What professionals know
The stack effect is driven by the density difference between warm indoor air and cold outdoor air. The pressure difference is proportional to both the height of the building and the temperature differential, following the relationship where stack pressure increases with column height and with the difference between indoor and outdoor absolute temperatures. A 30-foot column with a 50-degree Fahrenheit indoor-outdoor difference produces on the order of a few pascals of pressure at the extremes, modest in absolute terms but more than enough to move significant air volumes through the leakage area of a typical home.
Every building under stack pressure has a neutral pressure plane (NPP), the height at which indoor and outdoor pressures are equal. Below the NPP the house is under negative pressure and draws air in; above it the house is under positive pressure and pushes air out. Sealing leaks above the NPP lowers the plane and reduces both infiltration and exfiltration; sealing leaks low does the opposite. This is why attic air sealing is prioritized in cold climates.
The effect reverses in summer with air conditioning: cool dense indoor air creates a mild reverse stack, drawing hot humid air in at the top. In tall buildings this can drive substantial latent loads.
Because exfiltrating air carries interior moisture and infiltrating air carries whatever contaminants exist at low entry points, the stack effect couples directly to indoor air quality. Radon entry, crawlspace mold spores, and combustion byproducts all track with stack-driven negative pressure at the base of the home. Blower door testing quantifies total leakage (ACH50), and pressure diagnostics locate where the NPP sits.
The step-by-step fix
The full guide walks you through it in order:
- What you can do with this
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Education, not medical advice or a professional inspection of your property.