The common belief that cold air rises in a house is a misunderstanding of physics, often stemming from noticeable drafts. Cold air does not rise; it is denser and sinks downward. This sinking cold air and rising warm air create a continuous cycle of movement inside a building, causing the drafts and temperature imbalances homeowners experience.
The Physics of Air Density
Air is composed of molecules that are constantly in motion. When air is heated, the molecules absorb energy, causing them to move faster and spread farther apart. This molecular spacing means that warm air is less dense, or “lighter,” than cold air. Conversely, cold air molecules pack more closely together, making the cold air significantly denser and “heavier.” This difference in density causes warm air to float upward while the heavier cold air is pulled down by gravity.
Vertical Movement: The Stack Effect
This fundamental principle of air density is applied vertically within a house through a phenomenon known as the Stack Effect. In a heated home during colder months, the structure acts like a large chimney, where the column of warmer indoor air is less dense than the colder outdoor air. As the warm air rises, it exits the home through leaks and openings in the upper levels, creating a negative pressure zone at the base of the house.
This pressure imbalance causes the colder, denser outdoor air to be sucked into the lower parts of the building to replace the escaping warm air. The cold air then begins to warm, rise, and exit high, perpetuating a continuous cycle of air turnover. The greater the temperature difference between the inside and outside, and the taller the home, the stronger this pressure difference becomes, intensifying the Stack Effect. This constant movement of air is a major contributor to drafts, uneven temperatures between floors, and increased energy bills as conditioned air is continually lost.
Common Infiltration and Exfiltration Points
The Stack Effect manifests as energy loss at specific points in the home’s structure. Air infiltration, where cold air is drawn in, typically occurs at low points in the building envelope. Common lower leakage areas include gaps around basement window frames, unsealed crawlspace hatches, and cracks in the foundation or sill plate where the house meets the ground.
Penetrations in exterior walls for utilities also act as significant entry points for cold air. These include the spaces around electrical outlets, plumbing pipes, and cable wiring that pass through an exterior wall.
Exfiltration, the escape of warm, conditioned air, is concentrated at the highest points of the house, where the internal pressure is highest. The attic is the most significant area for this heat loss, often occurring through unsealed access hatches or pull-down stairs. Other high-level weak spots include gaps around recessed lighting fixtures, bathroom exhaust fans, and unsealed chimney flues. Sealing these specific high and low points is the most effective way to slow the Stack Effect, minimize drafts, and improve overall home energy efficiency.