What causes the stack effect in relation to indoor radon levels?

The stack effect refers to the phenomenon where warm air rises within a building, creating a pressure differential that allows air to enter through lower points like cracks and openings in foundations or walls. When outdoor temperatures are cold, the temperature inside the building is typically kept warmer, creating a significant difference between indoor and outdoor air densities. This temperature difference causes warm air to rise and exit through upper openings, which effectively creates a vacuum that pulls in cooler outdoor air through the lower areas of the home, including basements and crawl spaces.

As this cooler air enters from below, if it comes from soil that has been contaminated with radon gas, the indoor radon levels can increase. Thus, the combination of cold outdoor temperatures and warm indoor temperatures directly contributes to the upward movement of air, and consequently, can lead to elevated radon levels in residential spaces, particularly in poorly ventilated areas.

Other options, such as increased indoor humidity or excessive outdoor air pressure, do not specifically relate to the stack effect in the context of radon levels. Similarly, while sealing foundation cracks can impact radon entry, it does not describe the underlying mechanics of the stack effect itself. Thus, the interplay of temperature differences is what fundamentally drives this phenomenon in relation to radon levels indoors