Which action would be least effective in limiting stack effect?

• A. Sealing Lower Envelope
• B. Sealing Upper Envelope
• C. Installing An Exhaust Fan On The Top Floor
• D. Installing An Exhaust Fan On The Bottom Floor

Answer: (C)

Stack effect is driven by warm air inside a building rising and escaping at the upper levels, which pulls in cooler outdoor air from lower openings. To limit this buoyancy-driven air movement, you want to reduce the pathways for warm air to escape and the openings through which cold air can infiltrate and be heated.

Sealing the upper envelope cuts off the main exit route for warm air, so less buoyant air can leave the building, reducing the driving force of stack effect. Sealing the lower envelope reduces cold-air infiltration at the bottom, which means less cold air gets heated and then rises, again dampening the stack-driven flow.

Installing an exhaust fan on the bottom floor is less impactful for stopping stack effect because it doesn’t directly interrupt the upward movement of warm air; it may even draw air out in a way that doesn’t address the buoyancy-driven flow as effectively.

Installing an exhaust fan on the top floor would actually worsen or fail to limit stack effect, since it actively removes warm air from the upper levels, increasing the upward flow and the pressure difference that drives the stack effect. In short, that action enhances the very phenomenon you’re trying to curb, making it the least effective option.