By: Daniel Overbey Heat rises. The adage is one of the cornerstones of passive environmental systems in buildings. The typical anecdote is the hot-air balloon. By using a burner to thrust heated air into the (typically) nylon envelope, the balloon fills with less buoyant gas that lifts it and its passenger(s) into the air. But what if I told you that this oversimplified statement actually misrepresents the characteristics of heat? The truth is – technically, heat does not rise. Consider heat transfer via radiation – it happens at the speed of light through empty space. A great example of this is the Sun. Which way is “up” in outer space? Yet, the Sun warms the earth everyday. Consider heat transfer via conductivity – when one material transmits heat to an adjacent material by “touching.” A great example of this is when you grab a hot bowl of soup. The heat will conduct from the bowl into your hands regardless of orientation. There is no “rise” or “fall” of heat. Anywhere you touch the bowl, heat transfers. The misconception that heat rises stems from heat transfer via convection – the transfer of heat through fluids (gases and liquids) by way of molecular movement. While conduction is the transfer of heat by molecules vibrating in place, convection is the transfer of heat by molecules actually moving about. Highly energized molecules in the air have more energy than less energized molecules. Since the highly energized molecules bounce around more, they tend to “rise.” As the molecules lose energy, they bounce around less and end up “falling.” This is why hot air is less dense than cool air. The energized molecules need more space for their higher level of activity or movement.