In summer, the dense air is inside your home because that’s where the temperature is lower, especially if you’re air conditioning your home. The leakier your house is, the more temperature difference you’ll notice between the top and bottom of the house.Īll this happens because the warm air inside your home in winter is less dense than the cold air outside. As the warm air leaks out at the top, cold air leaks in at the bottom. When a cubic foot leaks out, however, it has to be made up by a cubic foot leaking in. Because of its lower density, the warm air will leak out the top of the house if there are leaks there. Your nice warm air finds way to leak out ( exfiltration) and cold air leaks in (infiltration). What happens in reality is that homes leak. Positive pressure inside the house with nowhere to go because there are no pathways. There’s still a pressure difference across the building envelope, but that’s OK if the air barrier’s good. If your house has no leaks, the warm air can’t escape and do its thing. So, in winter, the warm, low density air inside your house wants to rise…if it can. You’ll feel the stack effect pushing lots of warm air into the attic. Climb up into the attic and then put your face over the hole. Open your pull-down stairs or scuttle hole to the attic on a cold day when your home is warm. Try this experiment if you don’t believe me. But the low density air inside the house will move up and out into the cold, dense winter air when given the chance. Obviously, a house isn’t going to start floating up into the air like a balloon (although I recall with great fondness the Disney movies of my childhood that showed such magical events). The problem with stack effect in buildings is that buildings aren’t vacuum chambers. Temperature difference between inside and out (because density depends on temperature).Two factors affect how much stack effect a building experiences: The name for this phenomenon is stack effect. Yes, that’s due to heat, but density is the main factor causing the movement here. Warm air rises when it’s surrounded by cold air because of its lower density. The point here is that it’s easy to get confused by heat in the building science of air movement. Coyote’s anvil in the air above his head, and it turns him into a pancake. Now, imagine an object with higher density immersed in a fluid. Think of air bubbles in water, as shown in the photo above. When you have a lower density fluid immersed in a higher density fluid, the lower density fluid rises and the higher density fluid falls. When a mass of air takes up more space, it has a lower density. When we heat air, the molecules jiggle and zip around faster, which causes them to spread out. Air is the fluid we live in, and this time of year we spend a lot of money pumping heat into it in our homes and workplaces. When you’re dealing with fluids, you have to account for density and buoyancy as well. So, temperature difference is really what drives heat to move in any given direction. Put a torch to the top of a steel pole, and heat will travel downward by conduction. ![]() What the laws of thermodynamics tell us is that heat moves from areas of higher temperature to areas of lower temperature. Heat can move up, down, or sideways, depending on the situation. The problem is that sometimes people say this as if the flow of heat is driven by its wanting to rise. Everyone knows that, right? It’s absolutely true.
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