What makes The Stack Effect stronger?
Short answer, a higher “Delta T’ (and other factors).
Wat the wat? A higher what?
Here’s the english: “Delta T,” is used in the building sciences as shorthand (“short-speak”) for Difference in Temperature. That’s all it means.
Air is a fluid. One property of this fluid is that warmer air is more buoyant and so it rises. Cold air is denser (heavier in a sense) so it sinks. This simple principle is the driving force behind the Stack Effect. (Sometimes called, The Chimney Effect).
When is this force (The Stack Effect) more powerful? When there is a higher delta T.
For example, let’s say we have a house at 70° F. It’s a mild fall day and the outdoor temperature is 65° F outside. Our “Delta T” or Difference in temperature, is 5° F (70 – 65 = 5). The hot air is going to rise to the top (2nd story and attic) of the house and leak out. As it leaks to the outside it creates a negative pressure inside the house. Now, we have a “Delta P,” a difference in air pressure.
As that air leaves, new air needs to replace it. New air will leak in from the first floor or basement. This movement of air in and out of the house (and through the house) is driven by the air’s buoyancy. Overall this observable condition is the Stack Effect.
Let’s look at the same house a few months later in the winter. The indoor temperature is 70° F . The outdoor temperature is now 10°F. Our “Delta T” is now 60°F (70 – 10 = 60). A higher delta T means more buoyant air, this creates a greater difference in pressure (Delta P) as the force of the more buoyant(warmer) air escapes faster.
The greater the Delta T and the greater the height of a building the greater the force of buoyancy.
Just like a blustery day, the Stack Effect can move a significant amount of air through a building’s envelope. Leaky buildings consume large amounts of energy as their mechanical and ventalation systems condition (heat or cool) air that is continually exiting the building. What is special about The Stack Effect is that it works every hour of every day when there is a difference in temperature. It’s a near constant force unlike other air pressure forces in a home.
Another factor, the size of air leaks. Said another way, if we decrease resistance our fluid (ie air) will flow from areas of high pressure to low pressure more easily. Simply put this allows for the Stack Effect to move a greater volume of air through the building envelope. Take opening a window for example. This increases the air driven by the Stack Effect because it decreases air resistance. A larger volume of air is now able to flow through the building more easily. Decreasing resistance makes the Stack Effect stronger.
In a building, this is what we don’t want. We want to keep as much of our conditioned air as we can.
All Aboard The Merry Go Round
So what happens when we put it all together? Here’s one scenario to consider:
In the winter, cold air leaking into the first floor will make people feel cool. What’s the typical response? Turn up the thermostat naturally. The heating equipment warms the air, giving it more buoyancy. It rises and leaks into the 2nd floor above.
With this extra heat upstairs the building’s occupants may get overheated and feel uncomfortable. They crack a few windows to cool off. This decreases resistance, increasing the amount of air flow leaving the building. The “Delta P” downstairs is driven up causing more cold air to leak in. People downstairs feel colder, they turn up the thermostat again.
The cycle repeats and now you’re on merry-go-round-death-spiral of trying to heat the entire county in the winter with your cute little furnace. Good luck to your futile efforts, please tell us how expensive your energy bill was when you’ve succeeded.
Air Seal. Air Seal. Air Seal.
So what can you do?
The Stack effect is why air sealing is so important in houses. Many homeowners looking to save energy (at least in a heating dominated climate) will turn to strategies like beefing up the attic insulation. While important, this NEEDS to be done with air sealing. Air sealing seals up as many gaps and cracks as we reasonably can, as effectively as we can. This adds resistance to air movement decrease the power of the stack Effecting, keeping your conditioned air exactly where you want it. Inside the building.