What Is The Stack Effect In A Home?

As you’ve done research on your home energy efficiency, you may have come across the term “the stack effect.” Perhaps a contractor mentioned the stack effect offhand. But what is the stack effect, and why does it matter when discussing insulation and HVAC upgrades?

South Central Services has insulated hundreds of new and existing homes with spray foam. The stack effect is an important energy efficiency concept that explains how air travels in your home. In our service area, which is centered around Greencastle, the stack effect is quite prevalent.

By the end of this article, you will understand:

• How the stack effect works in the winter
• Why the process reverses in summer
• Why the stack effect matters
• Why air leakage contributes to the stack effect

The Stack Effect Helps Explain How Air Moves In Your Home.

For many people, the movement of air can be summed up by 7th grade science class: hot air rises and cold air sinks. While this principle is true, air movement is much more complicated than just moving up and down.

The movement of air has to do with both air temperature, density, and pressure. Cold air is more dense than warm air. Since hot air is less dense, it rises.

Another principle from science that is sometimes forgotten is that hot moves to cold. This is true for everything, including the air. Hot air is drawn to cold air, which drives how air moves.

When you live in a place with significant temperature differences in summer and winter, the stack effect influences how your home is cooled and heated. Air will move differently depending on the time of year and where heat is being drawn to.

Let’s learn about the stack effect and how it impacts your home in the winter.

In Winter, The Stack Effect Pulls Cold Air Into Your Home.

Pennsylvania winters have become less snowy in the last decade. Around Chambersburg, PA, there are fewer snow storms, and they happen later in the winter season. Even without snow, our temperatures can get quite low. We often have below-freezing incidents, especially in January and early February.

To keep ourselves warm, we turn up the heat as high as we’re willing to pay for. We wear socks, slippers, and long sleeves to make up the difference. We cover ourselves with blankets and try to stay comfortable.

But one of the reasons that we are often so cold in our homes during winter is due to the stack effect. All the hot air we pay for in winter is drawn to the cold air outside. It exits our homes through the ceilings.

The air that we lose has to be replaced somehow. Otherwise, the air in our house would quickly become like a vacuum. To maintain the air pressure in your home, the hot air that escapes is replaced with cold air. As air leaves through the ceiling and into the attic, new air is pulled in low in the home, often through the basement and crawl space. The replacement air is frigid and often dirty.

In Summer, You Experience The Reverse: Hot Air Pushing Into Your Home.

Pennsylvania summers tend to be hot, muggy, and sticky. Occasionally, we’ll have a day of dry heat. Regardless, summer days are consistently hot. A comfortable July day would be 85 degrees Fahrenheit, while the hottest days are over 100 degrees Fahrenheit.

The hotter it is outside, the cooler we want our homes to be. This means a more significant difference in temperature between inside and outside. The air in the attic is heated, often to 130 to 150 degrees Fahrenheit. This heated air pushes down into the living space through the ceiling. It also heats the conditioned space through radiant heat.

But the hot air that invades your home isn’t just heating your conditioned air: it’s actually replacing your conditioned air. As hot air pushes into your house, the conditioned air you pay for leaks outside at the same rate. In the summertime, cold air usually leaves your home through your exterior walls, basement, or crawl space.

While the stack effect explains how cold air infiltrates your home in winter, you experience the reverse in summertime.

Air Leakage Makes The Stack Effect More Prevalent.

The stack effect can be reduced to a low level if air-sealing and insulation are done correctly. The stack effect describes how air travels through a home in winter due to temperature differences. Since hot air is drawn to cold, air will flow through our homes in this way.

However, air leakage amplifies the stack effect. When hot air can easily leak from your home, there is a greater exchange of conditioned and unconditioned air.

To limit the stack effect, the best thing to do is put an airtight seal in the accessible areas of your thermal envelope.

An example would be an air-seal on your attic floor. In the winter, hot air could only rise as high as your attic floor. If the air cannot leak outside, the air pressure in your home stays the same, preventing cold air from being pulled down below.

An airtight seal in your basement band boards or crawl space can also reduce the stack effect. With an air-seal, cold air cannot easily be pulled into your home to replace any hot air that escapes.

The Bottom Line About How The Stack Effect Impacts You

All homes deal with the stack effect. It is a description of how scientific laws operate in a conditioned home. In the winter, the stack effect explains where all the hot air you pay for seems to escape to. In the summer, you experience the reverse, which makes your home much hotter.

You can limit the negative impact of the stack effect by investing in better insulation with an air-seal. Air-sealing helps to contain the hot air you pay for during winter.

Now that you understand how the stack effect works in a house, your next step is to:

• Learn more about the three methods of heat transfer
• Discover why air-sealing is so important
• Explore whether or not spray foam really makes a home more comfortable