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What Is the Stack Effect?

The stack effect impacts indoor air quality and your home’s energy efficiency.

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The stack effect refers to the movement of air in and out of your home based on prevailing temperatures and moisture. This phenomenon affects the air quality and utility costs in many homes. Designers and builders understand it. However, a lot of homeowners aren’t aware of it. 

Here we will demystify the stack effect and show you what impact it has on your home. This way, you will learn how to avoid some of the problems arising from it. 

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Understanding the Stack Effect 

Many homeowners are genuinely confused about the nature of air and heat flow around their homes. Some in the home improvement community are quick to point out that heat doesn’t rise, warm air does. 

But, if warm air floats up, doesn’t heat go up with it? 

Warm air typically rises because there’s less cool air. As air rises, it reduces the pressure at the base of your building. So, you now have warm air rising and wanting to escape from the building and lower air pressure. 

During winter, the stack effect will cause heat to rise together with the warm air that’s less dense than the cold air surrounding it. 

In summer, heat goes down as it follows the cool but dense air. This is in line with the second law of thermodynamics, which says that heat flows from hot to cold. Cold air from the outside rushes into the lower area of your home, forcing the lighter warm air up, and out via leaks or gaps at the top. The warm air will find its way out as it’s less dense. When a cubic foot of warm air leaks out, another cubic foot of cold air will leak in to replace it. The greater the leaks in your house, the higher the temperature difference between the top and the bottom section of your home. 

The negative pressure in your crawl space is due to lower pressure from the mass of air going out and as a result of the warm inside air. Because the cold outside air is dense, it’s exerting a positive pressure at the base of your home, which makes the inside pressure negative. It’s this pressure difference that forces cold air into your home and pushes warm air up. 

What’s Behind the Stack Effect? 

True, the stack effect has an impact on your home. But you can only experience it if these conditions are present: 

Entrance and exit: The stack effect occurs when there’s an entry point for outside air and an exit point for the inside air. If both exist, you’re going to experience the stack effect in your home. Sealing entrance and exit points helps prevent the stack effect. 

Upward moving warm air: The other thing that instigates the stack effect is the warm air that rises and the cool air that replaces it. This holds true regardless of the air temperature. Because the two types of air have different densities, they will arrange themselves such that the warm air rises to the top and the cool air settles at the bottom. 

Let’s say you have crawl space vents. The air in your crawl space might be warmer or cooler than the surrounding air. If the air is cool, it will settle at the base. But, once temperatures start rising, the air will warm up and rise through the living space before exiting the attic. 

Airborne Particles: Close to 50% of the air originating from the crawl space ends up in your living space. 

Why is that a problem? As well as bringing in moisture from the outside, the cold outside air might also pick up tiny airborne particles like allergens, dust mites, mold spores, and a host of other harmful microorganisms from the crawl space. When this air moves up into your home, it will spread these airborne particles across your living areas and affect your health. 

How the Stack Effect Affects You 

The stack effect does more than make indoor air conditioning uncontrollable. It will also instigate the following issues: 

Health problems: Your biggest concern is the fact that the stack effect brings with it allergens, insect droppings, and dust, which can all cause allergic reactions when they get into your living space. The severity of symptoms can range from mild signs like a runny nose and coughs to severe breathing problems. 

Mold and mildew infestation: As the cold air sweeps into the crawl space and goes up your home, it brings with it tiny mold spores. These microorganisms are small enough to float in the air. That means they’ll spread around your home and even attach to your damp walls, where they will grow quickly. 

While both microorganisms thrive in extremely humid conditions, you’re likely to see them grow if the crawl space remains open. 

High utility costs: An open crawl space will drive up your energy costs by 15-25%. Unregulated airflow means you’re going to experience swinging temperatures and moisture buildup, which will force you to use your HVAC or dehumidifier. Come winter, you will find that your home is colder than usual. In the summer, it will be too hot. Both will again require excessive air conditioning. 

Unnecessary repairs: Open crawl space vents are more than a nuisance. They allow moisture-laden air to get in and cause wood rot, which could leave you with structural problems that will compromise your home’s safety. 

Not only does the stack effect impact your home negatively, but it can also have a detrimental effect on your home’s efficiency and the health of your loved ones. You can resolve this issue with the help of a crawl space professional. 

Installing vent covers and encapsulating the crawl space with a 20-mil plastic vapor barrier can help prevent air exchange and all the undesirable effects of cold outside air. If you’d like to seal up your crawl space, schedule a free crawl space inspection and repair quote with JES Foundation Repair today.

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