The Stack Effect: Breathing Crawl Space Air

Because of something called the stack effect, having open crawl space vents or a dirt crawl space is much worse than just a moisture problem. In fact, more than 50 percent of the air you’re breathing on the first floor comes from the crawl space. And because of the stack effect, you might be breathing air that holds pieces of whatever’s in your crawl space right now. Here’s how the stack effect works.

stack effect

An Entrance and an Exit

First, the stack effect requires an entrance at the bottom and an exit at the top for air. The exit is present in pretty much everyone’s home; it may be due to opening an upper-level door or it might be the rafters in your attic. The entrance, however, isn’t present in all homes. If you have open vents or a dirt crawl space, however, air can come through either of those. 

The basic principle of the stack effect is that air comes through your open crawl space vents or dirt crawl space, warms up, and rises through your home. As it rises from the crawl space into the living space, it takes a variety of things from the crawl space into the rest of the home. That’s one of the reasons it’s so disconcerting.

Warm Air Rises

This is a basic concept in chemistry. Essentially, when air warms up, it becomes less dense than the same mass of cool air. When warm air and cool air are existing side by side, the warm air wants to move on top of the cool air, which takes the warm air’s place. When you apply this concept to the stack effect, it’s easy to see why it messes up your home.

When the air enters your crawl space, either by the crawl space vents or by the dirt floor, it may either be warmer or cooler than the rest of the air. If it’s warmer, as it probably is during the summer, it’ll immediately start rising to the top. If it’s cooler, it’ll start warming up because of all the warmer air around it. Either way, the air will rise to the top of the crawl space.

Particles Cycling Through Your Home

One of the reasons this is such a big deal is the fact that many of the particles in an unencapsulated crawl space are small enough to become airborne. That’s enough for many of them to start cycling through your home. This means they’ve gone from the ground in your crawl space to floating around in the air in your living space.

These particles can be very dangerous, especially if you have allergies or respiratory problems. They become even more dangerous if you have a wet crawl space because the particles in a wet crawl space tend to have more issues than the particles in a dry crawl space. If you have open crawl space vents, you almost definitely have a wet crawl space, so it’s definitely a concern.

How Big Can a Particle Be?

Some people wonder how big a particle can be before it’s no longer able to be airborne. The truth is that the weight of a particle matters much more than the size. Think about it: A large feather can often become airborne, but a very small marble can’t. However, especially in crawl spaces, the particles that end up becoming airborne are almost always very small, typically small enough to be almost invisible.

Most of the time, particles consist of small specks of dirt, bacteria, and mold spores. Occasionally, you may find something like a dust mite, which isn’t big enough to see with the naked eye but can still wreak havoc on your living situation. Think about all the things coming up from your crawl space and consider that even small particles can be dangerous.

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