The "Stack Effect" And Your California Home

How air movement in your home can lead to mold, rot and damage

Cold air moves in through the lower levels, is heated, and leaves through the upper levels.

Crawl spaces and basements are well known for being damp, dank spaces. Even when your home has no groundwater flooding issues and has perfect plumbing, the spaces can be plagued by moisture issues.

One major culprit for humidity in your home is a phenomenon called the "stack effect".

The stack effect refers to the process in which hot air leaves the home through your attic and upper levels. As it does, a vacuum is created below, and new air is pulled upwards through the basement, crawl space, and lower levels.

Condensation is one of the biggest causes of mold

Relative Humidity & Your Home

We know that air enters the home through the lower levels, moves upwards through the home, and exits through the upper levels.

What happens if the outside air that replaces the air leaving the attic is being pulled into the basement or crawl space through vents or other openings?

Basements and crawl spaces are naturally cooled by the earth around them which keeps a fairly constant temperature year-round. Any air that enters that space from outside will be cooled as well.

How Relative Humidity Affects A Typical Crawl Space

Imagine this: It's an 80 °F (27 °C) day with 80% relative humidity.

The higher the relative humidity number is, the closer the air is to becoming "full" of water. Warm air holds the most water and as it cools and "shrinks", it's able to hold less water. The relative humidity number shows how much humidity is in the air, relative to how much it can hold. Air at 80% relative humidity is 80% "full" of water. When the humidity rises above 100% outside, it rains.

Picture that humid air moving into your crawl space or basement. This cool, underground space drops the temperature of the air to 68 °F (20 °C). Because of this, the air's relative humidity goes up, even if no new water is added to the air. For every 1 °F the temperature drops, the relative humidity of the air will rise by 2.2%.

In this case, the relative humidity will rise by 26.4% (12 °F x 2.2%). Add the 80% humidity you already had, and you have 106.4% relative humidity. However, when the humidity reaches 100%, the air can hold no more water! So this extra humidity is dropped from the air as condensation, which is deposited on cool surfaces in the space, such as wood, metal or concrete.

Mold grows and spreads rapidly, making timely professional attention a necessity.

Mold Spores & Your Home

Mold needs moisture to survive, and it draws this humidity from the air around it. As the relative humidity in your basement or crawl space rises to 60% or higher, mold will live, thrive, and survive in the space.

As mold reproduces, it releases millions of allergenic mold spores into the air. In fact, The Journal of Property Management's research shows that a single square inch of drywall can contain as much as ten million spores!

Because of the stack effect, these mold spores will not be confined to your basement or crawl space. As air moves up from these spaces into your home, it will bring mold spores and humidity along with it. If you notice that you suffer from symptoms such as a runny nose, watery eyes, mild allergic reactions, a scratchy throat, fatigue, or headaches at home that seem to disappear once you leave the building, mold allergens may be the culprit.

Of course, mold and rot will also wreak havoc on your home. Damaged wood, structural issues, and ruined personal property are all consequences of mold, mildew, rot and humidity in a basement or crawl space.

Even when your home is protected from outside humidity, your home can still experience humidity when sources of standing water exist in your home.

Install a sump pump system to prevent groundwater flooding, and be sure to actively look for plumbing leaks-- especially in your crawl space.

Controlling Humidity & Mold in California

The key to controlling mold in your basement or crawl space is to eliminate the humidity that keeps it thriving.

At PRC CleanSpace we recommend starting by sealing off any vents, covering any exposed dirt and concrete, and installing airtight crawl space doors. Replacing drafty basement windows will also help hold back outside air.

Once this has been completed, it's a great idea to install either an energy-efficient basement dehumidifier or crawl space dehumidifier. This will remove any existing humidity in the space and keep it dry in the future.

We offer free basement & crawl space humidity control estimates in California, including Long Beach, Santa Ana, Anaheim and many areas nearby. Contact us today to get started!

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