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The Stack Effect

The stack effect refers to the process in which hot air leaves the home through your attic and upper levels


Lower levels of your home, like basements and crawl spaces, are dark and damp in nature and are prone to moisture problems. Unfortunately, moisture problems in the lower levels of your home can also create issues inside your home. 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.

For a free humidity control inspection, call or e-mail us today! We serve throughout central Missouri and Eastern Kansas, including Kansas City, Columbia, Moberly, and Springfield.

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 outside air 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.

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 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.

According to the Journal of Property Management, mold can reproduce as much as 10 million spores in just one square inch of drywall. If you take that across all the walls in your home, this can be an overwhelming issue and lead to allergy issues in your home.

Now think about the air you breathe in your home. Time Magazine reports that there can be hundreds of thousands of mold spores in a single cubic meter of air, and a person inhales 10-12 cubic meters of air each day.

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.

Any air at or below ground level is drawn upwards through the house. The stack effect is the motion of air being pulled into or pushed and out of your home. This is the result of the difference in temperature between the inside of your home and the outdoor climate. During the summertime, we crank up the air conditioning or fans to beat the heat, and we fire up the furnace in the winter to stay warm and cozy. This difference in air temperature between our homes and the conditions outside produces the stack effect, which pulls and pushes air through the home’s attic and crawl space. (Source: Clean Crawls)

You can’t actually see air or air movement. However, the Cold Climate Housing Research Center has several informational YouTube videos, including a great piece that is part of the “Your Northern Home” series that illustrates the stack effect.

If humidity, mold spores, and other allergens are present in your crawl space, you and your family are likely breathing them.

  • Mold

Mold spreads and thrives in the damp spaces beneath your house – meaning your family could be breathing in spores.

  • Pests and Rodents

Moist, humid crawl spaces are the perfect breeding ground for crickets, cockroaches, snakes, rodents, and termites.

  • Failing Insulation

Excessive moisture leads to soggy and falling insulation, which can be a hotbed for hazardous mold growth.

  • Efflorescence

When water encounters concrete surfaces and evaporates, chalky mineral deposits are left behind.

  • Wood Rot

Rotting wood beams weaken and buckle your floors over time, threatening the structural integrity of your home.

Controlling Humidity & Mold

Mold depends on moisture to thrive. In order to get rid of the mold in your basement or crawl space, you must attack the initial moisture problem at hand.

At Foundation Recovery Systems, 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.

After those have been taken care of, we recommend installing a dehumidifier in your basement or crawl space to remove any other humidity in the air and keep it permanently dry.

What are the benefits of fixing these problems?

If you choose to have one of our dehumidifiers installed, it will clean and filter the air, reduce odors and prevent mold and mildew growth. The air in the house will then be healthier, and your quality of life will be better.

By tackling any rotting wood or sagging floor joists and floors, the house will be structurally stable.

Families that invest in crawl space and attic insulation can reduce the stack effect’s impact and enjoy savings between 30% and 50% on their energy bills. By installing the correct kind of insulation in these critical areas of your home, you can stop the escape of energy through your attic and crawl space and significantly reduce your energy expenses. (Source: Clean Crawls)

How do we fix these problems?

The CrawlDrain™ perimeter drainage system collects seeping water from the walls and floor, then directs this water to a sump pump like the SafeDri ™ Pro Crawl Space Sump Pump System to be pumped out of the crawl space and away from the home.

The CrawlSeal™ moisture and vapor barrier is a true 20-mil liner that isolates the crawl space and home from the earth. It is completely waterproof and mold resistant.

Our low-profile crawl space dehumidifier fits perfectly into tight spaces, and it does the job to effectively clean and filter the air under your house. This powerful, yet energy-efficient unit also helps control moisture, humidity, odors, and mold growth.

Properly sealed crawl space vents and doors prevent outside air from entering your crawl space.

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FRS Service Area Map with Des Moines

Our Service Areas

Des Moines Location

2401 SE Creekview Dr.
Ankeny, IA 50021

Kansas City Location

211 SE State Route 150
Lee's Summit, MO 64082
(816) 774-1539

Moberly Location

1401 US-24
Moberly, MO 65270
(660) 202-8662

Springfield Location
1820 N Barnes Ave
Springfield, MO 65803
(417) 612-8286
St. Louis Location

1625 Larkin Williams rd.
Fenton, MO 63026
(314) 207-9995