How The Stack Effect & Your Crawl Space Affect The Air You Breathe
How Air Movement in Your Home Can Lead to Mold, Rot and Damage
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.
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. 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.
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 & humidity in a basement or crawl space.
Moisture allows mold to grow. When mold grows, a by-product of their metabolism is mold volatile organic compounds (MVOCs). These compounds are gaseous and enter the air, and are responsible for the mildewy or moldy odors associated with damp environments. Sometimes homes with a damp crawl space will smell musty or have an “earthy” odor. This moldy odor can infiltrate into the living or occupied areas of a home.
Controlling Humidity & Mold
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 an energy efficient dehumidifier. This will remove any existing humidity in the space and keep it dry in the future.