The Stack Effect: What is it and how does it affect you?
WHAT IS 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. Basements and crawl spaces are naturally cooled by the earth around them, which keeps a relatively constant temperature year-round. Depending on the season, the air inside your basement or crawl space will either be warmer or cooler than the air outside. During winter months, warmer air will enter the basement, crawl space, or lower level and exit through the attic or upper levels. In the summer, the stack effect is reversed: warmer air enters through the attic and upper levels, cools, and then travels down and out of the basement, crawl space, or lower level.
HOW THE STACK EFFECT CAN LEAD TO HUMIDITY AND MOLD
Imagine this: It is 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; as it cools and "shrinks," it can 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 relative humidity goes up, even if no new water is added to the air. This is because, 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. Moisture inevitably leads to mold growth. With the stack effect constantly circulating air throughout your home, this air also disperses mold spores, humidity, and odors. As humidity settles in your home, the likelihood of mold increases significantly.
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