The Stack Effect
Why Crawl Space and Basement Humidity Control Are Important

The stack effect is the movement of air into, through and out of buildings.

We all know that warm air rises. That, after all, is what gives a hot-air balloon its buoyancy. Warm air rises inside the house too and lowers the air pressure at the base of the structure. Air from outside the house is drawn in by the lower pressure though doors, windows and vents in the lower part of the building.

As that air is warmed within the house it too rises and the process repeats in a continuous cycle. This is called the stack effect because it is the same process which makes a chimney effective in expelling smoke and other gases from a building.

This phenomenon, sometimes also called the chimney effect, is a good thing as it ventilates the house but it has unwanted side effects. The lowest part of many houses is the crawl space and/or the basement. Crawl spaces and basements are cooled by the earth and so the air from outside will be cooled as it is drawn inside.

If the air from outside is high in humidity this cooling effect may cause liquid water to condense on surfaces inside the crawl space or basement. This effect is therefore a major reason why so many basements and crawl spaces are damp and humid.

Wet basements and crawl spaces are a breeding ground for mold. The damp conditions are also ideal for dust mites which thrive in damp, humid conditions. The mold itself provides food for the dust mites by breaking down organic compounds into simpler forms which the dust mites can digest.

The stack or chimney effect is therefore largely responsible for the billions of mold spores and dust mite droppings which are so uncomfortable to live with for many allergy sufferers.

Now, think about a chimney. When the warm air rises we see smoke. The visible smoke is made up of tiny particles of carbon we call soot. The stack/chimney effect carries particles up through the house in the same way but these are invisible; mold spores and dust mite droppings. What started as a problem in the basement or crawl space is now afflicting the whole house and its occupants, particularly those who are sensitive to these allergens.

This is why the stack/chimney effect, and its consequences are among the main reasons why it is important to control humidity in our basements and crawl spaces.

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