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How A Vented Dirt Crawl Space Affects Your Home

Tuesday, May 31st, 2011 by Nancy Videon

Moisture comes from two sources... the ground and the outdoor air

Water can enter a crawl space under the footing, through the block walls, cracks and openings or from plumbing leaks. Once in your crawl space, it sits there in pools (or in some cases, we refer to it as a lake). The exposed earth also contributes a lot of water vapor into a dirt crawl space.

The earth is damp, and as the damp soil dries, the water vapor flows upward into the house by what is called the Stack Effect. A house’s natural tendency is to draw air in from the crawl space and lower levels, and then upward into the living spaces. Building scientists say that 30% to 50% of the air you breathe on the first floor is air that came from the crawl space. Therefore, whatever is in your crawl space air is in your house and affecting you. If there is mold in the crawl space, there are mold spores upstairs. If there are damp odors in your crawl space…you guessed it…there are damp odor upstairs.

Previously the tactic was to add vents to a crawl space with the idea that air will flow in through the vents on one side and out through the vents on the other side. These vented crawl spaces, instead of becoming less damp, end up being more wet. Rather than letting moist air out, a house sucks moist air in, and then up into home!

Flooded Crawl Space in Georgia and South Carolina

Building scientists say that 30% to 50% of the air you breathe on the first floor is air that comes from the crawl space.

What is thought to be a logical solution to stopping the flow of water vapor up into the home, some home owners opt for spray foam insulation on the crawl space sub-floor. The biggest issue with this idea is that it does not address the main problem- moisture from the dirt beneath a home and humid air entering through the vents. The wood members in the crawl space are still exposed to ground moisture. There are only two primary ingredients needed for rot and mold to thrive: one is organic material, like the wood beneath a home, and the second is water.

Air that flows in through the crawl space vents brings its moisture with it. When air is heated or cooled, its relative humidity changes. The relative humidity of air goes up by 2.2% for every degree we cool it. Crawl spaces are cool due to the earth averaging a temperature of 55 degrees year round. When warm humid air enters into a crawl space, the air is cooled, and the relative humidity goes up. This high relative humidity leads to sweating HVAC ducts, mold, rot, and energy loss.

Condensation Building Up in Georgia and South Carolina
Insulation Problems in Georgia and South Carolina

“Sometimes the solution is not a solution, and worse than the problem” — Lawrence Janesky

Our mold removal company in Savannah has seen a few homes in which installation of spray foam on the sub-floor created a greenhouse effect, trapping the ground moisture in the crawl space, and leading to mold growth and structural decay. You may be solving one problem- water vapor flowing up into the home- but you may also be creating a new one!

To eliminate your vented dirt crawl space from having any negative effects on you home.

  • Fix the ground water leakage.
  • Isolate the house from the earth.
  • Seal the vents and other outside air leaks.
  • Condition or dehumidify your crawl space air.

The CleanSpace® System meets all the criteria for fixing your dirt crawl space.


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