House mold inspection: How can you prevent mold from forming under your house or in a crawl space? 

Are you going through a house mold inspection and coming across dampness under the house or in the crawl space? It's a good thing that house mold inspection caught this problem because you may want to be careful. Dampness in your crawl space can lead to mold and mildew growth, and in addition to being a health hazard, moldy conditions have the potential to cause rot, structural damage, and premature paint failure.

The good news about that house mold inspection is that moisture doesn't necessarily have to mean mold. At least not right now. Mold under the house or in a crawl space is in fact preventable. And according to the United States Environmental Protection Agency (EPA), the best way to control mold growth is to control the moisture; and wouldn't you have guessed it: the crawl space is one of the first places you should look when searching for moisture issues.

What to look for in a house mold inspection under the house or in the crawl space:

Common signs of moisture crawl space issues include wood rot, termite damage to subfloor materials, hardwood flooring "cupping," and visible mold growth on the wood surfaces underneath your home. Many crawl spaces have exposed earthen floors, which can allow ground moisture to seep out from underneath your home and increase the humidity levels - especially if you don't have a dehumidifier for crawl space.

What you can do to prevent mold growth under the house or in the crawl space:

Finding a moisture problem in a home during the house mold inspection before you buy it is a good thing. Mold growth can be prevented, and some solutions are as simple as installing a crawl space vapor barrier, addressing the grade of your landscaping, or properly guttering rainwater away from the property; however, a smart crawl space ventilation system and dehumidifier for your crawl space may be necessary to solve the issue.

The three most common solutions to this problem are:

  • Crawl space vapor barriers
  • Dehumidifiers
  • Crawl space ventilation systems

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Moisture Control

If you find moisture during a house mold inspection, Advanced Energy recommends adding interior and exterior drains for the crawlspace. Crawlspace drains should be separate from the foundation drains. Industry professional, Mike Trotter says, "Good exterior drainage is the first requirement of a crawlspace system."

Vapor Barriers/Liners

Once you've controlled the moisture issues, close the vents, and add a vapor barrier to the ground and the walls. The Advanced Energy research shows the minimum thickness required to stop the moisture is a 6 mil polyethylene liner.


It is good practice to place a meter in the crawlspace that will monitor the relative humidity. Relative humidity should be kept below 70%, yet not too dry. Advanced Energy's researcher Bruce Davis says they have run into problems where the relative humidity under the house is actually too dry. "You can over dry a crawlspace," he says. "If the wood moisture content is drawn down from 60% to 5% you will definitely see shrinkage and cracks in the floor. That is an extreme difference in relative humidity."

Pest Control

Another benefit of a closed crawlspace system is that lowering the wood moisture content lowers insect infestation. Advanced Energy studies saw a 70% reduction in insects.


You may experience an odor after the crawlspace is sealed. It is because the soil continues to decompose, while contaminates in the soil emit gases that rise and seep into the crawlspace. What can you do? Install a vent pipe and fan (similar to a radon mitigation system) to flush the soil gases to the outside of the house.

Ventilation, Ventilation, Ventilation

According to another Advanced Energy study, implementing additional ventilation to increase crawl space air circulation actually causes higher relative humidity levels, as well as ways for energy to escape. Plus, it exposes your home and invites pests from the outdoor environment into your crawl space.

The Advanced Energy field study shows consistent evidence that insulated crawlspaces outperformed ventilated crawlspaces. Here are the most important findings from the study on crawlspace moisture problems:

In sealed crawl spaces, wood moisture threshold stayed below 12% - which reduces the potential for mold growth and termite infestation affecting lumber framing.

Insulated crawlspaces were also dryer than traditional wall-vented crawlspaces: The daily average relative humidity level stayed below 70%.

When tested in a dryer climate, the sealed crawl spaces controlled normal moisture levels (around 50%); yet the vented crawl spaces kept 70% relative humidity for a only a few days.

Impressive Energy Savings: Homes with sealed crawl spaces and insulated floors saved about 20% in natural gas during the winter, and an added 15% annual energy savings compared to the control group.

More info? For more information about common places for moisture in your home, check out our related post: Crawlspace Moisture Problems