Crawlspace Moisture Problems

Did You Know? The 5 most common problems in a home with a crawlspace (mold, pests, cold floors, buckling floors, and wood rot) are all caused by abnormal moisture levels.

It used to be assumed that simply installing a few more crawlspace vents to bring in outdoor air fixed moisture problems; but did you know this can actually make crawlspace moisture problems worse?

According to an 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.

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What you can do about crawlspace moisture problems:

  • Moisture Control

    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.

  • Dehumidify

    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.

  • Post-Insulation

    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.

More Info? Learn more about the common areas for abnormal relative humidity levels: The Top 4 Most Common Places for Mold Growth In Your Home

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