Living with Mold Risks

How To Live With A Dirt-floor Crawlspace


Main Problems:

Moisture intrusion and organic materials that grow mold.

Air contamination that spreads mold throughout the residence.

Severity of Mold Risk:

Cost of Recommendation:



The Problems

Crawlspace Is More Susceptible to Mold Problems Due to Moisture Intrusions

All spaces built below ground level are very susceptible to moisture intrusion. And as we know, moisture (water) is necessary for mold to grow. This moisture intrusion may come from:

  • Hard rains or flooding brought in by gravity
  • A high water table bringing water through the flooring or walls
  • Excessive humidity from moist ground against the subgrade walls and flooring without a moisture/vapor barrier or with a compromised moisture/vapor barrier
  • Leaks from plumbing or spills on the ground floor or upper levels
  • A failed sump pump, which creates standing water

Other factors also contribute to making mold growth more likely. There is often very little ventilation in crawlspaces to push contaminated air out and bring clean air in. The organic building materials of these spaces make things worse since they provide a mold food source. All these factors contribute to mold, yeast, and bacteria growth. 

Mold-Contaminated Air Travels Up Through Your Residence

If the dirt-floor crawlspace does get a mold problem, that contaminated air will be carried up through the residence by what is known as the “stack effect”. The stack effect is defined as the movement of air into and out of buildings through unsealed openings, chimneys, flue-gas stacks, or other containers, resulting from air buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The stack effect helps drive natural ventilation and air infiltration. Bottomline — construction science tells us that mold in your crawlspace will travel up into the rest of your residence.

Mold Prevention & Recommendations

How To Live With A Dirt-Floor Crawlspace


Monitor Humidity & Moisture



Check for Water Intrusions  – Ensure rainwater, groundwater, watering of landscaping aren’t entering the crawlspace.

Monitor Humidity – Ensure humidity is kept below 50%. If humidity is high, or the crawlspace soil is periodically moist, a dehumidifier should be installed.

Our Recommended Hygrometer

if Needed

Install French drains and/or sump pump – If water intrusions do occur, install French drains to divert water away from the crawlspace, and/or a sump pump to remove standing water.

Wear respirator protection, N95 mask or P100 respirator, any time you enter a crawlspace.


Improve Airflow to Dilute Contaminants



Vent Crawlspace Air – Crawlspaces should be vented to allow radon, humidity, mold, and organic vapors to escape and to be diluted. However, care should be taken to ensure that the ventilating of the crawlspace does not result in humidity being added at levels greater than 60%. Crawlspace vents should be installed near the foundation and located on opposite walls of the crawlspace to allow for cross-ventilation. The vents should be large enough to allow for adequate airflow and kept free of debris.


Strongly Recommended

Add a Vapor Barrier between soil and crawlspace air – Crawlspaces should have a vapor barrier between the soil and the crawlspace air. This is often accomplished by laying down poly sheeting on soil with a thickness of at least 6 mil, but preferably a 20-mil thickness. The sheets should overlap each other by at least 4 inches, and be joined together with duct tape. We recommended adding a layer of lime to the top of the soil prior to laying the vapor barrier as this will inhibit microbial growth.


Inspect & Maintain Yearly



Wooden Structural Members ensure that the wooden structural members (subflooring, supports, etc.) do not show water damage or mold growth.

Insulation Condition – ensure all insulation is properly installed and in good condition. Exposed fiberglass insulation will hold on to moisture and mold. If the insulation is in poor condition or made of fiberglass, we recommend replacing with either encapsulated insulation or foam board insulation.

Foam board insulation at home depot.

Penetrations into Building – Ensure penetrations into the building (the subfloor, foundation, etc.) are completely sealed. Sealing is commonly done with spray foam or silicone sealant.

Ductwork Condition – Ensure that any ductwork in the crawlspace is sealed, not leaking, and well insulated. Poorly sealed or leaking ductwork means crawlspace air can enter your home bringing with it any contaminants including mold. Due to the Venturi Effect, even the tiniest holes are enough to leak air into your home. Patch with duct sealant and/or duct tape as necessary. Also, ductwork Insulation prevents condensation on your ductwork. 

Clean Debris – Keep your crawlspace clear of debris, e.g. leaves, trash, old insulation, etc.

Pests – Check for pests, such as rodents or insects, and eliminate if necessary.


Storage – Do not store items in the crawlspace that will later be brought into the house (e.g. – Christmas decorations, boxes, keepsakes, etc.)

Remove Visible Mold



Remove visible mold with Hydrogen Peroxide – If any staining or possible mold growth is observed on building materials, spray with a 12% Hydrogen Peroxide solution. Great caution should be exercised when using Hydrogen Peroxide, wearing appropriate PPE (personal protective equipment) and carefully following safety instructions. For the best results, apply a non-toxic sealant to all exposed wood once the Hydrogen Peroxide dries.

Our Recommended Hydrogen Peroxide Solution. Read their detailed instruction and guide here.


Periodically Test for Mold



Every 6-12 months check your crawlspace air for mold growth with the Mold Check-Up. If your results show significant mold growth, that could be an indication of mold in your crawlspace.

Guides for tackling mold

Living With Mold Risks

Guides for reducing mold and maintaining cleaner
environments in situations we can’t change.

Flat Roof



Visible Mold


Indoor Plants


Evaporative Cooler