Photograph: typical mold on floor joists and subflooring over a wet crawl space -  © Daniel Friedman Crawlspace Mold: How to Find, Test, & Prevent Mold in Crawl Spaces

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This article explains how to find and deal with mold in building crawl spaces. We explain how to spot crawl space mold contamination, when mold testing is appropriate, the risk of mold-contaminated crawl space insulation, the causes of crawl space mold, and how to clean or remove crawl space mold as well as how to prevent moldy crawl spaces in the first place.

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CRAWLSPACE MOLD: finding the problem mold, allergens, or other particles in crawl spaces

This crawl space inspection procedure for mold helps identify the presence of or locate the probable sources of mold reservoirs in buildings, and helps decide which of these need more invasive, exhaustive inspection and testing

The photograph at page top shows extensive fungal growth on the underside of subflooring and on wood framing supporting a wood floor over a very damp crawl space. Depending on the mold genera/species, if an airborne fungus is present at high levels in a crawl space, spores of that mold may move easily to upper levels in the building.

Photograph moldy fiberglass insulation which has fallen onto the concrete surface of a wet crawl space -  © Daniel Friedman
We very often find that insulation in crawl spaces has become a problem mold reservoir. Some of these molds move easily from the crawl area up into the living space.

For example, Penicillium sp. or Aspergillus sp. spores, which can be as small as about one micron in diameter, are so tiny and light that they can move much like a gas in a building.

These tiny spores rise in upwards-moving air currents as warm air, rising to upper building levels, creates a slightly lower air pressure in lower building areas such as basements and crawl spaces.

Depending on the tightness of construction, and the presence or absence of hot air or central air conditioning systems which themselves may increase indoor air particle movement, the rate of movement of particles from a moldy crawl space can vary widely.

In some instances we have found rather high levels of airborne mold spores and mold spores in settled dust that tracked directly to the moldy crawl space.

In other instances we found that there was not much air and particle movement upwards from a crawl space or basement, until specific triggering conditions occurred.

These included opening of a basement door, or in the case of crawl spaces, opening upper floor windows or turning on a whole house exhaust fan. These conditions led to a significant increase in movement of crawl space (or basement) mold from lower building areas into the living space.

First aid for moldy crawl spaces - what to do about mold in a crawl space

  • Do not enter or work in a moldy area without wearing proper respiratory and other personal protection. See CRAWL SPACE SAFETY ADVICE
  • Evaluate the history of water entry, dampness, ventilation, listing the factors conducive to fungal growth such as:
    • Present or past wet conditions, whether "once", episodic, or recurrent water entry
    • Marginal or no ventilation. Actually I prefer to stop trying to vent crawl spaces since when we want venting it's usually inadequate, and depending on weather conditions venting a crawl space can actually increase its moisture level and make matters worse. A preferred approach is to dry out, enclose, and seal a crawl space under a building, making it into conditioned space.
    • Exposed dirt. If the crawl space has a dirt floor, put down 6-mil plastic over the dirt to reduce moisture movement from the soil into the crawl space. Also see MOLD ON DIRT FLOORS
    • Presence of fiberglass or other fibrous insulation
    • Fix outside conditions that are causing wet crawl space conditions such as roof gutter or downspout spillage by the building foundation.
    • Fix inside conditions that wet crawl spaces, such as plumbing leaks or improper ventilation.
  • Look for visible mold:
    • Inspect the exposed sides of all framing, joists, girders, posts
    • Inspect the exposed under-side of subflooring of the floor overhead. See our warning about mold growth on pine boards discussed at Attic inspections above.
    • Special screening tests may assist in evaluating the condition in inaccessible crawl spaces.
  • Prevent mold movement into the living area: As with a moldy basement, keep openings between the crawl space and living space closed as much as possible.
  • Remove wet or mold-suspect insulation.
  • Test representative samples of mold before going to a major expense to be sure you're not just looking at cosmetic mold on framing lumber, or at something else that is not even mold.

    If a large area of visibly moldy material is present (more than 30 sq .ft.) professional cleaning and remediation are probably in order. In that case, a building inspection and preparation of a mold remediation plan is a good idea - that work should be performed by someone who has no conflicts of interest - in other words, not by the mold cleanup company or their agent.

    Follow any costly mold remediation job with a clearance inspection and test before you make final payment.
  • Review MOLD PREVENTION GUIDE Correct the Causes of Mold and Prevent Indoor Mold or other indoor environment problems

Risks of Mold Contamination in fiberglass insulation in a crawl space

Photograph moldy fiberglass insulation which has fallen onto the dirt surface of a crawl space -  © Daniel Friedman

These photos show crawl space fiberglass which had fallen onto the dirt crawl space floor.

We can pretty much count on this material to be mold contaminated unless the building was in an arid climate.

You can expect to find rodent contamination as well in insulation in this condition.

Photograph moldy and rodent infested fiberglass insulation can be expected where fiberglass isused in an open crawl space -  © Daniel Friedman
This photo shows crawl space fiberglass which the installer supported by chicken wire. The crawl space was pretty drafty which we might hope would keep it dry and less moldy.

However we might expect to find a nice mouse colony here, and depending on weather variations and crawl space flooding, this insulation too is at risk of becoming a mold reservoir.

We'd have preferred to use a solid foam insulation in conditions like this.

Key building water entry diagnosis and cure articles:

This article is part of our series: MOLD in BUILDINGS which describes how to find mold and test for mold in buildings, including how and where to collect mold samples using adhesive tape - an easy, inexpensive, low-tech but very effective mold testing method.

Reader Question about Mold in Dirt Crawl Space

Hi I have attached a few pics of the mold growth on my dirt crawl space each pic is isolated and no further spread is seen anywhere else. I was actually shocked to see this strange looking growth. I do keep the crawl vents open but there is no forced air movement at all. Please assist as to how to safely remove.

Mold fungi growing on dirt in a crawl space (C) InspectApedia GE

The house sustained a fire in may of 2014 and the ac system has been off since. There are fans running in house though. The only power to crawl space is to the sump pumps that discharge any water to the outside via pipes which none of the pics of the mold spores / mushrooms are even close to the sump liners of perimeter pipe in perimeter ditch.

There is no black plastic liner on dirt floor and no floor joist insulation, I removed all several years back when I dug my perimeter ditch and sump pits with the plastic liners. Thanks for any guidance! - G.E. 9/8/2014

Mold fungi growing on dirt in a crawl space (C) InspectApedia GE 


Nice photos - that blue fungus is not one I've seen before. I've posted all of your photos also at MOLD on DIRT FLOORS.

Most likely these grow where you see them because of a combination of moisture and organic material spilled on or in the soil.

Certainly you describe a damp or wet crawl area and one that was previously flooded (during fire extinguishment) - all of which invite fungal growth therein.

You can dig out (just an inch or so) the visible mold, toss it into the woods or into garbage, then put down plastic, and take other steps to dry out the area.

Probably more important would be mold growth on the exposed wood framing or in insulation if there is some in the crawl area. For that topic

or for more about the development of mold in fiberglass insulation see FIBERGLASS INSULATION MOLD

See CRAWL SPACE DRYOUT - live link just below - for procedures to prevent further mold problems in the crawl area.


Continue reading at CRAWL SPACE DRYOUT or select a topic from the More Reading links shown below.

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CRAWLSPACE MOLD ADVICE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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