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Crawl Space Mold Causes & Solutions

Crawlspaces are without question, the least pleasant portion of a home.  Rats, bugs, water damage, flooding, mold growth; the dark spaces beneath our houses take everything nasty that can occur and dump them in one place.  Like most problems, however, it doesn’t have to be this way.  Throughout this article, we’ll explore the most common problems and solutions found in this neglected area.

Primary Causes:

  • Saturation from Rain During New Initial Construction
  • Missing Vapor Barrier
  • Poor Ventilation (in certain climates)
  • Leak from above

Mold growth occurring during new construction:

A common scenario in wet climates such as the Pacific Northwest.  Houses built during the winter months can develop systemic mold problems if not dried correctly.  Dryout does not necessarily require dehumidifiers or heaters; often simple air movers and blowers can address the problem. Unfortunately, even the most basic dry out techniques are not implemented by contractors.  This leaves a crawlspace full of saturated lumber trapped above saturated soil; essentially a very large petri dish experiment.

Saturation occurs in two ways:

  1. Direct contact with rainwater and
  2. Secondary saturation due to moisture released from adjacent saturated materials, such as ground soil, concrete foundation, etc.
Common causes of mold in crawl space

Mold Growth in Crawlspace

Mold growth due to missing vapor barrier:

Primarily an issue in cool climates, missing vapor barriers contribute to the moisture load throughout the entire crawlspace.  Additionally, a missing vapor barrier can have a significant impact on the moisture load in the conditioned air space above.  The physics behind the problem is pretty simple.  If you recall from your high school science class, diffusion describes the movement of particles from areas of higher concentration to areas of lower concentration.  In the case of a missing vapor barrier, the concentration of water molecules in the soil is higher than the air, causing the moisture to migrate upward.

how to avoid mold in crawl space

Mold on Soil in Crawlspace

This particular crawlspace had a vapor barrier, though a previous contractor had shoved it off to the side.  Microbial growth on the soil won’t affect the structural integrity of the building, but it can have a profound effect on the indoor air quality.  The severity of the issues depends primarily on the dampness of the soil.  Some homes, especially those near the top of a hill, may have very dry soil and suffer few ill effects from a missing vapor barrier.  Most homes, however, encounter periodic dampness throughout the rainy season, necessitating a well-installed vapor barrier.

Crawl space flooding:

A vapor barrier doesn’t do much good when the water is floating on top of it… In reality, though, periodic crawlspace flooding typically doesn’t cause significant mold problems.  An exception should be made for hot/humid climates where the additional moisture load could lead to condensation on the underside of the floor assembly during cooling months.

  • Elevated water table
  • Improperly installed gutters and downspouts
  • Periodic surface flooding
  • Rainwater during initial construction
remove crawl space mold

Crawlspace Flooding

Crawl space as a storage unit?

Never a good idea.  The only exceptions I’ve found to this rule are conditioned crawlspaces with a concrete slab.  In a normal crawlspace, you’ll assuredly infuse everything you store with a musty odor.  In a problem crawl space, you’ll find your contents covered with mold growth and rodent droppings.


get rid of crawl space mold

Mold on Contents

Mold growth due to water leaks from above

Where does water go when a pipe leaks or a toilet seal fails?  If it occurs on the first floor of a home – directly into the crawlspace.  Of course, most people don’t want to enter a crawl space, so the moisture buildup often goes undetected.   Bathroom issues are the most common culprit.  Catastrophic incidents such as burst pipes wreak such havoc that the entire home is investigated.  Leaks from a toilet seal or shower pan are typically slow and can remain undetected for months.  Once the first sign of an issue is observed, extensive damage has often already occurred.

The photo below is a shower with the tile, pan and subfloor removed.  Much of the framing had completely lost its structural integrity.

what do i do about crawl space mold

Heavy rot and mold in crawl space.


Project Report > Inspection for Crawlspace Issues


  • Total of 4,140 sq ft.
  • Single Family Residence built in 1991

LOCATION: Back Bathroom


  • Rear portion of home had been flooded by a recently built retention pond behind the home.
  • Water damage was noted around the bottom 2-3 ft of sheetrock of the entire bathroom.
  • Minor mold growth was noted on the surface of the sheetrock along exterior wall primarily.
  • All baseboard trim was stained and water damaged.
  • Shower appeared to have had some moisture run under fiberglass pan.  This should be able to be cleaned / treated via opening the walls on either side of the shower.
  • Water damage was also noted in the ceiling and wall of the stall where the toilet was located.  No elevated moisture levels were noted at this time however, significant bubbling of paint and drywall tape were noted.
  • Carpeting leading out of bathroom requires removal.
  • Visible mold growth was also noted on contents located behind bathroom door.
  • Linen chest near door exhibited some visible mold growth and will need cleaning.
  • Vanity cabinet showed significant signs of water damage along the bottom edges and should be removed and disposed of.  Counter top and sink are in good condition and can be salvaged.
  • RH:48 % Temp: 60°F CO2: 518 ppm CO: 0 ppm


  • Shower does not require removal at this point.

LOCATION: Crawlspace

Crawlspace Ducting Issues


  • Good vapor barrier found throughout crawlspace.
  • Some pieces of insulation have fallen down due to missing or infrequent wire securing rods.
  • HVAC ducts did not appear to be sealed and allowed conditioned air to enter the crawlspace.
  • Many locations were noted where ductwork was not insulated.
  • Main HVAC trunk line did not appear to be suppored adequately and allowed contact with the ground.  This has resulted in insulation contacting standing water that occasionally infiltrates the crawlspace.  Visible mold growth was noted on the insulation at this time.
  • Evidence of rodent intrusion existed throughout the crawlspace.  Insulation on HVAC ducting appears to be contaminated by rodent feces and urine and should be removed and replaced.
  • Sump pump was present in crawlspace and appeared to be functioning well.
  • Drain from sump pump ties into exterior downspout drain and appears to spill water onto the concrete sidewalk whenever it operates.


  • Recommend having an HVAC contractor remove all HVAC insulation and seal all metal ductwork with high flow duct sealant.
  • Recommend supporting all ductwork to ensure it does not contact the ground.
  • Recommend having exterior downspout evaluated and repaired to prevent overflowing to occur from sump pump.

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