Mold in Crawl Space: Causes, Removal and Encapsulation

Crawl spaces have everything mold needs: moisture from bare soil, humid outside air, little light and plenty of wood to feed on. University of Georgia Extension lists water in the crawl space as a possible source of indoor humidity. Fixr notes crawl spaces grow mold for many similar reasons as basements, such as dampness and high humidity levels.

The main moisture sources:

• Bare soil. Ground moisture evaporates into the crawl space air. FEMA's mold guidance says to cover dirt in crawlspaces with plastic to prevent moisture from coming from the ground.
• Poor drainage. FEMA says the ground around the house needs to slope away to keep basement and crawlspace dry, and rain water from gutters or the roof needs to drain away from the house.
• Plumbing leaks. Supply and drain lines often run through the crawl space, and a slow drip can go unnoticed for months.
• Ductwork condensation. The EPA recommends making sure ducts are properly sealed and insulated in all non-air-conditioned spaces such as crawl spaces, because cold ducts sweat in humid air.
• Floods and storms. FEMA lists a damp basement or crawl space among the water problems that lead to mold. After a flood, read our mold after a flood in Texas guide.

In Texas, NOAA's climate summary says the Gulf serves as the primary source of moisture, which is most readily available to the eastern part of the state. Annual rainfall ranges from less than 10 inches in the far west to more than 60 inches in the extreme southeast, so a crawl space in Beaumont or Houston faces far more moisture than one in El Paso.

• A musty smell in rooms above, especially near floor vents or closets
• Visible fuzzy or stained growth on joists, beams or the subfloor
• Wet or sagging insulation between floor joists
• Standing water, damp soil or condensation on pipes and ducts
• Floors that feel soft or bouncy

The EPA notes you may suspect hidden mold if a building smells moldy, but you cannot see the source. TDLR, which licenses Texas mold professionals, lists crawl spaces among the hidden areas to inspect if you notice a musty odor. Our mold smell in the house guide can help you trace it.

Crawl space mold removal follows the same rules as anywhere else in the house: fix the water, then clean what can be cleaned and remove what can't.

Wood framing (joists, beams, subfloor). TDLR says mold growth should be cleaned from non-porous surfaces such as concrete, metal, glass, tile, and solid wood. The EPA's professional table lists wet vacuum, damp wipe and HEPA vacuum for wood surfaces, adding removal only for areas greater than 100 square feet. NDSU Extension says scrubbing may not completely remove mold growth on structural wood, such as wall studs, so it may need to be removed by sanding. See our mold on wood guide for more.

Insulation. Moldy batts between floor joists usually come out. TDLR says mold is difficult to clean from absorbent (porous) surfaces such as drywall, carpet, fleecy furnishings and insulation, so those should be removed and discarded. The EPA's water-damage table says to discard and replace wet fiberglass insulation.

Concrete and block. The EPA lists concrete or cinder block among materials that can be cleaned rather than discarded at small and medium sizes.

What pros do. Fixr says professional crawl space removal may include containment, air scrubbers, vacuuming, or cleaning the impacted surface areas with antimicrobial cleaners. For the full sequence, see the mold remediation process.

Before you put insulation back, let the wood dry. University of Georgia Extension says do not replace insulation and wallboard until wood framing moisture content is 15% or lower. That advice is written for walls after flooding, but the same logic applies to floor framing.

Crawl spaces are tight, dirty and often wet, and many have pests and exposed wiring. If you go under there yourself, the EPA's minimum gear for small areas is gloves, eye protection, and an N-95 respirator.

Cleaning without fixing moisture just resets the clock. There are three main levels of moisture control.

1. Ground vapor barrier

This is plastic sheeting laid over the soil. Texas A&M AgriLife Extension says at least 70 percent of the crawl space soil under pier and beam structures should be covered with a 6-mil polyethylene water barrier, which helps prevent moisture buildup in the subflooring. University of Georgia Extension's checklist says at a minimum, there should be a 6-8 mil. plastic, or vapor barrier, covering the soil.

2. Vented crawl space done right

AgriLife's guidance for pier-and-beam homes calls for 1 square foot of ventilation for each 150-square-foot section of crawl space area, and a crawl space with a minimum 18-inch clearance between floor joists and the soil. UGA adds that vents should be located near each corner, within the top 8 inches of the foundation.

3. Crawl space encapsulation and mold

Encapsulation goes further: a heavy liner sealed to the walls and piers, closed vents and some form of drying. EPA's Indoor airPLUS program, for homes it certifies, requires that crawl spaces without concrete slabs get a Class A vapor retarder as defined by ASTM E1745, installed by sealing or taping all seams and penetrations and sealing to walls and piers. In moist climate zones, it requires that basements and crawlspaces are served by a HAC system with RH controls or a supplemental dehumidification system, plus a floor drain with trap seal OR moisture monitoring system with audible alarm in the lowest area.

Two cautions:

• Get the details right. The EPA warns that vapor barriers must be located and installed properly or the building may have moisture problems.
• Remove the mold first. Bob Vila's cost guide notes that in existing homes mold must be dealt with before encapsulation, warning that unresolved mold or pests could become worse hidden under the vapor barrier.

Drainage

If water pools in the crawl space after rain, a liner alone won't solve it. Start outside: grade soil away from the foundation and route gutters away, as FEMA advises. UGA also suggests checking that any plantings around your foundation are not overwatered. Interior drains and sump pumps are the next step.

Table: Crawl space mold remediation cost and moisture control costs, national estimates. No source gave Texas-only prices. The encapsulation figures are older (2022), so expect current quotes to differ.

Fixr says crawl space removal usually costs less than basement work because crawl spaces are much smaller and more contained spaces. Structural repairs are extra: Fixr says repairing mold damage costs between $75 and $3,000, in addition to the remediation costs. More detail is in our mold remediation cost guide.

• The mold area is large. TDLR says that in most instances areas of visible mold less than 25 contiguous square feet in area may be cleaned or removed by people who are not licensed. Beyond that, hire a licensed remediation contractor. (Owners of residential buildings with fewer than 10 dwelling units are exempt for their own property.)
• The wood is soft. The EPA says molds can weaken floors and walls as the molds feed on wet wood. Sagging joists need a contractor or foundation specialist.
• Floodwater got in. The EPA says to call a professional if damage was caused by sewage or other contaminated water.
• The HVAC runs through it. The EPA says do not run the HVAC system if you know or suspect that it is contaminated with mold.

Texas law says a license holder may not perform both mold assessment and mold remediation on the same project. If you want the extent mapped first, hire a mold testing company, then a separate mold removal company. Find licensed companies in Dallas, Fort Worth, Corpus Christi or anywhere in Texas, and verify any license with our license check. Here's how we verify listings.