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Mold in a Crawlspace and How to Deal With It

By Sean Moss Oct 23, 2014

Living in the wet Pacific Northwest means that we have significantly more moisture coming down on us than other parts of Canada. For this reason, understanding potential moisture issues is important if you want to take care of your home, especially through the winter.

Although mold needs three main sources (moisture, food and moderate temperatures) to thrive, the single most important component is moisture. Take away the moisture and mold will not grow.

Since this article is about crawlspace mold, I will focus on why crawlspace mold develops, how to prevent and deal with it.

Moisture issues within the crawlspace originate primarily from the outside and occasionally from the living space above. Common sources include:

  • Damaged drainpipes
  • Pooling water against the foundation wall
  • Leaking downspouts and gutters
  • Broken sprinkler pipes
  • Foundation wall cracks
  • Foundation floor cracks
  • Land sloping against the foundation wall
  • Soil beds against the foundation wall
  • Large trees planted close to the home
  • Exterior crawlspace vents
  • Toilet, tub and shower leaks
  • Leaking pipes, drains and main water lines/shut-off valves
  • Missing exterior drains
  • Missing vapor barriers
  • Furnace condensate lines
  • Laundry venting into the crawlspace
  • Insulated crawlspace ceilings
  • High water table

Older homes with dirt floors always contribute to mold growth. The earth continually releases vapor moisture. In as little two days, when the conditions are ideal, mold can easily grow. Spores will drift into the living space above through air gaps from the crawlspace. This is why some houses with crawlspaces smell musty.

People can inadvertently contribute to crawlspace mold when they store documents, books and photos in cardboard boxes. Even worse, they can unknowingly contaminate their home after moving moldy items from the crawlspace into their living space. Subsequently, mold will grow on anything organic, such as dirt, dust, paper, under carpets, furniture, drywall, etc.

Another culprit for crawlspace mold is improper venting from laundry dryers into the crawlspace. At times the laundry duct is open or broken, releasing moist warm air into the crawlspace, further contributing to mold growth.

Efflorescence (a white powdery substance left behind on concrete, stone, brick, etc) is a telltale sign of moisture in the crawlspace. This is evidence of seepage, which could result in mold growth.

In damp crawlspaces, mold growth can usually be found on joists, studs, sill plates and insulation or any paper-based materials.

Another contributing factor for crawlspace mold is seepage from a high water table, or a home that has been built on a bog. This is not easily dealt with, as moisture will be constant. Try to avoid purchasing houses in these areas.

So How Do You Prevent or Tackle Mold?

  • From the outside, the home should have upgraded drain tiles installed, with a waterproof membrane to keep moisture out. All cracks on the foundation walls/floors should be properly repaired and monitored.
  • Sprinkler systems and soil beds (beside the foundation wall) should be removed. Perimeter stones rather than soil will encourage drainage, especially if the home is older and does not have a waterproof membrane around the foundation wall.
  • If the home does not have below-ground drainage, then divert all downspouts away from the foundation wall (ideally six feet to prevent pooling) by using downspout extensions or splash blocks. If the land slopes towards the home, call a landscaper to improve the situation.
  • If the home has clay, concrete or plastic corrugated drain tile (also known as “Big O”) call a drainage specialist to scope it for adequacy, service life, damage, clogs etc. In most cases, it will need to be replaced with a modern installation. Note: This is especially important if there are large trees growing nearby the foundation wall and or the home is older without waterproof membranes.
  • Contrary to conventional wisdom, crawlspaces should actually be conditioned, or heated like a basement, not vented. The reason for this is two-fold. First off, vents allow moist cold air into the crawlspace during the winter. Remember, we don’t want moisture in the crawlspace. Second, in the summer the vents allow warm air in. When the newly introduced warm air comes in contact with the cooler materials inside, condensation forms, resulting in moisture. So, regardless of the weather, vents contribute to crawlspace mold, so remove or cover the vents.
  • Take a good look at all pipes, drains and floorboards especially under bathrooms, kitchens, washing machines or water heaters. If you notice any leaks or moisture, call a plumber for immediate repairs.
  • When storing items in the crawlspace, place them in dry, sealed plastic bins. Never use cardboard boxes.
  • All exterior walls and floors should be thoroughly inspected for seepage or moisture issues. If you notice flooding (usually in the winter) then you will need to call a company specialized in dealing with basement or crawlspace flooding. A sump pump may be required to remove excess ground water.
  • If your dryer duct vents into the crawlspace, have it removed and properly vented outside. If the laundry vents through the crawlspace and to the outside, ensure that it is properly insulated and sealed at the penetration point through the wall.
  • If possible, crawlspace ceilings should not be insulated with fiberglass or other batts, as this too can contribute to condensation, leading to mold. The only time a crawlspace ceiling can be insulated is when the floor is dirt, does not have a vapor barrier, and the ceiling is completely sealed with an air barrier such as spray foam insulation. The goal is to create a complete environmental separation between the crawlspace and the living area above. In this case, the crawlspace walls will need to be vented as well. As mentioned, I do not recommend this type of solution. It makes maintaining pipes or inspections very difficult and does nothing to neither prevent mold nor address any residual mold on the ceilings. I have seen this from time to time in older homes, or when the homeowner experienced musty smells migrating from the crawlspace into the home.
  • The entire floor space should be covered with vapor barrier under a concrete slab. Walls should be properly insulated and vapor barrier applied. If the home has a dirt floor, it should be covered with a heavy-duty vapor barrier and completely sealed up the walls. A basement waterproofing or crawlspace repair contractor can help you with this.
  • If you have mechanical equipment in the crawlspace, have it maintained annually. Check for leaks.

If you have any reason at all to suspect mold or find it in your crawlspace, call a professional mold inspector or mold remediation contractor to help you pinpoint the moisture source as soon as possible. A mold remediation contractor should remove any mold growth immediately.

For more information on this or any other topics relating to mold or home inspections, contact me at 604-729-4261.

Sean Moss
Sean Moss is a home inspector focusing primarily on residential properties with specialized knowledge in mold and building envelope science. He has been featured in Vancouver Magazine, Richmond News and The Jewish Independent and he also shares his knowledge through articles and workshops. Call Sean on 604-729-4261, visit his website and see his rating on review website HomeStars.
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