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Why Your Crawl Space is Damp, Smells Musty, and Has Mold

Tuesday, August 13th by Kristina McGovern


Learn how moisture makes the crawl space damp, musty, and moldy - and what you can do to fix moisture problems underneath your home.

Maybe you can’t remember the last time you looked inside your crawl space, but if the air in your living area stinks and feels damp, your hardwood floors are buckling, your carpets are damp, or your allergy symptoms are getting worse, the cause could be a moisture problem in your crawl space.

4 ways moisture gets inside the crawl space

While the exact ways that moisture enters a crawl space will vary based on whether the crawl space is vented or sealed, there are four main causes of a crawl space moisture problem:

  1. Outside air through open vents

    Outside view of an open vent in a crawl space foundation

    In the past, builders tried to dry the crawl space out and prevent moisture problems by adding vents to the foundation, but more recent studies have shown that crawl space vents do more harm than good. The vents let in hot, damp air - and they can also let in water.

    What happens in a vented crawl space on a humid summer day?

    Let's say the air in the crawl space is 68 degrees F and the surface temperature of the walls, dirt floor, and floor joists is 62 degrees Fahrenheit (F). And the air coming in through the vents is 77 degrees F with 80% relative humidity.

    The relative humidity is how full of water the air is relative to the maximum amount of water it can hold at a given temperature. For every one degree that the air is cooled, the relative humidity goes up by 2.2%.

    The difference between the temperature of the crawl space surfaces (62 degrees F) and the outside air (77 degrees F) is 15 degrees F. When we multiple 15 degrees F by 2.2%, we get a 33% increase in relative humidity where the outside air comes in contact with the surfaces inside the crawl space.

    When we add 33% to the original 80% relative humidity, we get 113% relative humidity. At 100% relative humidity, the air can’t hold any more water and it releases moisture in the form of condensation.

    When the warm, humid air coming into the crawl space comes in contact with the cold surfaces in the crawl space, such as the walls, dirt floor, ducts, water pipes, floor joists, and sill plate, condensation forms and you can end up with mold, rot, and a musty smell.

    Lower humidity doesn't mean you're in the clear.

    Let’s say the air outside is 72 degrees F with only 60% relative humidity. The air is cooled to 62 degrees F when it comes in contact with the crawl space surfaces. That’s a difference of 10 degrees F, which will increase the relative humidity by 22% (multiplying 10 degrees F by 2.2%) to 82%. The problem is when the relative humidity is over 70%, we end up with mold, fungus, and rot.

    Mold and fungus caused by moisture are just a few of the ways a dirt crawl space negatively affects your health.

    What to Do: Seal the crawl space vents to keep damp outside air from coming into the crawl space. The crawl space walls and floor should also be covered with a vapor barrier to protect against water and moisture coming through the foundation walls and up from the dirt or concrete floor.

  2. Exposed dirt

    Interior view of a dirt crawl space

    Exposed dirt in the crawl space adds a lot of moisture to the crawl space air. The ground is damp and as the damp soil dries, the water evaporates upward into the crawl space. This invisible flow of water vapor is constant.

    What to Do: Add a vapor barrier over the dirt floor to stop water vapor from evaporating into the crawl space.

  3. Groundwater leakage

    A crawl space with a large puddle of water on the gravel floor

    Water from the soil around the foundation seeps and leaks into the crawl space in several ways, including under the footing, between the footing and the walls, through block walls, or through cracks in concrete walls. After the water gets inside, it forms puddles either on the exposed dirt or on top of the vapor barrier and eventually evaporates upward into the air and the house.

    What to Do: Stop water from pooling on the crawl space floor by installing a perimeter drainage channel and sump pump in the crawl space. There are also other steps to take to keep water out of the crawl space, such as cleaning the gutters and downspouts and fixing the grading around the house.

  4. Plumbing leaks

    Wet batt insulation between crawl space floor joists

    Water from a leaky drain pipe or water supply pipe can form puddles on the crawl space floor and wet fiberglass insulation. The standing water and increased moisture can lead to mold problems, rot, and attract pests.

    What to Do: Check for standing water below a tub, shower, toilet, or other plumbing fixture, or look for dripping water inside the crawl space when water is running inside the house. Call a plumber to fix any pipes that are leaking water into the crawl space. Adding a vapor barrier will protect against wet dirt and moisture caused by a plumbing leak. The TripleSafe and SmartSump sump pump from Basement Systems have airtight lids and floor drains that drain water from the top of the vapor barrier, such as in the event of a plumbing leak. Some local building codes require that a sump pump be installed in a sealed crawl space, so be sure to check with the crawl space contractor in your area.

The water or moisture problem in your crawl space is more than likely caused by a combination of issues. This is why it’s always best to have your crawl space inspected by a professional who knows what to look for and can recommend the best solution for your home.

An easy solution for a damp crawl space

While a crawl space moisture problem can be complicated, the solution is fairly simple - seal the crawl space to keep out moisture from the outside air and soil and install a dehumidifier to control the humidity level. Installing a sump pump and drainage system might be recommended to fix water seepage issues or to meet building code requirements.

If your crawl space is damp, musty, or moldy, learn about how our CleanSpace Crawl Space Encapsulation System can help your home or contact us for a Free Crawl Space Inspection from your local CleanSpace contractor.

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