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Crawl Space Humidity 101: A Complete Guide to Crawl Space Moisture Control

A crawl space home foundation differs from a basement or slab. The typical crawl space has a dirt or gravel floor and isn’t tall enough for an adult when standing straight up.

A crawl space raises the lowest habitable portion of the home above the base flood elevation (BFE). That means if there is a flood, the habitable areas of the home shouldn’t get wet.

Do you own a home with a crawl space? There are a few things you should know about maintaining the area beneath your home.

Maintaining proper crawl space humidity levels is important for discouraging mold growth and other unhealthy conditions.

Keep reading for a guide to proper crawl space maintenance and extend the health and life of your home.


Maintaining your crawl space at a level of about 55% humidity keeps the air healthy. Here are a few of the bad side effects of high humidity in your crawl space:

  • Wood rot
  • Mold and mildew growth
  • Insulation damage

Mold and mildew on the wood cause wood rot which destroys the stability of your home. So what can you do?

Measure the humidity levels in your crawl space with a digital thermo-hygrometer. You can find one as low as $15 dollars. Is the humidity above 65 percent? It’s time to take action.


There are several reasons water gets into the crawl space. Find and fix those problems before tackling the crawl space itself.


Bulk water is standing water in the crawl space. Do you see several inches or more of water?

If so, look for plumbing leaks and drainage issues. A common cause of bulk water in the crawl space is improper sloping of the ground outside the home.

If the ground slopes toward the house, there’s a problem. The ground should decline away from the house at a 5% grade or more. The ground sloping down toward the foundation causes water seepage into the crawl space.


If the ground itself is full of water, it evaporates into the crawl space air. Sometimes the soil appears dry but that’s because it’s evaporating into your crawl space.

After installation of a thermo-hygrometer, you’ll know if you’ve got a ground moisture evaporation problem.


Crawl space vents are an outdated fix for humidity in the crawl space. When it’s damp outside, the crawl space air is vented with damp air! That doesn’t help with a moisture problem.

On a hot, humid summer day, the air venting into the crawl space could be as much as 75% relative humidity and 85 degrees. When it hits the cooler air of the crawl space, it increases the relative humidity by about 2% per degree!

This means your crawl space humidity goes up by about 22 degrees. This causes condensation along the surfaces inside the crawl space. Thus vents aren’t an ideal solution for humidity control.

In the winter, your crawl space dries out but the vents waste a lot of heat. That’s heat you’re paying for!

What solutions are best for reducing crawl space humidity?


If there’s bulk water in the crawl space, identify the problem fast. If it’s soil sloped toward the foundation, get out there and grade the soil. Make sure the dirt slopes away from the foundation.

Do you suspect a plumbing leak? Get a professional and check out all the pipes under the house. Sometimes old copper pipes have leaks as small as a pinhole.

Over time, these tiny leaks add up to big water problems.

Sometimes, an outdoor issue such as shedding water from the neighbor’s home is the problem. If that’s the case, a sump pump is a good fix.


A sump pump is a pump in a bucket dug into the ground in the crawl space. The water drains to the lowest spot and ends up in the bucket. The pump then comes on and drains the water to the outside of the home.

If you’ve got a sump pump, check it a couple of times per year and make sure it’s working well.


What’s a vapor barrier? It’s a thick sheet of plastic that covers the floor of your crawl space. If you have a dirt floor in the crawl space, a vapor barrier is crucial.

The vapor barrier keeps moisture underneath the plastic and away from the wood joists of your floor. It also keeps the insulation dry.

Use a professional company for this step so you know it’s done right. The vapor barrier protects the air you breathe inside your home. You want it as clean and free of allergens as possible.


There’s one last step that’s sometimes necessary for maintaining proper humidity levels in the crawl space. That’s the installation of a crawlspace dehumidifier.

A crawlspace dehumidifier is installed in the crawl space, positioned for optimimum air circulation. It’s fan circulates the clean & dry air throughout the space. This keeps crawl space humidity levels down.


Keeping the right crawl space humidity level is crucial for the health of your home. The right humidity level discourages mold and mildew growth and keeps insulation dry.

Preventing mold growth is important since the air you breathe inside your home originates in the crawl space. Unhealthy air in the crawl space means unhealthy air in your daily living space.

Measure the humidity level in your crawl space first. If it’s too high, identify the problem and fix it. If there’s a dirt floor, install a vapor barrier.

If the humidity level is still too high, consider installing a crawl space dehumidifier.

Keep your home healthy for many years of trouble-free maintenance by keeping your crawl space dry!

Need help with your crawl space? Request a free inspection now!

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