Your crawl space humidity affects your entire home. Too much moisture creates mold, rots wood, and makes your house uncomfortable.
This guide explains the ideal humidity level for your crawl space. You'll learn how to measure it, what causes problems, and how to fix them.
What Is Crawl Space Humidity?
Humidity measures water vapor in the air. Scientists call this relative humidity (RH), which shows how much moisture the air holds compared to its maximum capacity.
Think of it like a sponge. A bone-dry sponge holds zero moisture. A dripping-wet sponge can't hold any more. Your crawl space air works the same way.
When humidity gets too high, water condenses on cold surfaces. You see this as droplets on pipes, beams, and walls.
What is the Ideal Humidity for a Crawl Space Humidity?
Keep your crawl space humidity between 30% and 50%. This range prevents mold growth while avoiding overly dry conditions.
Most experts recommend targeting 45-50% for the best protection. This level stops mold without making wood too dry and brittle.
How to Measure Crawl Space Humidity
Measuring the humidity in your crawl space is relatively simple. In fact, if you have a smartphone, you can do it right from your fingertips.
Use a Hygrometer
Traditionally, humidity is measured with a hygrometer. It’s a device that measures an environment’s relative humidity. Digital models cost $25-50 and provide instant readings.
When measuring, place the device in the center of your crawl space, away from walls and vents. Take readings monthly during normal weather. If weather conditions are particularly humid, or if rain has just happened, check more frequently.
Smart Device Monitoring Options
WiFi-enabled hygrometers send alerts to your phone. You install them in your crawl space and get warnings when humidity exceeds your set threshold.
These devices like the Acurite Weather Station cost $30-80 but make it easier (and safer) to keep track of humidity levels. You won't need to climb into your crawl space monthly.
What the Numbers Mean
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Below 30% humidity: Rarely a problem, but extremely dry air can actually make wood become brittle over time.
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30-50% humidity: Safe zone. Continue monitoring, but no action needed.
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50-60% humidity: Warning zone. Install waterproofing measures before serious issues occur.
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Above 60% humidity: Danger zone. Take immediate action to prevent mold and structural damage.
What Causes High Crawl Space Humidity?
Several things control moisture levels under your house. Understanding these helps you fix problems at their source.
Humid Climate
Areas with a humid climate inevitably cause high humidity in a crawl space. As the air passes into the crawl space, it leaves behind droplets of water and makes the air wet.
Think about what happens to your mirror during a long, hot shower. Condensation forms on the cooler mirror’s surface. The same thing happens to crawl space walls and wooden supports.
They’re cooler than the humid air outside, so water droplets form and then evaporate. This makes the crawl space’s air wet and thick.
Ground Moisture
Dirt floors hold a lot of moisture, especially after rain or snowmelt. All this water eventually evaporates right into your crawl space and causes humidity levels to rise.
Your local soil type affects how much water vapor enters your crawl space. For instance, clay soil holds more moisture than sandy soil.
Ventilation and Air Flow
Open foundation vents were once thought to allow for better airflow and lower humidity. Now we know they actually make humidity worse by making it easier for humid outdoor air to get inside.
Plumbing Leaks and Water Intrusion
According to the EPA, the average household’s leaks can account for nearly 10,000 gallons of wasted water per year. If you have a leaky pipe in your crawl space, it can lead to a ton of damage and high humidity levels.
Pipes are unusually exposed in a crawl space, so without the right waterproofing measures, they can also rust and fall apart. If this happens, an endless cycle of leaking, high humidity, and damage can occur.
Signs of Excessive Moisture in Crawl Spaces
While you can determine the exact humidity level in a crawl space with a hygrometer, you don’t always need one to know there’s a problem. There are some clear problem signs that point towards high humidity levels. These include:
- Musty smells in your home
- Higher energy bills than normal
- Mold growth
- Condensation on pipes, ducts, or concrete walls
- Rotting wood
- Increased pest activity
Keep an eye out for these problems. If you notice even one, there’s a good chance humidity in your crawl space is high.
How to Lower Crawl Space Humidity
You can fix humidity problems with the right approach. Start by addressing water sources, then control what remains.
Seal Openings
Replace old wood or metal crawl space doors with higher quality alternatives. Our ABS plastic doors won't rust, rot, or warp over time.
Their tight seals prevent humid outdoor air from entering. This simple upgrade often drops humidity by 10-15 percentage points.
Also, be sure to seal crawl space vents permanently. Modern building science shows sealed crawl spaces keep more moisture out than ones with open vents. We also offer crawl space vent covers.
Install a Vapor Barrier
Vapor barriers are a type of heavy plastic sheeting that blocks moisture intrusion. The sheet is attached along crawl space walls and over the crawl space floor to block any and all water vapor.
Professional-grade barriers use 10-20 mil thick material for durability. This single improvement can reduce crawl space humidity by 40-70%. They can be pricey, but are ultimately worth the investment as they prevent structural damage and moisture problems.
Use a Crawl Space Dehumidifier
Crawl space dehumidifiers remove moisture from the air directly. They work continuously to maintain an ideal humidity level in your crawl space.
| Learn More: What Does a Crawl Space Dehumidifier Do? |
Most residential crawl spaces need a dehumidifier with a 30-70 pint capacity. Set your crawl space dehumidifier to 45-50% for best results. The unit will cycle on and off automatically to maintain this humidity level.
Fix Drainage Problems
You need to direct water away from your foundation. Update faulty or clogged gutters and downspouts and ensure they dump water at least 10 feet from your house.
Also, make sure the soil in your yard is graded to slope away from your home. Homeowners often don’t notice there’s a problem, so keep an eye out for puddles that stick around right next to the crawl space. If you see any consistently, it’s time to do some landscaping.
Keep Humidity Levels Low With Quality Solutions
Don’t ignore high crawl space humidity. It can lead to mold growth, musty odors, and structural if you don’t act quickly.
If you’re looking for tools to keep humidity levels low in your crawl space, Crawl Space Door Systems, Inc can help. We have a variety of crawl space waterproofing products available for DIY installation.