Crawl spaces are easy to ignore. Without the utility of a basement, they’re often literally “out of sight, out of mind” for homeowners unless there’s an obvious problem. But if you want to protect your home from trouble down the road, there’s one aspect of your crawl space you should pay close attention to: Its humidity levels.
High humidity in a crawl space can result in problems like
- Dry rot in floor joists
- Foundation damage
- Pests like cockroaches
To avoid these problems, your crawl space should consistently have humidity below 60% and above 30%. About 55% relative humidity is ideal in most cases.
5 tips for keeping your crawl space dry
Humidity can be a challenge in your crawl space because there are many factors that contribute to higher levels of water vapor in the air. There are several ways to combat the problem.
1. Install a vapor barrier
One of the easiest ways to reduce crawl space humidity is to install a vapor barrier (a plastic sheet that lays over the ground) blocking moisture from rising up.
A plastic sheet with a minimum thickness of 6 mil (0.006 inches) is laid over the dirt and run up the foundation walls to a height of 6 inches, fastened in place, and sealed with tape and caulk.
2. Install a sump pump
A sump pump is a pumping system utilizing a shallow pit where water collects until it reaches a prescribed level and is pumped out, preventing water levels from rising above the ground.
A qualified plumbing professional can install a sump system in less than a day.
3. Use a dehumidifier or fan
Alternatively, you can improve ventilation in a windowless area like a crawl space with the addition of an extractor fan or a transfer fan.
4. Seal the crawl space
Crawl spaces can be vented for passive air exchange or non-vented; older homes tend to have vented crawl spaces. Venting a crawl space can reduce humidity if the humidity levels outside are low, but can have the opposite effect in very humid climates. If your home is located in a very humid area, closing the vents and sealing off other openings can be effective at reducing humidity levels.
Consider encapsulating a crawl space.
If you want to seal off your crawl space from exterior humidity, consider encapsulating it. This is an augmented form of vapor barrier that seals the space off entirely, keeping outside air (and pests) from infiltrating.
5. Check the grade and gutters
A properly designed home will divert water away from the foundation when it rains, but clogged gutters and incorrect grading of the ground around a house may be allowing water to pool near the home. Cleaning out the gutters and inspecting the drainage around the home can help prevent water from entering the crawl space.
Uncovering the source of water in your crawl space
Knowing where the moisture is coming from in the first place will help you choose a strategy to battle humidity in a crawl space.
Most humidity issues in a crawl space can be traced to some common causes:
- Plumbing leaks. If you notice water puddling in your crawl space under pipes, inspect them for leaks. These are most commonly found at connections where two or more pipes meet.
- Groundwater or runoff. If the water table under your house rises, the dirt can become saturated, raising overall humidity levels. A sump pump can help dry out the ground.
- Condensation. High humidity in your crawlspace causes water vapor to condense on colder surfaces, like ductwork or pipes. This water then drips down onto the floor.
- Infiltration. If water pools against your house after rainfall, it can seep through the foundation walls and leak into your crawlspace. Inspecting and repairing gutters and re-grading landscaping can alleviate this.
Common questions about crawl space humidity
Crawl spaces can be claustrophobic, mysterious places filled with dirt and spiderwebs. Many homeowners don’t know much about what’s normal in a crawl space. In this section, I will try to answer some of the most frequently asked questions about crawl space humidity.
Your crawl space should be dry. A well-maintained crawl space will have no standing water, no condensation issues, and moderate humidity levels. Any spikes in humidity or signs of standing water should be investigated and dealt with immediately.
The ideal humidity level for a crawl space is about 55%. The acceptable range is between 30% and 60%. Crawl space humidity can also be too low; levels below 30% can lead to wood shrinkage, which can also damage your home.
If your crawl space is experiencing higher-than-normal humidity due to a specific event—a plumbing leak or local flooding, for example—it may dry out over time. Most humidity issues in crawl spaces will require more direct interventions to rectify the source of the moisture.
Absolutely. A damp crawlspace is an ideal breeding ground for mold, and easily transmits mold spores to the living areas of the home. Mold can cause a wide range of negative health issues, including allergic reactions, upper respiratory issues, fever, and possibly more severe conditions. If your crawl space is damp, it’s imperative that you locate the source of the moisture and make repairs and changes as needed to reduce the humidity.
It’s easy to ignore your crawl space if there are no overt problems. But even something as seemingly minor as high or low humidity in the space below your main floor can have a devastating effect on your property and even your health. Monitor the humidity levels in your crawl space regularly, and take action to repair leaks and reduce those levels as needed.
- Centers for Disease Control and Prevention. (2022). Basic Facts about Mold and Dampness. cdc.gov
- Environmental Protection Agency. (2023). Mold Course Chapter 2: Why and Where Mold Grows. epa.gov
- Family Handyman. (2021). How to Install a Vapor Barrier in a Crawlspace. familyhandyman.com
- Hoke, A. (2017). Low Humidity Takes Toll on House. Tennessean.com
- Loveland, M. (2023). What is Crawl Space Encapsulation: Pros, Cons, and Everything You Need to Know. angi.com
- Miranda ML, et al. (2011). Crawl Spaces as Reservoirs for Transmission of Mold to the Livable Part of The Home Environment. Pubmed.ncbi.nlm.nih.gov
- Wallender, L. (2022). Crawl Space Vapor Barriers: Why You Need One & How to Install It. thespruce.com