# What Humidity Should a Crawl Space Be?

- **URL:** https://airsenseenvironmental.com/what-humidity-should-a-crawl-space-be/
- **Published:** 2026-11-12

A crawl space should stay below 60 percent relative humidity, the same ceiling the EPA sets for indoor air, and closer to 50 percent once the crawl space is sealed. If your hygrometer reads above 60 percent for days at a time, the crawl space has a moisture problem, and opening more vents rarely fixes it. This question matters more in St. Louis, MO than in drier parts of the country. Homes across St. Louis County, St. Charles County, Jefferson County and the Madison and St. Clair County side of the river sit through long, humid summers, and many older houses here were built over vented dirt-floor crawl spaces that pull that air straight under the floor. This guide covers the target numbers, how to take a reading you can trust, what a high reading means, and when a crawl space needs a dehumidifier. What is the ideal crawl space humidity level? The ideal crawl space humidity level is below 60 percent relative humidity (RH), and ideally between 30 and 50 percent. That is the EPA's recommendation for indoor air in A Brief Guide to Mold, Moisture and Your Home: "keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity." There is no federal standard written only for crawl spaces. The closest crawl-space-specific figure comes from the EPA's Moisture Control Guidance for Building Design, Construction and Maintenance, which tells builders to control "humidity in the crawl space to less than 65 percent RH" with dehumidifiers or, when the outdoor dew point is low enough, with ventilation. Together, those give three working numbers: Ceiling: 60 percent RH. Above it, conditions start to favor mold and wood decay. Target for a sealed crawl space: about 50 to 55 percent RH. This is common contractor practice, not a regulation, and it sits comfortably under the EPA ceiling. Floor: about 30 percent RH, the bottom of the EPA's ideal range for indoor air. Holding a sealed space in that band all year is the point of crawl space encapsulation. How do you measure crawl space humidity? You measure crawl space humidity with a hygrometer, a small humidity meter the EPA describes as "a small, inexpensive ($10-$50) instrument available at many hardware stores." A single reading tells you very little. A week of readings tells you a lot. A reliable reading follows four habits: Place the meter mid-space, away from vents, the access door and any dehumidifier outlet. Read it at the same time each day for a week, because one afternoon can mislead in either direction. Take readings after rain. A jump after a storm points to where the water comes from. Repeat in summer and in winter. A January reading says little about July. Write the numbers down. A contractor can do far more with a week of data than with "it felt damp." Why is crawl space humidity highest in a St. Louis summer? Crawl space humidity peaks in summer because warm outdoor air carries more water vapor, and a vented crawl space lets that air in to meet cool surfaces. When humid air touches a surface below its dew point, the water vapor condenses into liquid on ductwork, pipes, joists and the subfloor. The EPA's moisture guidance puts a number on when venting helps and when it hurts. Ventilation is only listed as a way to hold crawl space humidity under 65 percent "if outdoor air dew points are lower than 55˚F." Above that dew point, outdoor air brings moisture in rather than carrying it out. Check the dew point on a typical St. Louis afternoon in July and you can judge how often that condition holds. The ground adds to the load. The same EPA guidance notes that "damp basements or crawlspaces may add as much water vapor to the air in a day as all the other internal sources combined." A bare dirt floor evaporates moisture into the space every day of the year. What happens if crawl space humidity stays too high? High crawl space humidity leads to mold growth, wood decay, pest activity and musty air in the living space. The EPA states that mold needs moisture to grow and that "it is important to dry water-damaged areas and items within 24-48 hours to prevent mold growth." A crawl space that sits above 60 percent for weeks never gets that chance to dry. The damage shows up in four ways: Mold on joists, subfloor, insulation facing and stored cardboard. Wood decay in framing that stays damp for months. Wet, sagging insulation falling out of the floor joists. Pests such as termites and carpenter ants, drawn to damp wood and soil. Health effects are the reason this reaches the people upstairs. According to the EPA, "molds produce allergens (substances that can cause allergic reactions), irritants, and in some cases, potentially toxic substances," and mold can trigger asthma attacks in people with asthma who are allergic to mold. Anyone with health concerns should talk to a health professional. Does crawl space humidity affect the air upstairs? Yes, crawl space humidity affects the air upstairs because air moves from the lowest level of a house to the highest. The EPA's moisture guidance describes this stack effect: in cold weather, "buoyant warm air rises from the lower to the upper floors and then flows out near the top of the building," and replacement air "is pulled into the building at its base." In a house over a crawl space, the base is the crawl space, and air rises through gaps at plumbing, wiring and duct penetrations, the rim joist and the subfloor. A musty first floor is often a crawl space smell. That airflow makes crawl space humidity an indoor air quality question, which is why our guide to why crawl space encapsulation is a must in humid climates treats moisture control and healthy indoor air as one project. The same upward airflow carries soil gas, including radon, which is why moisture work and radon mitigation so often meet in the same crawl space. Does an encapsulated crawl space need a dehumidifier? Yes, an encapsulated crawl space needs active moisture control, and in most homes that means a dedicated crawl space dehumidifier. A sealed liner and closed vents stop most outside air and ground moisture, but moisture still arrives through foundation walls, small air leaks and plumbing, and a sealed space has no way to dry on its own. The EPA's moisture guidance says the specifications for a non-vented crawl space "are the same as for basements," with a plastic membrane allowed in place of a concrete slab. For basements, the same guidance says to "use air conditioning or dehumidifiers to reduce basement humidity during warm, humid seasons," and a sealed crawl space needs the same treatment. The two common options are: A dedicated crawl space dehumidifier with a humidistat, set to hold the target RH, draining to a sump or condensate pump. Conditioned air from the home's HVAC system, supplied in a controlled amount to the sealed space. Much of the field research on closed crawl spaces, including an Oak Ridge National Laboratory comparison of ventilated and sealed crawl spaces, is collected by Advanced Energy's crawl space program, which the EPA's moisture guidance lists as further reading on closed, insulated crawl spaces. Air Sense Environmental installs AprilAire crawl space dehumidifiers as part of its encapsulation systems. Either way, something has to actively hold the setpoint; the liner alone will not. What size dehumidifier does a crawl space need? A crawl space dehumidifier is sized by its moisture removal in pints per day, and the right capacity depends on the space and its moisture load, not on square footage alone. ENERGY STAR's dehumidifier guidance states that the capacity you need "depends on two factors: the size of the space that needs to be dehumidified and the conditions that exist in the space without dehumidification." For a crawl space, a contractor looks at four things on site: Volume. Floor area multiplied by height, not footprint alone. Air leakage. How well the vents, rim joist and penetrations are sealed. Moisture sources. Wet soil, seepage, plumbing and how the space behaves after rain. Temperature. ENERGY STAR notes that frost can form on dehumidifier coils when the air drops below 65°F, so cooler spaces need a unit rated for low temperatures. ENERGY STAR certified models remove the same moisture as similar conventional units on 20 percent less energy, so efficiency is worth asking about once the capacity is set. What does each crawl space humidity reading mean? Each crawl space humidity reading falls into a band that calls for a different response. The higher bands use ENERGY STAR's sizing-chart descriptions, and the lower bands follow the EPA figures above. Crawl space RH readingWhat it meansWhat to doBelow 30%Drier than the EPA's ideal range. Uncommon in a humid climate.Check the dehumidifier setpoint.30% to 50%Inside the EPA's ideal range for indoor air.Keep monitoring through the seasons.50% to 60%Under the EPA ceiling. Typical target band for a sealed crawl space.No action needed. Watch the summer trend.60% to 65%Above the EPA's 60% ceiling for indoor air.Find the moisture source. Check vents, gutters and grading.65% to 75%Above the EPA's 65% crawl space construction limit. ENERGY STAR calls 50% to 75% RH "slightly to moderately damp" with a musty odor.Schedule a crawl space assessment. Plan for moisture control.75% to 90%ENERGY STAR's "very damp": the space consistently feels and smells damp.Professional assessment soon. Look for mold and wet insulation.90% to 100%ENERGY STAR's "wet": walls or floor sweat, or seepage is present.Treat as a water problem first, then humidity. A reading that holds for a week matters far more than a single spike. Frequently asked questions Is 70% humidity in a crawl space too high? Yes, 70 percent humidity in a crawl space is too high. It is above both the EPA's 60 percent ceiling for indoor air and the 65 percent limit in the EPA's crawl space construction guidance, and it falls within the range ENERGY STAR describes as damp with a musty odor. Should a crawl space dehumidifier run in winter? A crawl space dehumidifier should stay on in winter but will run far less. A humidistat only starts the unit when humidity rises above its setpoint, and winter air holds less moisture, so most of the year's run time happens in summer. Can a crawl space be too dry? A crawl space can be drier than ideal, but in a humid climate it seldom is. The EPA's ideal range for indoor air starts at 30 percent, so a reading well below that usually means the dehumidifier setpoint is too low. What humidity should a crawl space dehumidifier be set to? A common setting for a crawl space dehumidifier is 50 to 55 percent relative humidity. That band sits under the EPA's 60 percent ceiling with room to spare, without running the unit harder than the space needs. Does humidity affect radon levels? Humidity does not create radon, but the moisture conditions and airflow in a crawl space can change how soil gas enters a home, and sealing a crawl space can raise or lower radon. The EPA recommends fixing a home at 4.0 pCi/L or higher, so test after any crawl space work. Our article on how humidity and radon mitigation work together covers the connection in more depth. Get your crawl space numbers checked If your crawl space reads above 60 percent, or you have never measured it, an assessment is the fastest way to a real answer. Air Sense Environmental inspects crawl spaces across the St. Louis bi-state metro, measures humidity and moisture on site, and designs crawl space encapsulation systems that hold the space in the healthy range. Call 314-664-9807 or request a free estimate.