What is the ideal crawl space humidity level?

It’s easy to make the argument that your crawl space is the most neglected area of your home. After all, we basically only go down there to diagnose plumbing leaks or store our garden supplies! Thankfully, crawl spaces are robust and don’t require much maintenance – although persistently high moisture levels may ultimately lead to mold, mildew, or even foundation problems!

This article will discuss how high humidity impacts your crawl space, its common causes, and the best crawl space humidity solutions.

The Goldilocks Zone of Crawl Space Humidity

Crawl space humidity should be between 30 and 50% relative humidity. A hygrometer is the device which measures relative humidity and these can usually be picked up affordably at home and garden centers or online.

Relative humidity (RH) means that this percentage varies in response to temperature. As temperature increases, the amount of water that air is able to contain increases, while the amount of water air can contain decreases as temperature falls. This means that temperature changes can impact RH even if the overall amount of water vapor in the air remains constant.

The dew point is the temperature at which the water vapor contained in the air will condense back into liquid water. You can see this effect on warm humid days on the outside of a cold glass. If the glass is below the dew point, dew will form on its sides. The same phenomenon can occur in your crawl space, with water vapor condensing on pipes or even foundation walls!

Dehumidifier installer columbia sc

What are the effects of high crawl space humidity?

High humidity levels lead to a variety of problems ranging from insect infestations (including termites, cockroaches, spiders, and dust mites) to mildew or mold growth, and even wood rot. Left unchecked, high crawl space humidity can cause severe problems.

Crawlspace insulation rotting due to humidity and old age

Can crawl space humidity be too low?

Outside of desert environments, it is uncommon for crawl space humidity to be too low. However, it is possible, and the results can be just as damaging as too high!

Wood is hygroscopic – meaning it can gain or lose moisture in response to the conditions it’s exposed to. If your crawl space humidity is too low (below 30% for extended periods of time) then your floor joists and foundation elements may warp or crack as they dry out.

Keep in mind that a short bout of dry air (or humid air for that matter!) isn’t going to be a problem. The main concern is your crawl space’s long-term moisture content.

What causes excessive crawl space moisture?

Moisture is a normal fact of life – all of the air we breathe contains moisture! But, too much of a good thing is problematic, and the same holds true for relative humidity levels.

Dirt floor crawl space

Up until recently, most crawl spaces were built with exposed dirt floors. However, homeowners have found out the hard way that moisture tends to evaporate endlessly from the earth, creating the perfect conditions for overly damp crawl spaces. This can be particularly problematic in humid environments or areas with many ponds, lakes, streams, or rivers.

Standing water

This is a serious problem! If you peer in through your crawl space entryway and see standing water, something is wrong and you need to contact someone for crawl space repair ASAP! This water can wick into foundation timbers, causing them to rot, not to mention the plethora of moisture problems that may result.

Outside air unable to enter through crawl space vents

Crawl spaces should be self regulating – any humidity that makes it into them can easily be vented outside. However, large wrap-around porches or dense vegetation around your home can limit airflow to your crawl space. This can turn a crawl space which would normally be happily dry into a musty, overly moist environment.

How can you reduce humidity levels in the crawl space?

Vapor Barrier

The simplest, longest lasting solution to excessive moisture in your crawl space is installing a vapor barrier. Vapor barriers are layers of polyethylene sheeting which are set on top of your crawl space floor, and then sealed along the edges with vinyl tape.

A properly installed, high-quality vapor barrier will provide at least 20 years of service and will help ensure that your crawl space stays at the ideal humidity level. While a vapor barrier doesn’t block 100% of inbound moisture, it reduces it to levels which can be passively regulated by natural air exchange between your crawl space and the outdoors.

Crawlspace and basement Vapor Barrier and insulation Greenville and Columbia SC

Sump Pump or Exterior Drainage

If you have standing water under your home then your crawl space air is almost certainly going to be too moist! Depending on the cause, you may need to install a sump pump or drains along your house.

It is best to address the conditions that are leading to the water accumulating, rather than simply continuously removing it.


Crawl space encapsulation is a big project and only makes sense in very specific situations. In particular, crawl space encapsulation should be considered when your crawl space’s humidity issues stem from poor air flow which cannot otherwise be resolved.

Encapsulation is similar to vapor barrier installation – except the waterproofing is taken to the extreme. In addition to waterproof sheeting on the floors, any and all openings to your crawl space will be sealed, including the crawl space vents.

A crawl space dehumidifier will be installed, further guaranteeing that the humidity levels stay at the right level. Almost always your encapsulation project will include a sump-pump or french drain system, to protect your crawl space from flooding.

One thing to keep in mind about encapsulation is that these are active water management systems. If the dehumidifier or sump pump malfunctions, humidity levels in your crawl space can rapidly soar.

Without the ability for air exchange, mold and mildew will grow shockingly quickly in a sealed crawl space. Accordingly, we recommend only choosing encapsulation when your situation requires it as these systems are costly, require routine maintenance, and may lead to expensive repairs down the road.


Keeping your crawl space humidity levels between 30 and 50 percent relative humidity is essential if you hope to avoid expensive foundation repair or mold remediation in the future. Checking your crawl space humidity levels is relatively simple, although it will require you to purchase a hygrometer and do a bit of crawling around under your home!

Not interested in crawling around in the dirt? We offer free onsite evaluations for crawl spaces in the South Carolina Midlands!

If you suspect your crawl space moisture levels are too high, we recommend contacting a reputable crawl space repair company. In most cases a simple vapor barrier is all that you’ll need to make your crawl space humidity problems a thing of the past.

Dragon Vapor Barriers & Insulation