Crawl Space Humidity: What’s the Most Optimal Level?

Josh Mitchell

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Josh Mitchell

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Holly Curell

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Over 40% of the air in your crawl space goes straight into your home.

That means that damp and humid conditions in your crawl spaces can lead to mold growth, bacteria, and pests thriving in your home.

Managing the humidity levels can stop this from happening, but you need to keep it at the right level.

Unfortunately, it’s not as simple as managing humidity levels indoors, but in this guide,

I’ll make it easier by explaining the humidity levels in your crawl space humidity and how to manage that humidity properly.

Ideal Crawl Space Humidity Levels Explained

crawl space humidity levels

The ideal crawl space humidity is 50-65%.

Your crawl space humidity levels will vary much more than other parts of your home, but keeping it around this level is best for preventing damp conditions, mold growth, and pests like termites from spreading into your home.

The ideal relative humidity of crawl spaces is higher than living spaces, which are recommended to be kept at 30-50%.

There are three main reasons for this:

  1. 1
    A crawlspace isn’t lived in, so you don’t have to worry about personal comfort.
  2. 2
    Crawl spaces tend not to be used for storage, so you won’t have personal belongings to protect from damp conditions.
  3. 3
    If you lower the humidity levels too much, then you can experience shrinkage. This can damage the wood under your home, leading to creaky floorboards and cracked drywall.

It’s normal for crawlspace humidity to be higher than in interior rooms as it’s outside your home.

However, that moisture in the air still needs to be regulated to keep your home safe and comfortable.

TL;DR: The most ideal humidity for crawl space is 50-65%. This is higher than the desired humidity levels for typical living rooms.

Dew Point Vs. Humidity

When considering the relative humidity in your crawl space, you must also consider the temperature and conditions you live in, as this influences the Dew Point.[1]

The Dew Point is another measure of the moisture in the air and takes into account temperature too.

This is important when considering outside areas like crawl spaces which aren’t regulated.

Basically, warm air can hold much more moisture than cold air.

Therefore, if you have hot conditions with a lower relative humidity, it can actually be worse than cold conditions with a higher relative humidity.

This is because there’s a higher dew point, meaning you’ll feel more humid, and your crawlspace is more likely to suffer from damp conditions.


If you live in a hot climate, you should keep your crawl space humidity at 50-60%. 

If you live in a colder climate, you can allow the humidity to go up to 60-65%.

What Should Crawl Space Dehumidifiers Be Set At?

Your dehumidifier should have a programmable humidity level and a humidistat to measure the humidity and turn the appliance on and off as needed.

You should set your crawl space dehumidifier to between 50 and 65% humidity, depending on the temperature and environmental conditions.

How Long Does It Take To Dehumidify A Crawl Space?

how long does it take for a dehumidifier to work

The first time you use a dehumidifier, it could take the dehumidifier up to 12 hours to reduce the humidity to the desired level.

It all depends on the relative humidity of your crawl space and the capacity of your appliance to handle the moisture.

Once you’ve removed the initial high humidity, your dehumidifier will just need to maintain the right humidity levels in the crawl space air.

This means it won’t have to work as hard, and it shouldn’t take as long.

However, you may need a crawl space dehumidifier set up permanently if you live in an area of very high humidity.

Specific crawl space dehumidification units with a built-in sump pump can be installed.

This allows them to operate continuously to maintain consistent humidity levels without manually emptying the tank.

TL;DR: It can take up to 12 hours for a new dehumidifier to reduce the humidity levels to your desired value in the crawl space.

4 Reasons Your Crawl Space Is Too Humid

1. Stagnant Water In The Crawl Space

Your crawl space can easily develop a few inches of stagnant water (or bulk water).

This can be caused by rain or crawl space flooding and is typical after a storm, or it can be caused by a drainage issue or plumbing leak.

The stagnant water will lead to increased moisture in the air and raise the crawl space humidity level.

Stagnant water can wreak havoc underneath your home, particularly on any exposed wood.

This can lead to increased wood moisture levels, wood rot fungus, and even damage to hardwood floors.

You can combat the humidity or install a vapor barrier.

Still, it’s crucial to determine the cause of the stagnant water and resolve the underlying issue wherever possible to reduce crawl space moisture levels.
will a dehumidifier in the basement help the whole house

TL;DR: Crawl spaces often develop stagnant water which can lead to increase moisture content.

2. Ground Moisture Evaporation

The ground beneath your home is full of water, which can also add to the crawl space humidity level.

The soil itself can seem dry, but it’s because the moisture is actually evaporating up into the damp crawl space.

This is a very common issue in high-temperature areas where there is more evaporation and less cool air.

It’s hazardous because it can increase wood moisture levels and negatively impact the structural integrity of your home.

Many people install a crawl space vapor barrier to prevent ground moisture evaporation.

TL;DR: Ground water evaporation is a natural phenomenon that can increase moisture content over time.

3. Outdoor Air Entering Through The Space Vents

Crawl space vents are designed to let air in and improve ventilation for a healthy crawl space, but they can actually cause humidity problems.

As the cooler air enters through the crawl space vents and comes into contact with the high crawl space humidity, it causes the water to be released from the air.

This creates condensation and can lead to standing water gathering in the crawl space.

TL;DR: Crawl spaces are hotter as compared to outside. As the cold air enters, it can cause condensation.

4. Exposed Dirt Crawl Space Floor

Exposed dirt floor and soil in your crawl space can make controlling humidity levels much more challenging.

A dirt floor gives the perfect surface to capture and absorb moisture from the hot air, causing the humidity level to rise.

The dirt won’t just cause moisture problems either, as it can lead to fungal growth and cause bacteria to spread.

This can result in poor indoor air quality throughout your home.

TL;DR: An exposed dirt floor absorbs and stores moisture from the air leading to rise in crawl space humidity.


It is worth noting that a dehumidifier does not clean the air. It does prevent certain microbes and bacteria from thriving by removing moisture, it does not purify the air. In order to clean the air you may need an air purifier.

What Happens If Humidity Is Too High?

Promotes Mold Growth

Crawl space humidity problems give the perfect environment for mold problems to develop.

The high moisture content and humid air lets the mold thrive and spread quickly, and it can even travel up into your home.

This can impact your health and present dangers or issues of concern for most people with pre-existing allergies or respiratory conditions.[2]

will a dehumidifier kill mold

Leads To Wood Rot

Wood floors and beams can become severely damaged when exposed to air with a high moisture content for a long period.

The wood moisture levels will increase as it absorbs the water vapor, and this can lead to wood rot.

High wood moisture levels in your crawlspace can damage the foundation underneath your home.

Dealing with it can be expensive and a real nightmare for homeowners. If you do think there are issues with your foundations, you should contact a professional.

Most of the time, they will provide a free inspection before you have to commit to any work.

Bugs And Pests

A high humidity level and moisture content in the air can encourage pests and bugs to gather in the crawl space.

If left unchecked, these termites and other critters can cause serious structural damage to your home’s foundation.

In the long run, this can be a serious problem, and the damage to the floor and walls of your house can be much more expensive to fix than just solving the humidity issues in the first place.

TL;DR: High humidity can not only directly damage wooden floors and beams, it can even provide perfect conditions for mold and bugs to thrive.

How To Reduce Crawl Space Humidity

Crawl Space Encapsulation

The main reason your crawl space can get so damp is that it’s not sealed off or protected from the elements.

Crawl space encapsulation creates a first line of defense against high humidity levels with a thin impermeable sheet of polyethylene lining the floors and walls.

The encapsulated crawl space prevents stagnant water from gathering through condensation and prevents evaporation from the soil upwards.

Installation can take some time, but having a sealed crawl space is the best way to reduce water problems, and it can even increase the energy efficiency of your home.

Sump Pump Too Pump The Water Out

Once installed, a sump pump will pump water out of your crawlspace and safely away from the foundation of your home.

Installing it in the lowest part of your home allows the water to flow into a bucket.

Once it reaches a certain level, the pump will automatically activate and the process will remove the moisture.

This is one of the solutions which can remove a lot of water quickly, but you will need to deal with the issues causing the moisture build-up too.

Crawl Space Dehumidifiers

If you’ve noticed high humidity in your crawl space, then it’s worth investing in a crawl space dehumidifier.

This appliance will regulate the air and actively remove moisture using a dehumidification process where it will heat and cool the air.

If you have a very wet crawl space, you can also connect it to your plumbing or a sump pump, so it actively drains without having to be emptied.


does dehumidifier ice up

Regular dehumidifiers are most effective in summer and can struggle in winter because the low temperatures can freeze the dehumidifier coils.

Dehumidifiers for crawl spaces are usually built for year-round use, so you can use them in summer or winter conditions.

Invest In A Vapor Barrier

A vapor barrier is similar to encapsulation but only involves lining the floor of your crawl space with impermeable material.

It’s considered the minimum moisture defense you should use in this part of your home as it prevents water evaporation upwards.

It can also reduce the risk of flooding from rainwater. If you live in an area with heavy rainfall, then a vapor barrier is extremely useful.

However, as the vapor barrier only lines the floor, it won’t be as effective as crawl space encapsulation.

TL;DR: From using a dehumidifier to encapsulation and sealing it off, there are many ways to prevent and remove moisture from crawl space.


Can My Crawl Space Be Too Dry?

Yes, if the humidity levels in your crawl space are too low, it can damage your home foundation by causing dry rot.[3]

Lower humidity levels in your crawl space can also lead to dry air in your home, leaving you uncomfortable.

Should I Put A Fan In My Crawl Space?

Installing an effective exhaust fan as an air conditioning system can help to remove wet air and stop humidity from circulating around your crawl space.

Just make sure the fan circulates and vents your hot air out without letting cold air in, as this can lead to condensation, which will cause your humidity levels to rise.

Should You Open Vents In The Crawl Space?

No, your vents should be closed and sealed off from the elements so that they are only used to remove damp air from inside the crawl space.

This will prevent cold air from entering the area and causing moisture issues.

What Should Not Be Stored In A Crawl Space?

Papers, books, photos, and electronics should never be stored in a crawl space because the humidity levels can cause damage.

Wrapping It Up

Keeping the humidity levels down to 50-65% in your crawl space will prevent mold and mildew growth and protect the foundation of your home.

Having a crawlspace dehumidifier installed will work effectively in many crawl spaces, but just remember that it’s dealing with the symptom, not the disease.

If you have any plumbing issues or leaks, you’ll need to determine what they are and fix them first before you can get the humidity down to optimal levels.


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Josh Mitchell


Josh Mitchell
My name is Josh and I am obsessed with home appliances. From portable AC units to heaters and air purifiers, I enjoy testing, learning and using these devices to improve the air quality inside my family home.

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Midea U Shaped Window Air Conditioner

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