Ideal Humidity Level For Crawl Spaces & How To Control It?

The crawl space is an important area in the home because it acts as a buffer between the house and the earth and is a good location for the ductwork that helps to keep your house comfortable. However, humidity problems are common in this area.

In general, the ideal crawl space humidity level is around 45% to 55%. Keeping the humidity in this range can improve the overall structural integrity and prevent harmful microorganisms from growing and thriving.

You might be wondering why you should keep the humidity at this level and how to control the humidity in your crawl space.

Continue reading to learn everything you need to know about “humidity in crawl spaces.”

What is the ideal crawl space humidity level?

Even though I mentioned the ideal humidity level at the outset, I want to expand a bit on that in this section. 

45% to 55% is an acceptable humidity for most crawl spaces because it can prevent condensation and dryness in most cases.

However, the dew point is a more accurate metric than the humidity level when it comes to how dry or humid the air is. In case you don’t know, the dew point is the temperature at which air must reach to cause condensation.

The air may still be dry even if the humidity level is above 55%, assuming the dew point drops to a certain level. This is because cold air tends to hold much less water vapor.

Why should you keep the crawl space humidity in check?

Since we don’t spend much time in a crawl space, many assume that controlling humidity is unimportant. However, it is not true.

When the air is humid, it can promote the growth of mold, fungus, and dust mites. These germs can travel to other rooms, which triggers health problems. The wood rot fungus can eat wood, compromising structural integrity. Repairing it is very expensive.

When the crawl space air is dry, the wood will shrink, which will also cause structural damage. In addition, the bacteria spreads faster in the air, posing health risks to us. 

Therefore, maintaining a proper humidity level is important for your crawl space and house.

Why is a crawl space so humid?

Albeit a crawl space can be too dry, especially in winter or arid regions, most crawl spaces suffer from high humidity issues. So why a crawl space is so humid?

Moisture evaporating from the ground

The crawl space is the space that sits close to the ground and has a dirt floor. The moisture in the soil will constantly evaporate into the air in your crawl space, increasing the humidity level. 

Bulk water issues

When there is standing water in a crawl space, something is wrong with the drainage or plumbing system.

When there is a slope between them, water from the backyard or ground can easily go down and accumulate around the crawl space. Water can seep into the crawl space and make it wet because waterproofing is frequently neglected.

Damp outdoor air

Outdoor humidity plays an important role in indoor humidity. In hot and humid climates, the indoor environment tends to be wet at the same time.

Crawl spaces, as you may know, have a lower temperature than the outside. When hotter outdoor air enters the crawl space through vents, it cools and condenses on colder surfaces because cold air cannot hold as much moisture as hot air.

How to maintain the crawl space humidity level?

Now that we already know the importance of proper humidity in crawl spaces, this section will guide you on how to regulate it. After all, not everyone can live in an area with the perfect climate.

Find the root cause and fix it

Look around your home to see if any water drains into the foundation or any plumbing leaks.

When you find it, hire a technician to repair or modify it, or do it yourself if you have the necessary expertise.

Install a sump pump 

A sump pump refers to the pump that is placed in the sump. It is a device that allows water to drain from the foundation outside the house. This prevents standing water in the crawl space, which inhibits mold growth.

Use a vapor barrier 

As previously stated, the main cause of damp crawl spaces is evaporation of ground moisture. To combat it, cover the floor with a thick layer of plastic sheeting to keep moisture in the soil.

You should ensure the material used is free of harmful chemicals since those substances will be released into the air and put you and your family at risk. 

Spray foam insulation 

foam insulation

Foam insulation is a common way to treat moisture issues in crawl spaces. It is used to fix the cracks, keeping water and anything else outside of your crawl space.

Apply it to joist boxes, space between floor joists and vents, and let it dry.

Use a dehumidifier 

Having a crawl space dehumidifier installed effectively keeps the moisture level under a certain level. It is often activated on summer days when the air is humid. 

However, the big disadvantage of using a dehumidifier is that it can only address the symptoms. The moisture level will rise again when the machine is off. Therefore, you should first address the root cause while using it.

Check here to know where to place your dehumidifier in a crawl space.


The ideal humidity level for crawl spaces is 45% to 55%. Maintaining the moisture at such a level can keep the mold, fungus, and dust mites at bay. 

To control crawl space humidity, first address the root cause, such as leaking pipes or drainage issues, and then take some measures, including using a sump pump, insulation, and dehumidifiers. 

Even if crawl spaces are not the places you often go to, their condition can affect the safety and health of your house and you. Therefore, you should pay more attention to it.

About The Author

Avatar photo

Liz Yang is the founder of Airsmartly. She has been working at home for a few years and realizes that the performance of the HVAC system plays such an important role in our life. She has tested dozens of products in person, like humidifiers, air purifiers, dehumidifiers, and ACs, and wants to share tips about using or troubleshooting these products with you. Her uncle is an HVAC expert with over 30 years of experience in the field, and often offers assistance when she is unsure how to handle a situation. He is also in charge of reviewing the articles on this site.

Leave a Comment