What Is Acceptable Moisture Level In Crawl Space?

Moisture in crawl spaces is often caused by duct-work condensation, high Humidity outdoors, and ground moisture evaporation. Having high levels of crawl space moisture for extended periods leads to structural damage. So, it is important to regulate the moisture level to keep it in the most suitable condition. You can measure the humidity level in your crawl space using a digital thermo-hygrometer. This is an affordable device that costs around $15. 

moisture in crawl space

What is the Suitable Moisture Level in a Crawl Space?

The moisture levels in wood vary depending on the Humidity of the environment. The acceptable moisture level for crawl space wood needs to be between 10% and 16%. This is the natural range of moisture for wood. Anything higher or lower than this could be a concern. Wood usually adjusts to the atmospheric moisture level. So, the best way to preserve wood is to allow it to have natural lead. The Equilibrium Moisture Contents (EMC) for most areas across the United States falls within this range throughout the year. There are certain variations in extremely dry and wet areas. But the overall seasonal variance is only 2 – 3% which shows that this is fairly consistent.

What Happens When the Moisture Levels Go Down?

People usually think that having dry wood in crawl space is not an issue to consider. But lower moisture level is not suitable for wood. It dries up the wood, causing cracks or splits. You may find this often on hardwood floors. Wood dries out when the Humidity drops below 30% for a long time. This can weaken the structural integrity of the house. But usually, the moisture level of the crawl space doesn’t go down unless you use a dehumidifier. 

What Happens When the Moisture Levels Exceed the Recommended Value?

There is a higher chance for humidity levels to increase in crawl spaces. Moisture content greater than 19% or humidity levels around 70% supports mold growth. This is the moisture level where certain organisms grow to decompose the wood. Mold also causes various health concerns and structural damage in the house due to the rotting of wood. It also damages the insulation, which is not an easy task to restore. 

How Does Moisture Relate to Humidity and Temperature?

As you are aware, temperature and Humidity are included in talking about moisture levels. The temperature levels are important in understanding moisture in crawl space. For example, consider a summer crawl space temperature of 70 ℉. At this constant temperature and various humidity levels, the Equilibrium Moisture Contents (EMC) of wood will be as follows.

Relative Humidity at 70 ℉EMC of Wood

As you can see from the above table, healthy wood moisture levels of 10% – 16% occur when the relative humidity lies between 55% and 80%. This is for the normal summer temperature of around 70 ℉.

What is the Suitable Relative Humidity Level?

A crawl space will usually have some variation in moisture levels throughout different climatic conditions. But the best humidity levels are around 75% and 80% in the long term. Having wood moisture between 10% and 16% keeps the wood safe from mold and rot. But this relative Humidity in wood is different from that inside the living space. The air inside your house should have a relative humidity of 30% – 50%. So, the household needs air conditioning in summer to remove Humidity and a humidifier in the winter to increase moisture. 

How to Maintain Acceptable Moisture Levels?

There are several ways in which you can keep the expected moisture levels in the crawl space. Here’s how to get rid of moisture under house at an affordable cost.

Install a Sump Pump

The sump pump is an effective way to remove water from the basement to prevent it from flooding during heavy rains and storms. It will discharge the water to a suitable location away from the house. Although the installation may require time and labor, it is a reliable way to keep dampness away from the basement or crawl space. You just need to check it once a few months to make sure that everything is working smoothly.

Vapor Barriers

A vapor barrier is a thick sheet of plastic that covers the crawl space floor. It acts as a barrier that keeps insulation and the wood dry. Therefore, this makes it easy to keep the moisture level of the crawl space below 60%. Professional installation of vapor barriers is the best way to get it done. This also provides clean air to the house. 

Change Wet Insulation

Insulation regulates the temperature inside the house and increases efficiency. A crawl space insulation usually lasts for about 40 years. But depending on the climatic conditions, you might even have to replace it after about 10 years in damp environments. The material cost is around $0.33 – $1.33 per square foot. With the addition of labor costs, the average cost will be about $575 for an area of 300 square feet.

Crawl Space Dehumidifiers

A dehumidifier is probably the easiest and cheapest way to maintain appropriate humidity levels. Usually, the dehumidifier is installed in a specific direction in the crawl space. This then provides proper air circulation to keep the air inside the house clean. Dehumidifiers reduce the possible rotting of the crawl space wood under the house. Advanced dehumidifiers use data from the temperature conditions to determine the operation of fans and the dehumidifier. They can also monitor the wood moisture through a special wood moisture sensor. Hence, the algorithm maintains acceptable wood moisture within the crawl space efficiently.

Benefits of Maintaining Proper Moisture Levels

Set the dehumidifier around 70% for the crawl space. Lowering the humidity level below 50% can cause various issues related to the dryness of the wood. The benefits of controlling the Humidity in the crawl space are as follows.

Secures Health

Too much Humidity in the house creates good habitats for bacteria, fungi, and mold growth. Having a dehumidifier in the crawl space removes moisture from the air that goes into the house. This improves the air quality inside the house and prevents dampness. So, the residents will not have health concerns related to mold and mildew.

Prevents Pests

Pests usually look for damp habitats in which they can find food and moisture easily. Less Humidity in the crawl space lowers the possibility of attracting pests into the house. You don’t need to worry about property damage caused by rodents and insects. 

Structural Integrity

Controlling the moisture level in a crawl space improves the overall structural integrity of the building. This is because it prevents wood from rotting due to dampness and from drying up due to lack of moisture. The wood continues to stay in the ideal condition for as long as you maintain the crawl space moisture level. The house is safe since the foundation is safe with healthy wood holding it up.

The Comfort of Living Space

Living in excessively humid spaces is really uncomfortable. It makes you hot, sticky, and sick. This is why most people use dehumidifiers to retain the ideal humidity levels inside the house. But if you put a dehumidifier in the crawl space, it controls the moisture levels even better. You can save money on utility bills while keeping the house at a comfortable temperature with minimal use of the air conditioner.

Energy Efficiency

Running the air conditioner for a long time increases your electricity bills. But having a dehumidifier in the crawl space reduces the need for air conditioning. It naturally lowers the temperature and keeps the air clean. So, maintaining the ideal humidity levels in the crawl space has the advantage of energy efficiency as well.

Frequently Asked Questions

Is It Normal to Have Some Water in the Crawlspace?

Finding water in the crawl space after heavy rain or storm is quite common. But large pools of standing water are a cause for concern. If you see several inches or more of water in the crawl space, check for plumbing leaks and drainage issues. Improper sloping of the ground outside the house can also cause water to pool up. 

Should I Open My Crawl Space Vents in the Summer?

The crawl space vents allow outside air to circulate under the house. This reduces the moisture build-up and prevents mold growth. You should open these crawl space vents in the summer to allow airflow across the area. But you need to close them in the winter so that humid air doesn’t flow inside the space.

How Do You Tell if There is Mold in Your Crawl Space?

Damp crawl spaces are more likely to have mold and mildew. Check for any signs of water stains on the baseboards with black, white, or greenish spots on the walls and insulation. The presence of mold also creates a musty smell that even causes a runny nose, cough, and eye irritation. Warped wooden floors and rotting wood are good indications that there could be mold growing in the space. 


Once the moisture in crawl space increase beyond the desired levels, it creates a suitable breeding ground for various bacteria, fungi, and pests. This increases the rate of wood rotting in your crawl space. The best way to preserve your wood structure is by maintaining ideal moisture and relative humidity levels. Try out these methods to keep your basement dry even if you don’t have a dehumidifier.