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Crew Installing Crawl Space Vapor Barrier

Crawl Space Humidity

The best way to keep humidity levels in your crawl space to an acceptable level is encapsulation and dehumidification.

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Condensation on wooden joist in crawl space

Although it may not be common knowledge, crawl spaces should be dry and free of moisture at all times. If not, it’s possible water and water vapor can lead to issues like mold, wood rot, and pest infestations. Humidity in your crawl space doesn’t just stay in the crawl space, it travels upstairs because of the stack effect. The Philadelphia area has a subtropical climate, so homes in the area battle humidity regularly. It’s important to address the humidity in your crawl space as well as the rest of your home.  

Let’s step back and define what humidity is. Humidity is the amount of water content in the air in relation to the maximum amount of water air can hold at a certain temperature. For example, ideal humidity levels in your crawl space should be between 30 and 60 percent, with a goal of 50-55 percent. Keeping these levels will deter mold growth, wood rot, and pests. When humidity levels reach 100 percent, it turns into water, typically found on cool surfaces in the form of condensation. 

Think of a cold glass on a warm summer day… the water droplets on the outside of the glass are condensation caused by humidity levels over 100 percent.  

Why Do I Have High Humidity in my Crawl Space? 

Dirt Crawl Space Floor 

A dirt floor crawl space is both easy and cost-efficient during the initial building phase. However, the dirt floor basically allows water to pass through easily. The issue with dirt crawl spaces, however, is it’s basically impossible to tackle a humidity issue without installing a high-quality vapor barrier first. This barrier locks out water coming from the ground and allows you to tackle humidity problem.  

crawl space with falling insulation and open vents

Open Vents 

Old building code required crawl spaces to have vents, but these vents were found to cause more harm than good, especially when it comes to humidity. The original idea involved 2-way airflow, air would flow in and out freely making moisture problems less likely. However, it was found that open vents actually increase the amount of moisture. Vents allow cool air outside air to meet warmer air inside the crawl space, this increases the humidity level. In an area like Philadelphia, where it’s often humid, condensation can cause problems in your crawl space. 


With Philadelphia’s more than average rainfall, crawl space flooding can become a serious problem rather quickly. Heavy rain that causes flooding puts force on crawl space walls, causing hydrostatic pressure against your foundation. This can cause walls to crack, allowing water to enter freely.  

How Can I Reduce Humidity Levels in my Crawl Space? 

The best way to keep humidity levels in your crawl space to an acceptable level is encapsulation and dehumidification. 


Encapsulated crawl space with dehumidifier

Encapsulation is the process of sealing your crawl space off from the outside and including it into the envelope of your home. This process involves addressing any water issues by installing drainage, such as a sump pump,  installing a high-quality vapor barrier, and ensuring all crawl space vents are sealed. This process seals out both water and humidity and creates a dry, healthy crawl space. 


Adding a dehumidifier is a good idea, even if your crawl space is encapsulated. Because of the stack effect, the crawl space gets a lot of air from the first and second floors of the home, this air is often humid since Philadelphia has a humid climate. An energy-efficient dehumidifier can ensure humidity levels stay at a proper level so mechanical items, like the furnace and air conditioner, don’t have to work harder, causing more money in the long run.  


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New Jersey

535 Route 38 East, 149E
Cherry Hill, NJ 08002


106 Park Drive
Montgomeryville, PA 18936