Crawl Space Ventilation Fans: Good or Bad Idea?

If you’ve ever been down in your crawl space, you know it isn’t the driest or most comfortable place. Over the years, many solutions have been put forward to make crawl spaces better.
One of the most common suggestions is to use crawl space fans. Unfortunately, these don’t actually solve anything (and can even make the problem worse). On this page, we’ll look at why crawl space ventilation fans don’t work and what you should do instead.
What Is a Crawl Space Fan?
When we talk about a crawl space fan, we’re talking about a mechanism that takes air from the outside and blows it into your crawl space. There are many different models available; some go on the crawl space walls, while others are placed on the floor or ceiling of the crawl space.
The Problem With Crawl Space Fans

A common misconception in the past (and even still sometimes today) was that airflow was the best way to keep a crawl space dry. For that reason, most crawl spaces have open vents along their walls to allow air to move in and out.
In theory, crawl space fans work alongside these open vents to create air circulation below your home. In reality, this isn’t what happens. Instead, unconditioned air from the outside transfers into your crawl space, essentially copying and pasting the outdoor environment to the area below your home.
Think about it: if the outdoor air is humid and a fan blows it straight into your crawl space, what will happen? This is the main reason crawl space fans are a bad idea.
Relative Humidity: The Main Problem

Relative humidity is the amount of moisture in the air compared to the maximum amount the air can hold at a certain temperature. It’s expressed as a percentage. For example, if the relative humidity is 50%, the air holds half of the moisture it could have at that temperature.
In Florida (especially during the summer), it’s not uncommon for the temperature to be in the mid-to-high 90s. Let’s imagine that the temperature outside is 95°F. At this temperature, air can hold approximately 35 to 37 grams of water vapor per cubic meter.

Now, most crawl spaces are much cooler than the outside area surrounding them due to having dirt floors and being out of the sun. If your crawl space is 62°F, then the air holds approximately 13 to 14 grams of water vapor per cubic meter.
When the warm, humid air from outside (95°F) enters your cooler crawl space (62°F), it brings more moisture than the cooler air can hold. Since the cooler air can only hold about 13 to 14 grams of water vapor, but the warm air can carry 35 to 37 grams, the excess moisture (about 20 grams) has nowhere to go and condenses into liquid water inside your crawl space.
We know that was a lot of information, but it’s crucial to understand so you can find solutions that work in your crawl space. Just remember that when warm air mixes with cool air in your crawl space, it creates moisture. Using a crawl space fan doesn’t prevent this from happening; it speeds up the process.
Issues Caused by Crawl Space Fans
When you use a crawl space fan, it’s more likely for moisture problems to arise. Rather than preventing issues, fans actually lead to:
- Mold growth
- Wood rot
- High humidity
- Wet insulation
- Sagging floors
- Musty odors
- High energy bills
- Water in the crawl space
Avoid using a crawl space fan to prevent these issues in your home!
How to Keep Your Crawl Space Dry
Crawl space fans transfer humid air into your crawl space and cause moisture issues. Instead, a much better solution is crawl space encapsulation.
Crawl space encapsulation is like putting a protective cover over your crawl space to keep it dry and clean. This process uses several key steps to seal the space from outside air and moisture.


- Seal the Vents and Openings: Close any vents, openings, or gaps to prevent outside air and moisture from entering the crawl space.
- Install a New Door: Install durable doors to seal any entry points to the crawl space.
- Set Up Drainage: Set up a drainage system to manage and redirect water away from the crawl space.
- Place Drainage Matting: Lay down drainage matting to direct the flow of water under the vapor barrier and prevent moisture buildup.
- Attach a Vapor Barrier: Install a thick plastic barrier on the floor and walls to block ground moisture.
- Install a Sump Pump: Install a sump pump to remove accumulated water from the crawl space.
- Include a Dehumidifier: Place a dehumidifier in the crawl space to control humidity levels and keep the air dry.
- Replace or Install Insulation: Replace or add insulation to the crawl space walls to maintain a stable temperature and reduce humidity.
These steps keep your crawl space dry, clean, and structurally sound. They’re a modern solution to crawl space problems, unlike open vents and crawl space fans, which have been proven to fail.
Protect Your Crawl Space With Alpha Foundations

Crawl space fans are ineffective at keeping crawl spaces dry. In fact, they can accelerate moisture issues below your home. For a lasting solution, consider installing crawl space encapsulation.
If you want to learn more about encapsulation or schedule a free inspection, contact Alpha Foundations today. We’re Florida’s leading crawl space repair experts, serving homeowners since 2002! Protect your crawl space the right way with us.
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