Pros and Cons of a Crawl Space Vent Fan

Written By: Shaye Glisson
Published on: October 8, 2012
Updated on: August 5, 2026

For a very long time, people have considered crawl space fans the best option when it comes to crawl space humidity fixes. You may have heard that a crawl space fan can fix your crawl space humidity problems. In fact, a so-called expert may have even led you to this idea.

Unfortunately, this has led to a variety of people who believe that crawl space ventilation and crawl space fans can actually help with existing crawl space moisture. Don’t believe this rumor. Read on to learn more about why open vents, in any capacity, can actually do more damage to your crawl space.

What Is a Crawl Space Fan?

Many people don’t even know what they mean when they’re talking about a crawl space fan. This can lead to a lot of confusion when talking about ventilation, fans, and the best way to keep your crawl space dry and healthy.

In simplest terms, a crawl space fan takes air from the outside and blows it into your crawl space. The idea is that because it makes sure air is constantly moving in your crawl space, you won’t have to worry about water accumulating in your crawl space, and that will help you avoid common moisture concerns.

However, a crawl space fan is probably a bad idea, regardless of why you’re using it. When you use a crawl space fan, you also have to have crawl space vents. You’re using unconditioned air from outside your home and pumping it into your home.

Crawl Space Humidity by the Numbers

The reason crawl space vents don’t work is simple math. Consider that the temperature in your crawl space is 65°F, and the temperature outside is 95°F — a perfectly reasonable thing to assume on a hot day in Virginia. If you have a crawl space vent, air from the outside is coming into your crawl space, which means that 95°F temperature is coming into contact with the 65°F temperature.

For every degree the air cools, the relative humidity goes up by 2.2%. Consider a theoretical situation where your crawl space is currently at 0% humidity, which is near-impossible. Even in this situation, that means your crawl space humidity goes up to 66%, which is high enough for some types of mold to grow. If your crawl space is at 50% humidity, which is a healthy humidity level, then the humidity’s just gone up to 116%.

When the relative humidity level in any area is above 100%, that water has to go somewhere. Outside, the excess water turns to rain; inside, it’s likely to condense and form puddles in the crawl space. Crawl space condensation, whether it’s on the walls, floor, or pipes, will end up causing damage of some kind.

Crawl Space Ventilation Problems

There are many concerns that arise because of crawl space vents. Here’s a list of the ones that should be at the top of homeowners’ minds:

Crawl Space Moisture

The most obvious concern with crawl space vents is the one you’ve already heard about in this article. Crawl space moisture can be a significant problem with open crawl space vents, especially because of the issues with relative humidity in different temperatures of air.

Remember that crawl space moisture has to go somewhere; it doesn’t just hover in the air forever. Any amount of crawl space moisture can be damaging because it creates an environment that fosters mold and mildew growth. Moisture in your crawl space can also lead to dry rot.

Water Coming in From Outside

When it rains, or there are flooding issues in your area, you might find that water comes in from the outside with open crawl space vents. Open crawl space vents aren’t great at filtering water from the outside, which means it’s extremely common for water to come into the crawl space and impact the area.

Some crawl spaces can close them at times or put a covering on them occasionally. However, it’s common for homeowners to forget about these options, which typically leads to a lot of issues with water coming in anyway. The only way to fix it is to permanently cover the crawl space.

Crawl Space Pests

When you have a constant opening in your crawl space, you’re inviting pests into your home. It doesn’t help that you typically keep your home at a very comfortable temperature for humans, which is also usually a very comfortable temperature for pests that might be roaming around your neighborhood.

Even if the crawl space vents have very small openings, which aren’t big enough for animals like raccoons, that doesn’t mean pests can’t come through. Extremely small openings don’t do much to mitigate tiny pests like ants and mosquitos, and even pests like snakes often do a great job of coming in through tiny openings.

HVAC Overloads

One of the reasons crawl space vents lead to higher levels of energy usage is probably because you’re basically pumping your heating and cooling out of the building with an open crawl space vent. Remember that your home’s heat and cooling go into your crawl space as well as the rest of your home, which means it’s going to “leak” from the vents.

This will always lead to higher energy bills for the same reason that it would if you kept your home’s windows open at all times. You want to keep the warm or cool air that you’re creating inside your home. Instead of opening a crawl space vent, it’s important to close the vent and instead circulate the air in your crawl space through other means.

The Stack Effect

If you’ve never done any research into crawl spaces and their impact on homes, you may not know about the stack effect. The stack effect applies when you have an entrance for air in your crawl space and an exit for air in your attic. Air will come in through the crawl space, warm up, then rise through the home and exit out of the attic. This process will continue to repeat as long as there’s an entrance for air in your crawl space, such as a crawl space vent.

The stack effect can be very damaging in many different homes. Because crawl space vents typically cause lots of moisture to build up, they also tend to cause plenty of mold and mildew, and pests can leave their droppings all over the crawl space. Essentially, the crawl space takes anything that can be airborne, which sometimes includes droppings and mold spores, and brings it up into your living space.

The Problem with Crawl Space Vents

No matter what additional endeavors you undertake in attempting to fix your crawl space moisture concerns, you’re never going to be able to address the crawl space moisture problems if you still have vents in your crawl space. At the end of the day, you need to fix the crawl space vents if you want to avoid crawl space moisture as a whole.

Studies regularly find that crawl space vents almost always stay above 70% humidity, which is high above the generally recommended 50% humidity level. The only way to fix it is to remove the source of the problems. Covering crawl space vents is possible, but you need to ensure you’re doing it the right way.

How to Curb Crawl Space Humidity

Closing Crawl Space Vents

Open crawl spaces cause high electric bills because vented crawl space lets in outside moisture

First off, you need to make sure you have a fully closed crawl space. If you’re dealing with poorly sealed crawl space doors or vents, you’re going to have moisture coming in through these areas. No matter how much dehumidifying you do, you’re never going to be able to get rid of the crawl space moisture in these situations.

That’s why closing crawl space vents and doors should be your first step. If you have crawl space vents, you need to install crawl space vent coverings, and if you have crawl space doors that open to the outside, you need to make sure the insulation around the doors are solid. It’s important to invest in high-quality fixes for these so they actually work.

Crawl Space Vapor Barrier

The next step should be to install vapor barrier liners. A vapor barrier liner goes on the ground, up the walls, and around any supporting struts in your crawl space. The intent behind the vapor barrier liner is relatively obvious from the name: it’s supposed to trap water vapor outside the barrier, making sure it stays in the ground instead of in your crawl space.

It’s important to note that not all vapor barrier liners are created equal. Although technically, you can get away with using a thin vapor barrier liner, such as a 6-mil liner, you really need a 20-mil liner like CrawlSeal™ to provide the maximum amount of protection. Plus, with a strong liner like a 20-mil liner, people will be able to go into your crawl space to inspect it in the future without damaging the liner.

Call the Experts at JES

Discovering the right solution for crawl space humidity is crucial for maintaining your home’s structural integrity and air quality, so don’t let moisture issues persist. Schedule a free inspection with a JES expert today to explore your crawl space repair options, tailored to address the specific humidity challenges of your home. Take the first step towards a safer, drier basement environment now.

Frequently Asked Questions About Under House Fans

Technically, nothing’s stopping you from putting a fan in your crawl space. However, it’s important to note that putting a fan in your crawl space definitely won’t fix moisture and humidity problems. In fact, many vent fans actually allow external air into the crawl space, which often leads to added moisture and even worse problems.

Whole house fans, which vent air from outside into your home, work well only on days that the outside climate is better than the inside climate. Unfortunately, that very rarely happens. That means whole-house fans are really only useful in very specific climates, often only during a small part of the year.

Especially if you’re having crawl space moisture problems, you might be wondering whether the answer is to open up some ventilation and let your crawl space “breathe.” In fact, the opposite is true; crawl space moisture often comes from too much ventilation. Crawl space encapsulation is a much more effective fix.

There are many reasons you might have moisture under your house. If you currently have a damp crawl space and no idea why it’s probably a good idea to turn to the crawl space experts from JES. You can schedule an inspection to learn more about what’s going on under your crawl space and why you’re having problems.

Pros and Cons of a Crawl Space Vent Fan

Table of Contents

Pros and Cons of a Crawl Space Vent Fan

Table of Contents

What Is a Crawl Space Fan?

Crawl Space Humidity by the Numbers