Do Crawl Space Ventilation Fans Work?

A practical solution to controlling the moisture that builds up in crawl spaces is to install a ventilation fan. After all, fans are used in water damage restoration dry floors and walls.

Though, do crawl space ventilation fans work? In a word, no. Here’s why. Crawl spaces are built to be ventilated. Builders have designed crawl spaces that way since the early 1950s. Newer homes, however, have more efficient heating and air conditioning units.

Cold air in the ducts mixes with the warmer air in the crawl space and creates moisture.

Why Ventilating Outside Air Doesn’t Work

Adding a ventilation fan actually makes this situation worse. The fan draws air in from the outside on the premise that moving air will lower moisture. However, it serves to add more humidity to the moisture already building up. On top of that, a vented basement crawl space poses efficiency issues.

Increased Humidity Instead of Less

By design, crawl spaces are cooler than the surrounding air outside and in the structure. In fact, the temperature varies by 10 to 20 degrees. On a temperate day with a high of 75 degrees and 60% humidity, the crawl space will stay around 55 degrees.

So, when the ventilation fan draws the outside air in, that air will cook down as well. At that point, the humidity in your crawl space increases by 2% for each degree the air cools. So a 20-degree drop means a 40% increase in humidity.

From there, the math is simple. Your crawl space air has reached 100% humidity. Air can’t hold more than that, and the result is condensation throughout your crawl space. Further, mold begins to grow at 60% humidity, which makes is a big problem for those living in warm and humid climates.

Any insulation in your crawl space absorbs the humidity and condensation around it. From there, the moisture leeches into the subfloor, creating the perfect environment for mold.

Thus, the extended answer to the question about ventilation fans is that they do more harm than good. That’s not to say a different type of fan can’t help control humidity. The recommended way to manage humidity in crawl spaces is to encapsulate and dehumidify.

Reduces Energy Efficiency

A vented crawl space makes more sense in the summer. In the winter, open vents mean that cold air enters your home through the basement. Cold air enters through the crawl space and flows over your hot water pipes, heating ducts, furnace, and water heater.

As a result, all the related utilities have to work harder to maintain the comfort of your home. If you live in a cooler part of the country like New England, this can lead to a drafty home.

Encapsulating Your Crawl Space

The U.S. Department of Energy recommends encapsulating a crawl space with a vapor barrier. To do that, you’d like the entire space with a barrier and then seal it to stave off moisture from the ground and outside air. Then, install and run a dehumidifier year-round to keep your space dry and as free from moisture as possible.

Encapsulating is the best way to control moisture issues. Doing so protects your hardwood and floor joists from buckling and rotting. It also improves the energy efficiency of your home by up to 18%. You’ll find a comfortable reduction in drafts rising from the floor during colder months.

Crawl spaces do require ventilation of some kind. The following options address that.

Using Supply Air from Your HVAC System

Many times, contractors will vent some air from your HVAC system through the encapsulated crawl space. Often, the duct system is already there, so this process is easy and cost-effective. The drier air from the HVAC helps dry out the crawl space.

If you know something about heating and air, then you know that it will create a positive pressure in the crawl space. The house will be under slight negative pressure. Rest assured, that the pressure effect is negligible. As long as your home’s heating and air unit can accommodate the extra square footage, this method works well.

Some recommend about 1 cubic foot per minute (or cfm) for every 30 square feet of crawl space. The International Residential Code (IRC) recommends 1 cfm for every 50 square feet of crawl space floor. If your system runs continuously, it can send enough dehumidified air through the crawl space to keep the humidity under 70%, which is a healthy amount.

Use an Exhaust Fan to Vent Air Outside

Another idea to combat humidity is to install an exhaust fan to run some of the supply air from the HVAC system outside. While this is a fan, it’s much smaller than a ventilation fan. The IRC describes an exhaust fan as one that operates continuously.

It cycles air at a rate of 1 cubic foot per minute, or 0.47 L/s for every 50 square feet (or 4.7m2) of crawl space. Per the IRC, it should also include an airway to an area like a transfer grill or duct. Finally, the perimeter walls should be insulated.

As an example, a crawl space of 2000 square feet needs a 40-cfm exhaust fan. It draws a small amount of air from your crawl space. The key is to be sure that air is drawing correctly from the air supply rather than from the outdoors.

Add a Dehumidifier

Some say that adding a dehumidifier system to an encapsulated crawl space is the best method of controlling moisture under your home. A dehumidifier does a formidable job of controlling moisture. Professionals install a humidistat with a fan system to keep the dehumidifier operating efficiently. All three components combine to form an effective, moisture-fighting system.

In Summary

Do crawl space ventilation fans work? No. Though, some sort of ventilation is beneficial for controlling moisture levels in the crawl space under your home. Contrary to common belief, a ventilation fan actually does more harm than good.

Basements have been vented for decades, and that won’t change. However, more and more contractors stand by encapsulating crawl spaces instead. Encapsulating is more effective at combating moisture build-up, as is adding an exhaust fan or dehumidifier system.