Crawlspace Fans: Everything You Should Know

Written By Adam Paul - Waterproofing Expert

A crawlspace fan is a mechanical ventilation unit designed to circulate air through the void space below the building. The main purpose of a crawlspace fan is to ventilate the air in the void space below the building, preventing excessive moisture buildup that leads to structural damage and unhealthy conditions.

Crawlspace fans are mainly of 5 types. CFM in crawlspace fan is the measurement of how much air a crawlspace fan can move in one minute. The ideal CFM for the crawlspace fan is 70 to 250 CFM for the residential crawlspace. You can calculate the CFM of the fan using the crawl space dimension and air changes per hour.

Crawlspace fans are available in different materials; among them, stainless steel crawlspace fans are the best. You will need 15-90 watts of electrical power and 110-120 volts to operate the crawlspace fan properly. And also, crawlspace fans should power on according to the outdoor humidity levels and temperatures.

We highly recommend that you to place the crawlspace fan to promote effective cross ventilation within the void space below the building. The number of crawlspace fans depends on the floor area and layout of the subfloor space.

As well as, you should know that crawlspace fans are not fully waterproof. But they are moisture or weather resistant. Crawlspace fans are essential even if the area is waterproofed and a dehumidifier is installed. Otherwise, it may cause severe water problems and damage to the crawlspace. Aqviz experts always recommend that you buy a proper, and quality rated crawlspace fan for efficient ventilation in the subfloor.

What is a Crawlspace Fan?

A crawlspace fan is a mechanical ventilation unit designed to circulate air through the void space below the building. crawlspace fan draws in fresh outdoor air and exhausts stale, moist air, helping to maintain dry and healthy conditions inside the crawlspace. These fans are especially beneficial in areas with high humidity or limited natural airflow.

These are the 5 advantages of using a fan in the crawlspace

Crawlspace fan controls the moisture buildup and prevents dampness in the crawlspace.
Crawlspace fan reduces mold and mildew growth by improving air circulation.
Crawlspace fan prevents wood rot in joists and beams by keeping the structure dry.
Crawlspace fan improves indoor air quality by ventilating stale and musty air.
Crawlspace fan discourages pests in the subfloor area by creating an unfriendly environment.

What is the main Purpose of Crawlspace Fan?

The main purpose of a crawlspace fan is to ventilate the air in the void space below the building, preventing excessive moisture buildup that leads to structural damage and unhealthy conditions. By moving air in and out, it maintains a balanced humidity level inside the crawlspace.

More than that, crawlspace fans prevents condensation, reduce odors, protects insulation, enhance the energy efficiency and extending the lifespan of the wood materials in the crawlspace.

What are the Crawlspace Fan Types?

There are five main crawlspace fan types, each designed for different ventilation needs and space conditions.

Through-wall fans: Through wall fans are installed in the foundation wall to move air directly outside. They are ideal for small to medium crawlspaces with easy wall access.
Inline duct fans: Inline duct fans are connected to ducts for air movement over longer distances crawlspace. Inline duct fans are suitable for large or complex crawlspaces with multiple zones.
Floor vent fans: Floor vent fans are installed into floor grilles above the crawlspace. Floor vent fans are best for homes needing upward airflow into living spaces.
Exhaust fans: Exhaust fans pull air out from the crawlspace to reduce humidity quickly. Exhaust fans Work well in high-moisture environments crawlspaces.
Supply fans: Supply fans push fresh air into the crawlspace. Supply fans are effective in dry climates crawlspace to create positive pressure.

What is the CFM in Crawlspace Fan?

The CFM(Cubic Feet per Minute) in crawlspace fan is the measurement of how much air a crawlspace fan can move in one minute. It indicates the fan’s airflow capacity and helps determine its effectiveness in ventilating a subfloor area. A higher CFM means more air is exchanged, which is crucial for moisture and odor control in the crawlspace.

CFM ratings come from laboratory testing under standardized conditions during manufacturing. The ideal CFM depends on crawlspace size and conditions. Aqviz experts highly recommend a fan rated between 70 to 250 CFM for the most residential crawlspaces. And also larger or damp crawlspaces may require fans with 300+ CFM capacity.

How to Calculate the CFM in Crawlspace Fan?

To calculate the required CFM (Cubic Feet per Minute) for a crawlspace fan, you need to determine the crawlspace volume and desired air changes per hour (ACH). The standard recommendation is 3 to 5 ACH for most residential underfloor spaces.

Formula:
CFM = (Crawlspace Volume × ACH) ÷ 60

Step-by-step example:

Measure the crawlspace dimensions:

Length = 30 ft
Width = 25 ft
Height = 3 ft

Volume = 30 × 25 × 3 = 2,250 cubic feet

Select the ACH (use 4 for moderate conditions):

CFM = (2,250 × 4) ÷ 60 = 150 CFM

So, a crawlspace of 2,250 cubic feet would need a 150 CFM fan for adequate ventilation. Adjust the ACH based on moisture levels and climate conditions.

What Fan Material is Good for Crawlspace?

Crawlspace fans are made from various materials, including galvanized steel, aluminum, ABS plastic, polypropylene, and stainless steel. Each material has different resistance levels to moisture, humidity, and environmental exposure.

When exposed to dampness and high humidity in the crawlspace, galvanized steel fans may corrode over time unless coated. Aluminum crawlspace fans resist rust but can oxidize and weaken. ABS plastic and polypropylene crawlspace fans are lightweight and resist moisture, but can degrade under prolonged UV exposure if used near vents. Stainless steel crawlspace fan offers the best resistance to rust, corrosion, and both UV and humidity, but tends to be more expensive.

For long-term performance in humid, enclosed foundation voids, Aqviz experts recommend using stainless steel fans. Stainless steel crawlspace fan withstand the moisture and temperature swings found in most crawlspaces, making them the most durable and maintenance-free option.

How Much Power Does a Crawlspace Fan Need?

A crawlspace fan consumes between 15 to 90 watts of electrical energy during its operation, but it depends on the fan size, motor efficiency, and airflow (CFM). And also, crawlspace fans run on 110-120 volts AC in residential setups.

To use crawlspace fans efficiently, choose models with energy-efficient motors (like brushless DC motors), and install humidity or timer controls to avoid continuous operation. Sealing air leaks and insulating the subfloor area also reduces fan runtime. Using fans with a built-in thermostat or smart controller can further reduce power usage by activating only when necessary. Matching the correct CFM to the crawlspace size avoids oversizing, which saves energy without sacrificing ventilation performance.

When to Power on Crawlspace Fan?

Crawlspace fans should be powered on primarily during the spring, summer, and early fall, when outdoor humidity levels and temperatures rise. During these seasons, excess moisture builds up in the underfloor space due to ground evaporation, condensation, and poor air circulation. Running fans in the crawlspace helps to remove humid air, reduce mold growth, and prevent structural damage.

In winter, crawlspace fans should be turned off or used minimally, especially in cold climates. Ventilating during cold weather can introduce cold, damp air that leads to frozen pipes, heat loss, and energy inefficiency in the crawlspace. If using fans year-round, install a humidity sensor or thermostat controller to automatically regulate operation based on environmental conditions.

Where to Place a Crawlspace Fan?

Crawlspace fans should be placed to promote effective cross ventilation within the void space below the building. The ideal setup includes installing one fan on one side to draw in fresh air and another on the opposite side to exhaust stale air. This setup ensures airflow across the entire subfloor area.

Fans can be mounted on foundation walls, vent openings, or between support piers, depending on the crawlspace layout. In long or divided crawlspaces, place multiple fans at opposite ends to overcome airflow blockage. Avoid placing fans near plumbing lines or electrical boxes to prevent moisture-laden air from condensing in these sensitive zones. Always install fans at least 6-12 inches above ground level to avoid pulling in debris or standing water.

How many Fans do I need for the Crawlspace?

The number of crawlspace fans needed depends on the floor area and layout of the subfloor space. As a general rule, one fan with 100-150 CFM capacity is sufficient for every 1,000 to 1,500 square feet of crawlspace, assuming a typical height of 3 feet and moderate humidity conditions.

For example, if your crawlspace is 2,000 square feet, you’ll likely need two fans, each rated around 150 CFM, or one larger fan with at least 300 CFM. In long or segmented crawlspaces, place multiple fans at opposite ends to promote cross ventilation. Use more fans if the area has high moisture, poor air movement, or blocked zones. Proper fan placement is just as important as quantity to ensure even airflow throughout the entire space.

Is Crawlspace Fan Waterproof?

Crawlspace fans are not fully waterproof but are typically moisture-resistant or weather-resistant, depending on their design and material. Most quality crawlspace fan models come with sealed motors, corrosion-resistant housings, and IP-rated electrical components to handle the damp and humid environment inside the foundation void.

However, direct exposure to standing water, flooding, or continuous dripping can damage crawlspace fan components over time. For wet crawlspaces, choose fans rated for high-humidity applications or with IPX4 or higher protection. Always install fans above ground level and away from water-prone areas to prevent moisture damage and ensure safe, long-term operation.

What if there is no Crawlspace Fan?

Without a crawlspace fan, stagnant air and moisture accumulate in the void space below the building, leading to several serious problems over time.

These 5 problems occur when there is no crawlspace fan.

Moisture buildup increases humidity levels, promoting damp conditions in the crawlspace.
Mold and mildew growth spreads across wooden beams, insulation, and joists in the crawlspace.
Wood rot weakens structural framing, especially in long-term dampness in the crawlspace.
Musty odors rise in the crawlspace and they affect indoor air quality.
Pest infestations in the crawlspace.

Do you need a Dehumidifier with a Crawlspace Fan?

Yes, a dehumidifier is often needed with a crawlspace fan. Especially in crawlspace areas with high humidity, poor drainage, or limited airflow. While crawlspace fans help move air and reduce surface moisture, they may not be enough to lower relative humidity below 60%, which is the safe threshold to prevent mold, mildew, and rot.

Does a Crawlspace Fan Need for a Waterproof Crawlspace?

Yes, a crawlspace fan is still needed in a waterproofed crawlspace to manage crawlspace humidity and maintain healthy air circulation. Waterproofing prevents liquid water intrusion through walls, floors, and seams, but it doesn’t eliminate moisture from soil evaporation, internal plumbing leaks, or condensation.

Is Solar Powered Crawlspace fan good for Crawlspace?

Yes, a solar crawlspace fan can be a good option for ventilating a crawlspace, but its effectiveness depends on sunlight availability, fan design, and crawlspace conditions. Solar fans are energy-efficient because they operate without drawing power from the home’s electrical system, making them ideal for remote locations or reducing utility costs.

Where to buy a crawlspace fan?

At Aqviz, we recommend sourcing crawlspace fans from reputable online vendors known for quality, warranties, and reliable support. Here are our top five picks:

Amazon.com
homedepot.com
Lowe’s
Tractor Supply Co.
tractorsupply.com

Specialty suppliers like Crawl Space Ninja and Sylvane offer professional-grade fans, including AprilAire, Tjernlund, and AC Infinity models, with expert guidance