Site Loader

Residential Building Code for Crawl Spaces

Crawl spaces have evolved into two main categories: Vented and Unvented (also referred to as Closed). The International Residential Code (IRC) has specific regulations for moisture control for both vented and unvented crawl spaces. For the purposes of this blog, we will be referring to the 2021 building codes. Not all states have adopted the newest building code, so check your local building codes for your specific requirements.

The crawl space often houses ducting, plumbing, HVAC, and electrical wires. Compared to a house built on a slab foundation, this allows for easier access to repairs and inspections for leaks, electrical problems, termites, and other issues that may arise. A major advantage to this type of foundation is the considerable cost savings compared to a basement construction and the ability to build in areas with a high water table. However, a crawl space is not without its own issues. Depending on location and construction, many crawl spaces are prone to moisture issues. Building codes are designed to address these potential problems.

What is a Crawl Space?

A crawl space is a type of foundation found in home construction. There are three common types of foundations: slab, crawl space and basement. The crawl space supports the integrity of the home. By definition, it is an unoccupied and unfinished space between the ground and the first floor of the home. It is typically small with a low ceiling, so you most likely need to crawl in it, hence the name. The purpose of this type of foundation is to raise the home off the ground to provide a buffer between the ground and the living space. This type of foundation was originally designed for homes in damp areas.

Unvented Crawl Space

ATMOX Picture of Unvented Crawl Space

Due to the potential disadvantages of the traditionally vented crawl space, the industry moved toward closing the crawl spaces and building or retrofitting crawl spaces to be unvented or closed. This is not without its own disadvantages. An unvented crawl space has a continuous perimeter foundation wall that is sealed. In other words, there are no vents to the outside present. Some new construction is designed this way, and in other cases, older homes have been retrofit by sealing all vents to the outside.

An unvented crawl space, as defined by the International Residential Code, is a crawl space without vents. It is required to have a 100% floor vapor barrier sealed and taped 6″ up the wall and attached to the stem wall or insulation for moisture control. Additionally, all seams must overlap six inches and be sealed with tape.

For purposes of moisture control in the residential building code, in conjunction with the vapor barrier requirement, an unvented crawl space must also provide at least one of the following:

  1. Dehumidification
  2. Conditioned Air Supply
  3. Continuous Exhaust Fan
  4. Plenum (Existing structures only)

Residential Building Code Requirements

Let’s look at the three most common moisture control options, the IRC requirements for each, and the advantage of using ATMOX products in all three scenarios. 


  • Code requirements for sealing and taping vapor barrier for an unvented crawl space
  • Code requires a dehumidifier
  • Dehumidifier sizing is in accordance with manufacturer’s specifications
  • No requirements as to the placement of insulation

ATMOX Advantage with Dehumidification


The ATMOX controller can further enhance the use of a dehumidifier. Combined with other ATMOX products, it has the power to improve drying capabilities, improve air quality, evenly distribute airflow, increase your energy efficiency, and extend the life of your dehumidifier. The controller uses sophisticated logic in its software to improve conditions through proper and beneficial ventilation and dehumidification. Furthermore, the ATMOX controller sends alerts to notify you of potential issues in your space.

ATMOX sensors measuring temperature and humidity are placed away from the dehumidifier to ensure the conditions further from the unit are consistent with the entire crawl space. Data from these sensors are used to direct operation of components.

Furthermore, ATMOX controller sends alerts to the ATMOX display to provide you with information and notify you of potential issues in your crawl space.


ATMOX circulation fans extend the reach of the dehumidifier. The dehumidifier has a fan to move air. However, it will dry the air closest to it first and may not reach far corners. Crawl spaces are rarely a perfect rectangle and may have sections or corners that are not getting consistent air movement. The ATMOX internal circulation fans move air to these hard-to-reach areas.

Dehumidifier and Outside Air

A dehumidifier is a great moisture reduction tool, however, it is uses a significant amount of energy. If desired and selected during setup, the ATMOX controller can use both the dehumidifier and outside air to control moisture in the crawl space. The controller accomplishes this using a dew point comparison of both inside and outside air.

When the outside air is beneficial, the controller will operate ATMOX louvered intake and exhaust fans to use outside air for moisture reduction and for exchanging air to improve air quality. If the conditions outside are not optimal for drying, the controller will activate the dehumidifier when inside conditions warrant the need for drying. However, the use of outside air is very energy efficient, so reducing the usage of the dehumidifier will increase your energy efficiency while effectively drying your space. This will also exchange air within the space to improve air quality with fresh air.


  • Code requirements for sealing and taping vapor barrier for an unvented crawl space
  • Conditioned air supply sized according to code that includes a return air supply to the living space
  • Insulation on perimeter walls required

ATMOX Advantage with Conditioned Air

ATMOX controller combined with the ATMOX products has the advantage of increasing drying through air movement and circulation through the space for even drying of the space.

The ATMOX controller uses highly accurate sensors to direct the operations of the ATMOX components. Furthermore, ATMOX controller sends alerts to the ATMOX display to provide you with information and notify you of potential issues in your crawl space.

The controller directs the ATMOX internal circulation fans to balance temperature and humidity levels within the space. Depending on where conditioned air is supplied to the crawl space, the air movement may not reach all areas of the crawl space. These fans assist in better circulation of the conditioned air.

Using conditioned air as a drying tool is very effective when the air conditioning is operating. There can be times in the spring and fall when the air conditioning is not needed within the house. During these times, the crawl space may not get the drying power that it needs. If desired and selected during setup, the ATMOX controller can supplement drying with outside air during these times when outside conditions are generally beneficial to a crawl space. In these cases, the ATMOX louvered intake and exhaust fans draw in outside air at the optimal times.

Continuous Exhaust Fan

  • Code requirements for sealing and taping vapor barrier for an unvented crawl space
  • Continuous exhaust fan with ventilation rate “equal to 1 cubic foot per minute for each 50 square feet of crawl space floor area”
  • Air pathway to the living space required
  • Insulation on perimeter walls required

ATMOX Advantage with Continuous Exhaust Fan

The ATMOX controller has a setup option to operate the ATMOX continuous ventilation fan. A benefit to using the fan with the controller is that there is a low temperature cutoff to avoid extreme cold conditions. With this option there is some air exchange in the crawl space through the creation of slight negative pressure on the home.

Additionally, the ATMOX display provides you with information and alerts on your crawl space conditions.

Depending on where the air from living space is coming into crawl space and exiting through ventilation fan, the air movement may not reach all areas of the crawl space.The ATMOX circulation fans help to balance temperature and humidity throughout the space to prevent moisture buildup. The fans accomplish this by distributing airflow evenly through the space.

Vented Crawl Space

A vented crawl space, also known as a traditional crawl space, has a continuous perimeter foundation wall with vent openings.

Residential Building Code Requirements

According to the IRC, vented crawl spaces have vents around the perimeter. Here are some of the requirements for the residential building code:

  • There must be a vent 3 feet from every external corner.
  • Vapor barrier not required.
  • If a vapor barrier is present:
    • then, there are no requirements for taping or sealing or coverage of vapor barrier.
    • then, fewer vent openings required.
    • then, the requirement for vents three feet from every corner not required “provided that the vent openings are placed to provide cross ventilation of the space.”
  • Vents with operable louvers allowed.

A vented crawl space has the advantage of using fresh outside air to prevent stagnant air. Stagnant air can lead to stale odors and moisture buildup. Although a vented crawl space allows for fresh outdoor air, the amount of air and rate of ventilation may not be significant enough – especially in many warm and humid climates. There is likely a need for assistance to create sufficient air exchange.

Types of of Crawl Space Ventilation

Mechanical or active ventilation is the process of assisting air movement and circulation through the crawl space. Mechanical ventilation uses fans to push and pull air throughout the crawl space. This increases air movement and circulation within the crawl space. The air movement is necessary for moisture control in your crawl space.

Additionally, controlled ventilation is ideal to use the outside air for drying when it is beneficial and circulating the air throughout the crawl space to provide consistent conditions. When referring to controlled ventilation, we are referring to the process of monitoring the inside and outside conditions specifically, the temperature, humidity, and dew point. This allows you to use the outside air to dry the crawl space without worry of bringing in air that may increase your moisture. This can be a common issue with fans on a humidistat.

ATMOX Advantage with Vented Crawl Space

The combination of ATMOX products with a vapor barrier allows for the ATMOX to better control the conditions within the crawl space. ATMOX only uses select vents for controlled air exchange while closing vents not used for intake or exhaust points. This controlled ventilation allows for air exchange when outside air is beneficial and closed when outside air is not better. If needed, a dehumidifier can be installed and controlled by the ATMOX controller. The ATMOX controller directs operation of all installed components to effectively reduce moisture in your crawl space.

The highly accurate ATMOX inside and outside sensors measure dew point, temperature and humidity of the crawl space air and outside air. This information is important to ensure proper ventilation because the outside air is not always beneficial. The ATMOX controller’s logic system uses this data to determine the best time for operation of the ATMOX components.

The ATMOX intake and exhaust fans will operate when the controller determines the best times for drying. When outside air is within an acceptable temperature range and dew point, the fans will operate. Furthermore, the outside air is not only a very effective drying tool, it is energy efficient so great for your pocketbook! This exchanging of air prevents moisture buildup and improves air quality.

The ATMOX circulation fans move air throughout the crawl space to reduce areas or pockets of stagnant air. Hard-to-reach areas may have higher moisture levels. Moving air to these areas helps to balance temperature and reduce the moisture throughout the space.

The ATMOX display provides easy access to information from your crawl space. The display provides you with information on your crawl space and alerts you with potential issues.

The ATMOX Solution

As you can see, there are many types of moisture control options within the residential building code. However, building code is sometimes not sufficient. The right solution for California may be completely different than one for Georgia. Climate plays a real factor in your moisture control solution. Regardless of your geographic location and climate, ATMOX has the advantage of enhancing all options within the building code.

ATMOX Picture ACE Crawl Space Controller for Moisture Control

Post Author: ATMOX TOM