Crawl Space Ventilation and Vent Basics


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No doubt you've seen those screened punch-out openings on the side of house foundation walls before and wondered what they were about. These are crawl space ventilation vents, and they are crucial to the proper functioning of your house. Your comfort—and your home's long-term maintenance—hinges on how well your crawl space is ventilated.

How Does Crawl Space Ventilation Work?

In a home where the entire structure or part of the structure is elevated slightly above the ground, yet not over a basement, the gap between the bottom of the structure and the ground is known as a crawl space.

A crawl space type of foundation is common in warm, moist climates where it is advantageous to raise the structure slightly off the ground to avoid moisture. Crawl spaces are also sometimes found in conjunction with basement foundations where a portion of the house—such as a porch—is separate from the main structure and has an independent foundation. 

If any single element is most destructive to homes it is moisture. Moisture is responsible for a host of problems that erupt in a home—wood rot, along with various strains of mold and mildew. Left unabated, moisture alone can bring down a house in short order.

Light, heat, and airflow are the best weapons against crawl space moisture. Because it is difficult to provide light and heat to a crawl space, promoting good airflow is the best solution.

To avoid damage from moisture that might rot away beams, joists, and flooring, building codes have long required that crawl spaces be properly vented so that good airflow can help keep these spaces dry. This airflow is normally provided by a series of rectangular, screened vents inserted into the concrete block foundations surrounding the crawl space. 

International Residential Code (IRC) Requirements

Virtually all code requirements for all aspects of home construction are enumerated in the International Residential Code (IRC). Unless local and state rules override them, the rules listed in the IRC are the basis for all code requirements for residential housing. 

IRC Section R408.1, Ventilation

The IRC prescriptions for venting crawl spaces are contained in section R408, in several paragraphs. The first paragraph of IRC section 408 provides the standard requirement for ventilating crawl spaces: 

The under-floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement) shall have ventilation openings through foundation walls or exterior walls. The minimum net area of ventilation openings shall be not less than 1 square foot for each 150 square feet of under-floor space area, unless the ground surface is covered by Class 1 vapor retarder material. Where a Class 1 vapor retarder material is used, the minimum net area of ventilation openings shall be not less than 1 square foot for each 1,500 square feet of under-floor space area. One such ventilating opening shall be within 3 feet of each corner of the building.

Translated, this code requirement means that you need 1 square foot of screened vent space penetrating the perimeter foundation for every 150 square feet of space in the crawl space.

For example, if your foundation size is 30 feet by 30 feet (900 square feet), you will need vents that have combined square footage of 6 square feet. This could be achieved by six 1-foot by 1-foot vents, or three 1-foot by 2-foot vents. 

If, however, you cover the bare ground in your crawl space with an approved vapor barrier material, you only need 1 square foot of vent for every 1,500 square feet of space. 

The code also requires that there be one ventilated opening near each corner of the building. This is necessary to ensure a good cross-flow of air. 

Section 308.2, Types of Crawl Space Vents

This second paragraph provides details of how these crawl space vents should be structured. Each vent must be at least 1 square foot in size and allowable materials include:

  • Expanded sheet metal plates not less than 0.047 inches thick
  • Perforated sheet metal plates not less than 0.070 inches thick
  • Cast-iron grill or grating
  • Extruded load-bearing brick vents
  • Hardware cloth of 0.035-inch wire or heavier
  • Corrosion-resistant wire mesh, with the least dimension being 1/8-inch thick

If you are using a ground vapor barrier, the vents can be of the type that has moveable louvers.

Section 308.3, Unventilated Crawl Spaces

This paragraph makes provisions for situations in which builders and homeowner prefer to omit vents in crawl spaces, usually because they wish to prevent thermal heat loss or to prevent insects and other vermin from gaining access to the crawl space.

Builders are now allowed the option of creating non-vented crawl spaces, provided they follow the following procedures:

  • Mechanically circulating air should be established between the upper conditioned area of the home and crawl space.
  • The air-circulating device must move at least 1 cubic foot of air per 50 square feet of crawl space area.
  • The crawl space floor area must be completely sealed with a vapor-retarding material.
  • The edges of the vapor retarder must be lapped up against the inner foundation walls. Overlapped separate sheets must be lapped at least 6 inches. The seams must be sealed.
  • All crawl space walls must be insulated to appropriate R-values for the regional climate.