Crawlspace Ventilation Requirements


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In a home where the entire structure or part of the structure is elevated slightly above the ground but not over a basement, the gap between the bottom of the structure and the ground is known as a crawlspace. This type of foundation is common in warm, moist climates where it is advantageous to raise the structure slightly off the ground to avoid moisture. Crawlspaces 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. 

In order to avoid damage from moisture that might rot away beams, joists, and flooring, building codes have long required that crawlspaces 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 crawlspace. 

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. 

The IRC prescriptions for venting crawlspaces are contained in section R408, in several paragraphs. Here are some of the key elements of this section of the IRC.

Section R408.1, Ventilation

The first paragraph of IRC section 408 provides the standard requirement for ventilating crawlspaces: 

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.

What this means, essentially, is that you need 1 square foot of screened vent space penetrating the perimeter foundation for every 150 square feet of space in the crawlspace. For example, if your foundation size is 30 by 30 (900 square feet), you will need vents that have combined square footage of 6 square feet. This could be achieved by six 1 x 1 vents, or three 1 x 2 vents. 

If, however, you cover the bare ground in your crawlspace 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 good cross-flow of air. 

Section 308.2, Ventilated Openings

This second paragraph provides details of how these crawlspace vents should be structured: 

  • Each vent must be at least 1 square foot in size.
  • Allowable materials include:
    • Perforated sheet metal plates not less than 0.070 inches thick
    • Expanded sheet metal plates not less than 0.047 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 Crawlspaces

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

In the most recent edition of the IRC, builders are now allowed the option of creating non-vented crawlspaces, provided they follow the following procedures:

  • Mechanically circulating air is established between the upper conditioned area of the home and crawlspace. The air-circulating device must move at least 1 cubic foot of air per 50 square feet of crawlspace area.
  • The crawlspace floor area must be completely sealed with a vapor-retarding material. This means lapping the edges of the vapor retarder up against the inner foundation walls, overlapping separate sheets by at least six inches, and sealing up those seams.
  • All crawlspace walls must be insulated to appropriate R-values for the regional climate.