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Section R408.1 specifies that the crawl space (under-floor space), which is the area between the bottom of the floor joists and the earth under the building (not including the basement), must have ventilation openings through the foundation walls or exterior walls.
Crawlspace code requirements and rules that govern under floor space venting, vapor barriers, grade levels, and access openings. This article is based on the IRC or international residential code on under-floor space.
The amount of venting required and the location of the vents depend on the total square footage of the crawlspace and how the ground is treated. A vapor retarder placed over the earth reduces the minimum required ventilation area and eliminates the requirement for vented openings within 3 ft. of all building corners.
Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m 2) of crawl space floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.11 of this code. 2.2.
Your crawl space needs a ventilation system that allows constant air movement through the space without bringing in additional moisture, which can spur mold growth. The IRC code state that there must be 1 square foot of screened vent area for every 150 square feet of crawl space.
Crawlspace ventilation guidelines: Should you ventilate the crawl space, close off crawl space vents, install crawl space vent fans or dehumidifiers or what? This article describes best practices as well as common codes and standards for venting (or not venting) the space beneath buildings.
If your crawl space is vented, you’ll need to install vents on all the walls. The size of vents must be 1 foot for every 500 square feet or 1 foot for every 1,500 square feet when they house Class 1 vapor barriers that contain a 6-mil thick sheet of polyethylene.