Frequently Asked Question

How do you determine how many foundation vents are needed?
Last Updated 7 years ago

National codes required 1 square foot of free area in the foundation for every 150 square feet of crawlspace area.

To determine how many foundation vents are needed, take the square footage of the crawlspace and divide by 150. Take that figure and multiply by 144 to cover to square inches. Next, divide that number by the NFA rating of the specific foundation vent being used (NFA ratings available HERE)

Example - 1000 Square Feet of Crawlspace and the 189 is the preferred vent:
1000 / 150 = 6.66 Sq Foot of Ventilation Needed
6.66 Sq Feet X 144 = 959.95 Square Inches Needed
959.95 / 74 = 12.97

You would want to round up to 13 and use 13 of the 189. To achieve a balanced system around the home, it may be desired to round up to an even number, or 14.

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