North Georgia Home Inspector

International Code Council Certified Building Inspector


Crawl-space Ventilation 1

Posted on September 2, 2016 at 3:25 PM

Does your house have enough crawl-space ventilation?


I doubt it!


Check this example:


Assume a 1500 square foot (30’x50’) house, with a crawl-space.


The most common method of providing crawl-space ventilation is through openings in the foundation walls, called foundation vents, or brick, vents.


Unless you have a PROPERLY installed vapor retarder in your crawl-space, crawl-space ventilation is required at a rate of 1 square foot of ventilation for each 150 square feet of crawl-space floor area.


So, a 1500 square foot (30’x50’) crawl-space, requires 10 square feet of free vent area.


Free Vent Area for Available Foundation Vents:

8x16 temperature activated automatic closing: 65 square inches (.45 square foot)

8x16 manual damper: 36 square inches (.25 square foot)

8x16 manual damper: 45 square inches (.31 square foot)


Ten square feet of free air vent area equates to twenty three automatic 8”x16’ crawl-space (brick) vents, or forty manual damper vents. Twenty three vents 16” long equals 30’-8” of ventilators. Equally spaced around the building, locates the vents at approximately 7’ on center. That’s one vent every seven linear feet of foundation wall, or four in each 30’ wall and eight in each 50’ wall. Manual damper vents require 53’4” of wall or one vent every four feet.


Does your house have that quantity of crawl-space vents?

I doubt it!


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