How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
How much ventilation is required for the under-floor crawl space of a home?
Thursday, June 21, 2018
Homes with an elevated floor and an under-floor crawl space are required to have ventilation openings of at least 1 square foot for every 150 square feet of floor area, and a minimum of one ventilation opening within 3 feet of each corner (to avoid dead air space at corners). Here’s how it is stated in the International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC):
The math is easier to calculate if you use a slightly higher standard of 1 square inch of ventilation for each square foot of floor area (1 square foot = 144 square inches).
Because homeowners often don’t recognize the importance of crawlspace ventilation for a Florida home, sometimes they get obstructed or covered up over the years. If you look closely at the photo at the top of the page, you can see that leaves have blown into the recesses of the screened decor blocks used for ventilation, which significantly reduces the available opening. Also, mulching of the foundation plantings over the years can raise the soil level over low vents, like the photo below.
The building code allows four exceptions to the ventilation requirement:
- No ventilation is required if you cover the ground in crawl space with a vapor retarder barrier which extends up the stem wall and is taped at overlapped seams and along the wall.
- Mechanical exhaust ventilation rated at 1 cubic foot per minute for each 50 square feet of crawl space.
- Conditioned air supplied from the home a/c system, delivered at rate of 1 cubic foot per minute for each 50 square feet.
- Plenum in crawl space (existing homes only). A plenum is collection area for supply air to the home.
The most common ventilation defects we find in homes with an under-floor crawl space are lack of vents at some corners, followed by vents that have been filled in or covered over. Usually the square area of the vents is sufficient.
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To learn more about exterior walls and structures, see these other blog posts:
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