How To Look At A House

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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:

  1. 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.
  2. Mechanical exhaust ventilation rated at 1 cubic foot per minute for each 50 square feet of crawl space.
  3. Conditioned air supplied from the home a/c system, delivered at rate of 1 cubic foot per minute for each 50 square feet.
  4. 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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Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at for $19.95.
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To learn more about exterior walls and structures, see these other blog posts: 

Should I buy a house with a crawl space?

What does a home inspector look for in the crawl space under a home? 

Is white mold on wood in a crawl space dangerous?

• Does code require water pipes in a crawl space to be insulated?  

Is the crawl space under a house required to be ventilated if the floor is concrete? 

Is it safe to go under a mobile home?

What does a home inspector look for when examining a mobile home crawl space? 

What is the code requirement for the height and width of an access opening to the crawl space under a house? 

 Why does venting a clothes dryer into a garage, attic, or crawl space cause problems? 

• When were receptacle outlets in the crawlspace under a home first required to be GFCI? 

• What is the minimum height above ground for wood floor joists and girders/beams? 

 Why is it a mistake to store lumber in the crawl space under a house? 

What are the minimum access openings for a termite (WDO) inspector in Florida for a crawl space and attic, and within these spaces? 

 Is it acceptable for an air conditioning condensate drain line to terminate under the house? 

When were receptacle outlets in the crawlspace under a home first required to be GFCI?

        Visit our EXTERIOR WALLS AND STRUCTURE page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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