Crawl Space Venting issue

Inspected a new home today. Over 3000 sq. ft., 2 stories, on a crawl space. The CS only had 4 foundation vents. There was a 100% vapor barrier, but it was not sealed/taped and did not extend up the walls. It was not a conditioned crawl space. The builder’s rep was trying to tell me that the 1 SF of ventilation for each 150 SF of crawl space rule was reduced if there was a 100% moisture barrier, but I can’t find that in the IRC.

Does anyone know anything about reducing the amount of ventilation required if a moisture barrier is installed? Again, this is NOT a conditioned crawl space.

With rough calculations, I believe this CS should have at least 11 foundation vents. Comments?

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The builder is correct in that much less venting is required with a ground vapor barrier.

I don’t have the requirements in front of me but, from memory, if 1 sq. ft. of venting per 150 sq. ft. of floor then with a ground vapor barrier it was something like 1 sq. ft. per 1000 to 1500 sq. ft. of floor area. Some areas are allowing no vents if the crawl is conditioned now.

One square foot of net free vent area is recommended for every 1500 square feet of floor area covered by a polyethylene vapor retarder. Ratio is 1:150 if vapor retarder is not used.
*Net free vent area is that area unobstructed by screens, louvers, or other materials.
*Heated crawlspaces and basements do not need vents.
Earth floors should be covered with a 4 - 6 mil. polyethylene vapor retarder.
*Providing at least two crawlspace vents will allow for a flow of air in and out of the crawlspace.

I’ve now been told, but have not confirmed, that the ratio of 1 per 1500 SF was in the 2003 IRC. It is not in the 2006 IRC, which is now widely used as a reference.

This house was built in 2008 when, I’m told by the AHJ, the 2003 code was their reference (it’s now the '06 revision). The AHJ also said to me “we strongly discourage it” even though it was allowed.

I’m thinking of explaining in detail that the home was built when this practice may have been permissible, but that it is not now and just cautioning them them that the construction method is not typical and may lead to moisture issues in the wood long term.

Correct. It is not in the 2006. Refer to R-408.2 “Openings for under-floor ventilations”.

How I addressed it. Had to touch on the code issue because that was the issue.

"The crawl space is considered to have inadequate ventilation by today’s standards for new construction. We counted four vents for the entire crawl space. Today’s standards for new construction, the 2006 International Residential Code (IRC), require ventilation openings of not less than 1 square foot for each 150 square feet of crawl space and 1 vent within 3 feet of each corner of the building. A 1500 sq. ft. crawl space, for example, would require 10 sq. ft. of ventilation. The size of the average vent is 16 x 8 inches. Therefore, a 1500 sq. ft. crawl space would require approximately 11 average size vents. We did not conduct sizing calculations on this home. However, the 2003 IRC may have been in use when the house was originally permitted. The 2003 IRC, which most jurisdictions no longer use, allowed ventilation to be reduced to 1 sq. ft. for each 1500 sq. ft. of crawl space provided a 100% vapor barrier was installed. You should inquire with the local building code enforcement authority and the builder to determine which rules were in place at the time of permitting this home.

Our view is that a home of this size with only 4 foundation vents is atypical (in 4 years, as a professional home inspector we have never seen so few vents in a new home) and we do not endorse this building practice. The moisture content in the wood structure was well within acceptable limits at the time of the inspection (not unusual for the low humidity conditions of winter months). Inadequate ventilation can lead to high moisture content in the wood floor structure, which can contribute to the fungi growth and decay in the long term and you are advised to monitor the conditions in the crawl space over the course of ownership, particularly during the humid summer months. You may wish to consult with a crawl space specialist or licensed general contractor for a second opinion."

Up here, we sometimes see an exhaust fan on a duct to the outside wall, controlled by a humidistat. Seems like a fool-proof solution to me.
I don’t know if this is allowed in Dixie :p.

John Kogel

That is a good retrofit for an older home and I see it used effectively; often installed after I point out issues with ventilation during an inspection. However, that is not an option for a new home. It either has to have ventilation meeting the ratio requirements or be an encapsulated crawl space.

Encapsulated. Good point. On new construction, there’s no excuse for not giving the poly a skim-coat of concrete before the floor joists go in.