Question 34
Domain 1: Footings, Foundations, and CrawlspacesWhat is the minimum required thickness of a vapor retarder in a crawlspace per the IRC?
Correct answer: B
Explanation
The IRC requires a crawlspace ground cover vapor retarder to be at least "6-mil polyethylene" or an approved equivalent. This minimum thickness limits moisture migration from the soil into the crawlspace and is the standard code-prescribed barrier.
Why each option is right or wrong
A. 4-mil polyethylene
B. 6-mil polyethylene
IRC Section R408.3 requires the exposed earth in a crawlspace to be covered with a continuous Class I vapor retarder, and the code specifies a minimum thickness of 6 mil. The material must also be lapped at least 6 inches at joints and extended up the foundation walls at least 6 inches, which is why a 6-mil polyethylene sheet is the minimum compliant answer.
C. 10-mil polyethylene
D. 15-pound building paper