Crawl Space Ventilation Requirement in WA
Ventilated crawlspaces do have without adequate ventilation, moisture from the ground and air can start to build up, which can cause serious problems with the homes wood and inner environment.
To prevent this, the International Building Code states that structures need 1 ft² of ventilation per 150 ft² of crawl space, or 1/150, but this ratio can be reduced to 1/1,500 where the ground surface is treated with an approved vapor retarder. For those of us in Washington State, the use of a 6 mil. vapor barrier is common, so our *Residential Code allows a 1/300 ratio.
2018 Washington State Residential Code （WRC）Section R408.1 https://codes.iccsafe.org/content/WARC2018P1/chapter-4-foundations#WARC2018P1_Pt03_Ch04_SecR408