A new unvented crawl space has a compliant sealed Class I ground vapor retarder and insulated perimeter walls. It has no continuous mechanical exhaust, no conditioned-air supply, no qualifying existing-structure plenum arrangement and no dehumidification. Under 2021 IRC R408.3, what is the finding?
- AGround cover alone completes the unvented crawl-space requirements
- One of the listed R408.3 moisture-control methods is still required
- CPerimeter insulation alone replaces every listed method
- DThe code prohibits all crawl spaces without exterior vents
Where to look it up
https://core-docs.s3.amazonaws.com/documents/asset/uploaded_file/3836/Lennox/3042182/2021_International_Residential_Code.pdf
Why B is correct
R408.3 requires the prescribed continuous ground vapor retarder plus one of its listed methods. Those include continuous mechanical exhaust with its stated rate, air pathway and insulation conditions; conditioned-air supply with its stated rate, return pathway and insulation conditions; a compliant plenum in an existing structure; or dehumidification sized to manufacturer specifications. The question excludes every listed route, so the ground cover and insulation alone are insufficient. Dehumidification is an allowed route; it is inaccurate to restrict compliance to exhaust or supply or to equate every option with outdoor-air exchange. Source: https://core-docs.s3.amazonaws.com/documents/asset/uploaded_file/3836/Lennox/3042182/2021_International_Residential_Code.pdf