Determine required interior vapor retarder class by climate zone under IRC R702.7
domain: codes.iccsafe.org · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Confirm that a vapor retarder on the interior side of exterior wood-framed walls is not required in Climate Zones 1-3 under the base provision
Apply the Class I or II vapor retarder requirement for Climate Zones 4 (except Marine 4), 5, 6, 7, and 8 on the interior side of exterior walls per the applicable table
Check whether the specific wall assembly (e.g., with exterior continuous insulation) qualifies for a different/no vapor retarder class under the assembly-specific exception tables
Where a 'responsive' (variable-permeance) vapor retarder is used, confirm it meets both the dry-cup Class I/II criteria and the greater-than-1-perm wet-cup criteria for inward-drying assemblies
Confirm the locally adopted IRC edition, since vapor retarder provisions changed significantly between the 2018 and 2021 IRC
Known gotchas
Vapor retarder class requirements interact with exterior continuous insulation R-value; check the applicable exception table rather than assuming base class rules apply unconditionally
The 2021 IRC significantly revised vapor retarder provisions from earlier editions; guidance based on pre-2021 code cycles may not match a jurisdiction on 2021 or later
Marine Zone 4 is treated like Zones 5-8 for vapor retarder purposes, not like the rest of Zone 4
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