The climate-zone table
IBC 2021 Table 1404.3(2) says which vapor retarder classes may go on the interior side of frame walls, by climate zone.
- Zones 1 and 2: Class III only. Class I and Class II are not permitted.
- Zone 3: Class II and Class III. Class I is not permitted.
- Zone 4 except Marine: Class II permitted, Class III allowed per Table 1404.3(3), Class I not permitted.
- Marine 4 and Zones 5 to 8: Class I and Class II permitted, Class III allowed per Table 1404.3(3).
The warmer the zone, the more permeable the retarder must be.
The polyethylene trap
Specifying sheet polyethylene (Class I) in warm zones is the common error. "Class I is the most restrictive, so it is always acceptable" is wrong: the table prohibits it outside Marine 4 and Zones 5 to 8.
Scenario: a spec calls for 6-mil sheet polyethylene as the interior vapor retarder in frame walls in Zone 3. Polyethylene is Class I under Table 1404.3(1), and Zone 3 permits only Class II or Class III. It is not permitted. Adding kraft-faced batts (Class II) behind it does not cure a prohibited Class I layer.
Zone 4 and the "required" trap
No zone requires Class I. Class I is permitted in Marine 4 and Zones 5 to 8; it is never mandatory, and non-marine Zone 4 does not permit it at all.
Zone 4 (not Marine) permits Class II, allows Class III per Table 1404.3(3), and does not permit Class I. Don't confuse it with its neighbours:
- "Class I and Class II permitted" is the Marine 4 and Zones 5 to 8 row.
- "Only Class III" is the Zones 1 and 2 row.
- "Class II only" misses that Zone 4 also allows Class III per Table 1404.3(3).