Three classes by perm rating
IBC 2021 Table 1404.3(1) sorts vapor retarders into three classes by perm rating. A lower perm rating means less water vapor gets through.
- Class I: 0.1 perm or less. Sheet polyethylene, nonperforated aluminum foil.
- Class II: greater than 0.1 and up to 1.0 perm. Kraft-faced fiberglass batts, vapor retarder paint.
- Class III: greater than 1.0 and up to 10 perms. Latex paint, enamel paint.
There are only three classes. There is no Class IV.
Traps on the boundaries
The common error is swapping the Class I and Class II boundaries, reading Class I as "1.0 perm or less." Class I stops at 0.1 perm.
Read each range as "greater than the lower number, up to and including the upper number." That settles the edge cases:
- A membrane testing at exactly 1.0 perm is Class II. Class II includes 1.0; Class III starts above 1.0.
- A material testing at exactly 0.1 perm is Class I.
A material does not have to reach 0.1 perm to be classified at all. 0.1 perm or less is only the Class I range; the other two classes cover higher ratings.
Traps on the materials
- Kraft-faced batts are Class II, not Class I. Calling kraft facing Class I is a common error, and the table does list it as a classified vapor retarder.
- Latex and enamel paint are Class III. Vapor retarder paint is a special product and is Class II; ordinary latex is not.
- Polyethylene and nonperforated foil are the Class I examples.
How class is tested
The vapor retarder class definition in IBC 2021 Section 202, used by Section 1404.3, relies on ASTM E96 Procedure A, the desiccant method.
The trap: Procedure B is the other E96 procedure (the water method), but the definition names the desiccant method. Two other standards show up as distractors:
- ASTM E2178 measures the air permeance of air barrier materials, not vapor permeance.
- ASTM C887 is the standard for surface-bonding mortar used in foundation dampproofing.