Vapor Retarder Class by Climate Zone: A Practical Guide to IRC R702.7
Few envelope details cause as much confusion on US jobsites as the interior vapor retarder. Some crews still staple polyethylene behind drywall out of habit. Others leave it out everywhere because they heard poly causes mold. The International Residential Code (IRC) lands between these two. Since the 2021 edition, Section R702.7 sets out which vapor retarder classes are permitted on the interior side of frame walls in each IECC climate zone, and in some zones it prohibits the low-perm classes outright.
Vapor retarder classes: what the perm numbers mean
The IRC and the International Building Code (IBC) group vapor retarders into three classes by water vapor permeance, measured in perms. The ratings are conventionally based on the ASTM E96 dry-cup (desiccant) method:
- Class I: 0.1 perm or less. Sheet polyethylene and unperforated aluminum foil.
- Class II: more than 0.1 and up to 1.0 perm. Kraft-faced fiberglass batts and vapor retarder paints.
- Class III: more than 1.0 and up to 10 perms. Latex or enamel paint.
Painted drywall is not automatically Class III. Ordinary drywall with latex paint can test well above 10 perms, so if your design relies on a Class III layer, use a product rated at 10 perms or less. Kraft facing is also more complicated than its class suggests: its permeance rises a long way as relative humidity goes up. Smart retarders are designed to behave this way.
What the 2021 IRC requires by climate zone
Under the 2021 IRC, a Class I or II vapor retarder is required on the interior side of frame walls in climate zones 5, 6, 7, 8 and Marine 4 (4C). Basement walls, the below-grade portion of any wall, and construction where moisture or freezing will not damage the materials are exempt. The IBC carries parallel provisions for commercial buildings in Section 1404.3.
The bigger change in 2021 was a vapor retarder options table, Table R702.7(2), which sets which classes may be used in which zones:
- Zones 1 and 2: Class I and Class II are not permitted. Class III is permitted.
- Zones 3 and 4 (except Marine 4): Class I is not permitted. Class II and Class III are permitted. A Class II retarder used with exterior foam plastic sheathing must meet Table R702.7(4) and test above 1 perm by ASTM E96 Procedure B (the wet-cup water method).
- Marine 4 and zones 5 to 8: Class I and Class II are permitted. Class III is permitted only under the conditions in Table R702.7(3). If a Class I interior retarder is paired with a Class I exterior layer, the design must be approved.
A key footnote permits Class I and II vapor retarders that measure above 1 perm under ASTM E96 Procedure B on the interior of any frame wall in any climate zone. This is how the code recognizes responsive ("smart") retarders.
Class III with exterior continuous insulation
Exterior continuous insulation (ci) warms the sheathing, which lowers the risk of condensation in the wall. Table R702.7(3) lets you use a Class III interior retarder in colder zones when there is enough exterior ci:
| Climate zone | Class III permitted with ci over 2x4 wall | Class III permitted with ci over 2x6 wall |
|---|---|---|
| Marine 4 | R-2.5 or more | R-3.75 or more |
| 5 | R-5 or more | R-7.5 or more |
| 6 | R-7.5 or more | R-11.25 or more |
| 7 | R-10 or more | R-15 or more |
| 8 | R-12.5 or more | R-20 or more |
These values only cover moisture control. They count toward the Chapter 11 energy requirements but do not replace them, so the thermal envelope still has to meet the energy code. Check which code edition and local amendments apply to your project. Many jurisdictions are still on earlier editions, and the IRC itself has been updated since 2021.
Quick reference by climate zone group
| Zone group | Typical climate | Interior vapor retarder (2021 IRC frame walls) | Practical notes |
|---|---|---|---|
| 1, 2 | Hot-humid and hot-dry South | Class III only; Class I and II not permitted unless above 1 perm by Procedure B | Avoid interior poly and vinyl wallcovering; let walls dry inward |
| 3, 4 (not Marine 4) | Mixed-humid, warm-marine, mixed-dry | Class II or III; Class I not permitted | Vapor drive reverses with the seasons; variable-perm retarders suit these zones well |
| Marine 4, 5 | Pacific Northwest coast (Marine 4), cool (5) | Class I or II required, or Class III with ci per Table R702.7(3) | Class II or responsive retarders are usually safer than poly with air conditioning |
| 6, 7, 8 | Cold and very cold | Class I or II required, or Class III with ci per Table R702.7(3) | Winter outward drive dominates; airtightness is critical |
The hot-humid South: why interior poly and vinyl cause trouble
In zones 1 to 3, especially humid areas along the Gulf Coast and in the Southeast, the main vapor drive for much of the year runs from the hot, humid outdoors toward the cool, air-conditioned interior. A low-perm layer on the room side traps that moisture inside the wall instead of letting it dry inward. Building Science Corporation has linked vinyl wallcoverings on the inside of air-conditioned buildings to mold, and hotels are the classic case. Vinyl wallcovering acts as a vapor barrier, and when wall cavities are depressurized and connected to outdoor air, hot, humid air is pulled in and condenses behind it.
Why smart vapor retarders suit mixed and humid climates
In mixed climates, vapor drive changes direction with the seasons. It runs outward in winter when the house is heated, and inward in summer when it is air-conditioned. A fixed-perm Class I layer guards against the winter case but blocks summer drying. A variable-permeance ("smart") retarder has low permeance when the air around it is dry, which is typical of heating-season conditions on the warm side of the insulation. Its permeance rises when relative humidity is high, so the assembly can dry toward the interior in summer or after a wetting event.
Airtightness matters more than diffusion
Air leaks carry far more moisture than vapor diffusion does. In most wet walls and ceilings, the cause is warm, moist air leaking through gaps at outlets, top plates and penetrations, not vapor diffusing through the drywall. Get the continuous air barrier right first, then choose the vapor retarder class to suit the climate. Our guide to continuous air barrier requirements in US energy codes covers the air side in detail, and Passive House projects take airtightness further still, as we explain in Passive House in the US: PHI vs Phius.
Where Passive Purple fits
Passive Purple is a fluid-applied, water-based airtight and variable vapor control membrane. It is tested as an air barrier material at 0.02 L/s·m² at 75 Pa (ASTM E2178) and is Passive House Institute certified. You can spray or roll it onto drywall, CMU, concrete, brick and steel, and paint over it once it has cured. For wood or OSB substrates, check fire and finish requirements for your project with us and your code official. Its vapor resistance varies with humidity, with an Sd value of about 0.8 to 40 m (EN ISO 12572).
The usual approximation for converting Sd to US perms is perms ≈ 3.3 ÷ Sd (in meters). That puts Passive Purple at about 0.08 perms (3.3 ÷ 40) at its tightest, in dry conditions, and about 4 perms (3.3 ÷ 0.8) at its most open, in humid conditions. In other words, it falls in the Class I range when dry and in the Class III range when humid. Applied on the warm side of the insulation, it limits outward diffusion during the heating season, when the risk is greatest, and opens up to allow drying when humidity rises.
These figures are approximate. European Sd testing (EN ISO 12572) and ASTM E96 use different test conditions, and US code classification, including whether a product counts as a responsive retarder under the Procedure B footnote, depends on ASTM E96 data and the authority having jurisdiction. Moisture behavior also depends on the whole wall: cladding, sheathing, exterior ci, interior humidity and local climate. For anything beyond a routine assembly, and in any hot-humid project, we recommend a project-specific hygrothermal analysis (for example, WUFI). Intelligent Membranes offers a condensation risk analysis service to help with this.
Get help with your assembly
If you are detailing walls or roofs for a specific climate zone, our team can review the assembly, provide CSI MasterFormat specifications for Passive Purple, and support substitution requests. For joints, penetrations and cracks, use Passive Purple Brush alongside the main membrane. To request a condensation risk analysis or ask a technical question, contact us or call +1 (347) 220 8403.