In building science, vapor control is both essential and misunderstood. Where rigid Class I vapor barriers once ruled the conversation, today’s energy-efficient enclosures demand more nuance. Enter the humidity-dependent vapor retarder, often referred to as a smart vapor retarder. These materials adapt to changing environmental conditions, becoming more vapor-open when drying is needed and more closed when protection is essential. But how do we know they actually work as promised? That’s where ICC-ES AC528 steps in.

In the International Residential Code (IRC) and International Building Code (IBC), the term “responsive vapor retarder” is in our view very loosely defined to describe materials that change permeability in response to humidity levels. This term lacks clarity and scientific rigor. It categorizes products that are not very smart, responsive or helpful for the insulated building envelope. This can and will lead to failures as these products only respond minimally or not at all to environmental conditions. None the less, the IRC2024/IBC 2024 codes rely on them to do so in section IRC section R702.7 vapor retarders (and in chapter 2: definitions) and IBC section 1404.3 Vapor retarders:

“RESPONSIVE VAPOR RETARDER - A vapor retarder material complying with a vapor retarder class of Class I or Class II but which also has a vapor permeance of 1 perm or greater in accordance with ASTM E96, water method (Procedure B).”

Smart vapor retarders are only as trustworthy as their tested performance and AC528 is the gold standard. At 475, we’re proud to carry the first products to meet this benchmark: INTELLO PLUS and INTELLO X. When you're specifying or installing vapor control layers, don't settle for assumptions—demand AC528 compliance.

What Is AC528?

AC528 is the Acceptance Criteria for Humidity-Dependent Vapor Retarders, developed by the ICC Evaluation Service (ICC-ES), the technical arm of the International Code Council (ICC). It provides a comprehensive, standardized framework for evaluating whether a smart vapor retarder performs as claimed under varying humidity levels and complies with key sections of the IBC, IRC, and IECC.

This isn’t just a lab standard. It’s a pathway to code compliance, offering credibility and consistency for specifiers, code officials, and high-performance builders alike.

What Does AC528 Test For?

AC528 outlines both material properties and performance criteria that smart vapor retarders must meet, including:

INTELLO PLUS & INTELLO X: Certified to AC528

INTELLO PLUS and INTELLO X, manufactured by Pro Clima and distributed in North America by 475 High Performance Building Supply, are fully evaluated under AC528 and carry the ESR-4854 report confirming compliance.

In contrast to responsive vapor retarders, a humidity-dependent vapor retarder, INTELLO PLUS and INTELLO X which are meticulously defined, independently tested and quality controlled smart vapor retarders that have been evaluated in ICC-ESR 4854 to AC528 (Acceptance Criteria for Humidity-Dependent Vapor Retarders). The test and quality controlled production of these Pro Clima world leading smart membranes will change its vapor permeability based on relative humidity as intended. This means architects, facade engineers, builders, contractors, prefab manufacturers can safely rely on them in design, hygrothemal modelling and in long term performance in building envelopes —INTELLO will tighten when relatively dry to block vapor migration outward into the assembly, and can open up when needed to allow inward drying. This dual behavior enables assemblies to dry safely and remain protected year-round.

Highlights from ESR-4854:

• Humidity-Dependent sd-values (INTELLO PLUS):
  • 25% RH: 42.3 m (very vapor closed) –0.07 US perms
  • 71.5% RH: 1.84 m - 1.78 US Perms conforming to the DIN-68800 HYDROSAFE® Rule
  • 90% RH: 0.43 m (very vapor open) - 7.6 US Perms
• Air Barrier system: <0.008 cfm/ft2 at 75Pa per ASTM E2357 - that's 5x tighter than 0.04 cfm/ft2 required for a airbarrier system
• Fire behavior: Flame spread index <25, smoke developed <450

These metrics confirm that INTELLO PLUS and INTELLO X offers low vapor permeability when exposed to normal interior winter conditions up to 70% RH (for winter) and high drying potential when humidity could increase (critical for summer to allow for inward drying)—precisely the kind of intelligent vapor control modern assemblies need. Especially when code, architects and clients are increasing the insulation values and airtightness in buildings - highly insulated assemblies need more drying reserves!

Why This Matters for Building Professionals

1. Code Compliance

With AC528 compliance and an ICC-ES Evaluation Report (ESR), smart membranes like INTELLO Plus and INTELLO X can be confidently specified for projects and approved by code officials.

2. Risk Reduction

Especially in assemblies like:

• Unvented walls and roofs
• Assemblies that have been exposed to moisture during the construction phase that will need to dry out
• Below grade walls or basements that can only dry to the interior
• In climate zones with seasonal vapor drive reversals
  • Outward direction in winter when building is heated
  • Inward direction in summer (hot/humid outside - cooled air that is relatively dry inside) 

Vapor variable retarders, especially ones that have a humidity curve confirmed that is humidity dependent and validated by ICC ESR reduce condensation risk while still allowing assemblies to dry.

3. Accurate Modeling

The use of ISO 12572 sd-values allows hygrothermal tools like WUFI to model these materials accurately—far superior to generic Class III assumptions.

The building code’s broad use of the term “responsive vapor retarder” blurs the line between inconsistent, underperforming products and truly high-performance smart membranes. While INTELLO PLUS and INTELLO X technically meet the definition of a responsive vapor retarder, it’s important to recognize that they operate on a much higher standard. As fully tested, AC528-compliant materials, they are in a league of their own. We urge the industry to adopt AC528 as the benchmark—because not all so-called “smart” vapor retarders are created equal. And when your building envelope depends on this functionality, performance must be proven—not assumed.

Get In Touch

Want help designing or modeling your wall and roof assemblies with smart vapor control? Get in touch with our team. We’re here to support every layer of your enclosure.