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.

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 Validate?

AC528 outlines both material properties and performance criteria that smart vapor retarders must meet.

INTELLO PLUS and INTELLO X have been evaluate for code compliance to this AC with ICC-ESR 4854.

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.

Highlights from ESR-4854:

• Humidity-Dependent sd-values (INTELLO PLUS): at 70% RH complies with ETA/HYDROSAFE ® rule
• Class II Vapor Retarder at standard conditions (ASTM E96-A)
• Air Barrier System: <0.008 cfm/ft2 ASTM E2357 (so 5x tighter than 0.04 cfm/ft2 required for a airbarrier system
• Fire Behavior: Flame spread index <25, smoke developed <450 - Class A material per ASTM E84

These metrics confirm that INTELLO PLUS and INTELLO X offer 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! Furthermore, the ETA (European Technical Approval) has also validated the vapor permeance curve is durable after age testing, confirming the long-term durability of ProClima's smart vapor control membranes.

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:

• Walls with vapor retarding elements on exterior (foam, OSB, ZIP) and unvented roofs (flatroofs, asphalt singles etc)
  
• Projects that have build when damp, ie have construction moisture that should dry out
• Below grade walls or basements that can only dry to the interior
• Climate zones (mixed and temperate climates) 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, especially when airconditioning space)

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.

Responsive Vapor Retarder

In the International Residential Code (IRC) and International Building Code (IBC), the term “responsive vapor retarder” is in our view very “losely” defined to describe materials that change permeability in response to humidity levels. However, this term lacks clarity and scientific rigor. It categorizes products that are not very smart, responsive or helpful for the insulated building envelope. Nor are such products third party verified for their specific behavior or even produced for this purpose by manufacturers. This can and will lead to failures as these products as they may 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).”

In contrast, a humidity-dependent vapor retarder, INTELLO PLUS and INTELLO X which are defined, independently tested and quality controlled smart vapor retarders that have been evaluated in ICC-ESR4854 to AC 528 (Acceptance Criteria for Humidity-Dependent Vapor Retarders).  The test and quality controlled production of these ProClima 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 tightening when relatively dry to block vapor migration outward (into the insulation), and can open up when needed (when humid to allow inward drying. This dual behavior enables assemblies to dry safely and remain protected year-round.

The building code’s casual use of “responsive” risks lumping together underperforming or inconsistent products with high-performance smart membranes. We do note that INTELLO PLUS, INTELLO X and DB+ do qualify as "responsive vapor retarders” as well - but since they are in a class of their own. We urge the industry to acknowledge why AC 528-compliant materials should be the standard—because not all “smart” vapor retarders are created equal - but if your envelope relies on them - they need to manufactured and perform intended!

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.