In the world of high-performance building, few topics are as critical—and as often misunderstood—as vapor control. The way moisture moves through a wall assembly has long-term implications for durability, indoor air quality, energy performance, and occupant health. The distinction between vapor-open and vapor-closed assemblies is a fundamental one that every building professional should understand, especially as we respond to increasing environmental challenges and occupant expectations.

What Is a Vapor-Open Assembly?

A vapor-open (or vapor-permeable) wall assembly is designed to allow water vapor to move freely through the building enclosure. This vapor diffusion ability is key to managing moisture that inevitably enters wall systems—whether from bulk water intrusion, air leaks, humidity, or condensation.

These assemblies are built using vapor-permeable and often hygroscopic materials like cellulose, straw, wood fiber, cork, and wool. These bio-based insulations not only allow for vapor diffusion but can also buffer and redistribute moisture, slowing its movement and allowing the assembly to dry naturally over time. This prevents mold growth and material degradation, both of which can compromise building performance and human health.

Crucially, vapor-open assemblies are designed to be airtight. Air leakage is one of the most significant contributors to moisture-related problems, as it carries large quantities of vapor into wall assemblies. An airtight building envelope ensures that thermal performance and indoor comfort are maintained while enabling moisture to diffuse and escape as needed.

Vapor-open design is particularly beneficial in mixed-humid climates or regions with distinct seasonal variations, where walls must be capable of drying to both the interior and exterior depending on the time of year.

What Is a Vapor-Closed Assembly?

A vapor-closed assembly, in contrast, restricts water vapor from passing through the enclosure. Materials such as closed-cell spray polyurethane foam, foil-faced rigid foam boards, and polyethylene vapor barriers are common examples. These systems create strong thermal and air barriers, but they prevent any drying if moisture gets behind the vapor-closed layer.

In theory, keeping vapor out sounds like a good strategy. But in practice, no wall is perfect. Moisture will enter—whether from flashing failures, wind-driven rain, air leakage, or plumbing mishaps. In a vapor-closed system, trapped moisture has no way out. This can lead to mold growth, material rot, indoor air quality problems, and ultimately, structural damage.

This issue is particularly problematic in wood-framed construction, where organic materials like framing lumber and sheathing are vulnerable to decay in the presence of moisture. Because damage in vapor-closed assemblies is often concealed, it can progress undetected until significant repairs are necessary. Additionally, these systems can be challenging and costly to inspect and remediate.

Why Moisture Management Is Essential

Building scientists widely agree that moisture intrusion is inevitable over a building's lifespan. The question is not whether water will enter an assembly, but whether the assembly has the ability to dry out safely and predictably. This principle underpins the growing emphasis on designing for drying potential, not just moisture resistance.

Drying capacity is a key resilience metric for modern construction. Assemblies that can release moisture efficiently are more likely to maintain their structural integrity and indoor air quality over time, reducing maintenance costs and extending service life.

Smart Vapor Control Layers

Advanced vapor-variable membranes, also known as "smart" vapor retarders, offer a high-performance solution to balance airtightness with drying potential. These materials change their permeability depending on the relative humidity of the surrounding environment, providing dynamic vapor control. We regularly use membranes from 475 High Performance Building Supply in both our straw panel, new construction, and retrofit work. By combining these products with bio-based insulation, builders can construct assemblies that are both robust and adaptable.

INTELLO is a smart interior vapor retarder that becomes more vapor-open under high humidity conditions. In winter, it blocks interior vapor from entering the wall. In summer or when moisture accumulates, it opens up to allow drying back to the interior.
Mento and Adhero are exterior weather-resistive barriers (WRBs) that are vapor-permeable, airtight, and water-resistant. Adhero is fully adhered, enhancing durability and eliminating fastener penetrations that could compromise water resistance.

Why This Approach Matters

Using vapor-open, airtight assemblies with smart vapor control layers leads to:

Greater durability: Assemblies dry out faster and avoid long-term degradation.
Improved indoor air quality: By reducing conditions that allow mold to grow.
Lower maintenance costs: Due to fewer moisture-related failures and longer-lasting materials.
Climate resilience: Assemblies adapt to changing humidity and weather patterns, making buildings more robust in the face of climate stress.

In short, these systems meet the triple goals of durability, health, and sustainability.

Our Approach

At Rare Forms, we integrate vapor-open, airtight construction principles into all of our projects. Our compressed straw panels provide excellent thermal insulation, carbon storage, and moisture buffering—all essential elements of high-performance enclosure design. We frequently pair these panels with vapor-variable membranes to create assemblies that are prepared for the realities of building use, weather exposure, and climate variability.

Our goal is to deliver buildings that don’t just perform well on paper, but actually hold up over time, promote occupant health, and support environmental stewardship. We believe that smarter building starts with better moisture design—and that means building vapor-open, airtight, and resilient from the start.

By embracing these principles, we can construct homes and buildings that last longer, perform better, and support both people and the planet.