When people think about protecting a building from moisture, they often focus on keeping water out. While that’s certainly important, an equally important question is: What happens if moisture gets inside the wall? No building envelope is completely immune to moisture. Construction moisture, humidity, small air leaks, and seasonal environmental changes can all introduce water vapor into a wall assembly. The goal isn’t simply to block moisture—it’s to manage it. That’s where vapor permeability becomes an essential part of building envelope design.
Understanding Water Vapor
Unlike rainwater, water vapor is invisible. It naturally moves through building materials as temperature and humidity change between the interior and exterior of a building. Under the right conditions, this vapor can condense into liquid water when it encounters a cooler surface inside the wall assembly. If that moisture cannot dry, problems such as material deterioration, reduced insulation performance, and mold growth may develop over time.
Why Drying Matters
Modern wall systems are designed with the understanding that some moisture will eventually enter the assembly. Instead of trying to create a completely sealed wall, building scientists focus on creating assemblies that resist bulk water intrusion, manage air movement, control vapor movement, and dry efficiently when moisture is present. Allowing a wall to dry helps reduce long-term moisture accumulation and contributes to a healthier, more durable building envelope.
What Does “Vapor Permeable” Mean?
A vapor-permeable material allows water vapor to pass through while continuing to resist liquid water. Think of it like a high-performance outdoor jacket—rain stays outside, but moisture generated inside can escape. Within a wall assembly, vapor-permeable weather-resistive barriers help support drying while still protecting the structure from rain and wind-driven moisture.
Climate Matters
Vapor movement doesn’t always occur in the same direction. In colder climates, vapor often moves from the warm interior toward the colder exterior during winter. In hot, humid climates, the opposite can occur. Because environmental conditions vary throughout the year, many wall assemblies benefit from materials that allow drying in multiple directions rather than permanently trapping moisture. Selecting products with the appropriate vapor permeability is an important part of designing a resilient wall system.
Where ALTA® Fits
ALTA® 360 is engineered as a vapor-permeable, drainable weather-resistive barrier that supports the overall moisture management strategy of the building envelope. While it helps resist liquid water from the exterior, its vapor-permeable membrane allows moisture vapor within the wall assembly to escape, supporting the wall’s natural drying process. Combined with its integrated drainage layer, ALTA 360 contributes to a wall system designed to effectively manage incidental moisture. Rather than relying on a single layer to solve every challenge, ALTA is designed to work alongside flashing, insulation, air control layers, and exterior cladding as part of a complete building envelope system.
Moisture Management Is About Balance
Successful building envelopes aren’t designed to simply block moisture—they’re designed to manage it. That means controlling rain, limiting uncontrolled air movement, directing water toward drainage pathways, and allowing the wall assembly to dry when necessary. Understanding vapor permeability is one of the keys to creating durable, resilient wall systems that continue to perform for years to come.
Looking Ahead
In Part 5 of our Building Envelope Basics series, we’ll explore why flashing is one of the most critical—and most overlooked—components of the building envelope, and how proper detailing around windows, doors, and transitions helps keep water where it belongs: outside the wall assembly.