As building codes continue to evolve and structures become more energy efficient, understanding the role of each building envelope component has never been more important. Two of the most commonly discussed systems are air barriers and weather-resistive barriers (WRBs). Although these terms are sometimes used interchangeably, they serve different purposes within a wall assembly.
Knowing the difference helps builders, architects, and contractors design wall systems that manage both moisture and air movement more effectively.
What Is a Weather-Resistive Barrier?
A weather-resistive barrier is designed to protect the wall assembly from liquid water that gets behind the exterior cladding. Wind-driven rain, snow, and incidental moisture can all find their way past siding, brick, or other exterior finishes.
A properly installed WRB acts as the wall’s secondary line of defense by directing water downward toward flashing and drainage pathways instead of allowing it to reach the structural components of the building.
Many modern WRBs are also vapor permeable, allowing moisture vapor within the wall assembly to escape while still resisting liquid water intrusion.
What Is an Air Barrier?
An air barrier has a different job. Its primary purpose is to reduce uncontrolled air movement into and out of the building.
Air leakage affects more than energy efficiency. Moving air can also transport moisture into wall cavities where condensation may occur under the right conditions.
By reducing uncontrolled air movement, an effective air barrier helps improve occupant comfort, supports HVAC performance, and contributes to the overall durability of the building envelope.
Can One Product Perform Both Functions?
In many wall assemblies, a single product may contribute to more than one control layer when properly designed and installed. Some weather-resistive barriers also provide air barrier performance as part of a complete wall system.
However, performance depends on the product itself, the surrounding components, and proper installation. Every seam, penetration, transition, and flashing detail must work together to maintain continuity throughout the building envelope.
Why Drainage Still Matters
While controlling water and air are both important, wall assemblies should also provide a way for incidental moisture to drain away from the structure.
Drainable weather-resistive barriers help create a pathway for bulk water to escape behind exterior cladding, reducing the opportunity for moisture to remain trapped within the wall assembly.
This drainage capability becomes increasingly valuable as modern buildings are designed to be tighter and more energy efficient.
Where ALTA® Fits
ALTA® 360 is engineered as a drainable weather-resistive barrier that helps manage liquid water while supporting the overall moisture management strategy of the building envelope. Its integrated drainage layer promotes water evacuation behind cladding while the vapor-permeable membrane helps support drying potential within the wall assembly.
When incorporated into a properly detailed building envelope, ALTA contributes to a system where water management, drainage, and drying work together to improve long-term building performance.
Looking Ahead
Understanding the difference between air barriers and weather-resistive barriers is an important step toward designing more resilient wall assemblies. While each performs a distinct role, they are most effective when integrated into a continuous building envelope where every control layer works together.
In Part 3 of our Building Envelope Basics series, we’ll explore why drainage planes matter and how they help protect wall assemblies from long-term moisture damage.