I’ve spent most of my career saying yes. By that, I mean when an opportunity arose—even if I wasn’t entirely sure I could handle it—I said yes first and figured it out later. That mindset has taken me to career heights I never anticipated. Most recently, my Stucco Stop column editor asked if I could do a webinar on water-resistive barriers. My answer? Yes.

Having worked in the field as a lather and plasterer and served as a technical advisor, I knew a thing or two about WRBs. Initially, I thought I could cover the topic in under ten minutes; after all, WRBs seemed simple—they resist water. But after research and a stubborn refusal to fail, I put together a full presentation, which I later adapted for this column. So, if you already know everything about WRBs, you can stop here. But if you enjoy diving into the details of an often-overlooked exterior cladding component, read on. 

Understanding Water

Water exists as a solid, liquid, or gas. For our purposes, we’ll focus on liquid and vapor forms and their impact on framed wall construction. Water naturally follows the path of least resistance. In building construction, if you don’t guide it, water will eventually penetrate your structure. That’s where the WRB comes in—it directs water away from your walls and protects the building envelope. 

What Is a Water-Resistive Barrier?

A WRB is a thin membrane designed to resist liquid water that has penetrated behind the exterior cladding. It protects the wall assembly and allows it to dry, acting as a critical layer in framed wall construction. Dating back to the early 1800s, WRBs were originally asphalt-impregnated felts, later standardized under ASTM D226. 

Photos: WWCCA

WRBs fall into three categories: sheet goods, liquid-applied, or integrated systems. Their effectiveness depends on proper installation with complementary materials, such as flashings, typically applied in a “shingle” fashion to direct water downward and away from the structure.

Even the best WRB isn’t a guarantee against water intrusion. Moisture can enter from outside—through wind-driven rain, cladding failures, or leaky windows—or from inside, through everyday activities like cooking and breathing. Proper ventilation, whether passive or mechanical, complements the WRB’s function. Think of WRBs as one line of defense in a holistic moisture management strategy. 

WRBs in Building Codes

The International Building Code and International Residential Code define when and how WRBs must be used. 

• IBC Chapter 14, Section 1402.2: Requires a weather-resistant exterior to keep the elements out and maintain indoor comfort. WRBs are referenced as part of this protection, guiding water that enters the wall assembly to an exit point. 
• IBC Section 1403.2: Mandates at least one continuous WRB layer from top to bottom, integrated with flashings at openings. 
• IBC Section 1404.4: Specifies that flashings must work with WRBs to direct water away from critical wall components. Flashings can be metal, vinyl, plastic, self-adhered membranes, or liquid-applied products. 

Climate zones further affect WRB installation requirements: 

• Zone B (Dry): Two layers of 10-minute grade D paper or equivalent, or a single layer combined with a non-water-absorbing barrier. 
• Zones A & C (Moist/Marine): Same as Zone B but must include a drainage space ≥ 3/16 inch with a minimum 90 percent drainage efficiency.

The IRC mirrors these requirements, with exceptions for masonry, concrete walls, and small detached structures like tool sheds. 

ASTM Standards

WRBs are referenced in several ASTM standards, including: 

• D226 – Defines asphalt-impregnated sheets. 
• E2556 – Specifies vapor permeability for mechanically attached sheet goods. 
• E1354 – Fire testing for combustible exterior wall materials. 
• C926/C1063 – Stucco application and lathing requirements. 

Integrated WRB sheathing products combine structure, insulation, and water resistance into a single product. However, in cases requiring two layers, an additional WRB layer must still be installed. 

Key Takeaways

• Cement and concrete walls do not require WRBs. 
• Flashings must integrate with WRBs and direct water to a drainage plane. 
• WRBs perform best installed vertically in a single plane, terminating at a drainage point. 
• WRBs are water-resistive; wall assemblies are weather-resistant. 
• Two WRB layers are required for framed walls. 
• No water intrusion testing is mandated by code. 
• Climate zones dictate drainage requirements. 

I want to thank W&C for giving me the opportunity to say “yes.” The process of preparing this presentation and writing this column reinforced how much detail goes into proper WRB design and installation. Saying yes once again opened the door to new knowledge and experience.