When Water Hits the Wall: Why CMU Walls Need Air and Water-Resistive Barriers

Concrete masonry unit (CMU) walls remain a trusted choice in commercial and institutional construction because of their fire resistance, durability, and structural performance. Schools, hospitals, universities, and civic buildings continue to rely on CMU assemblies to meet long-term performance goals.

While CMU walls are resilient, they are also porous. Without a continuous air and water-resistive barrier (WRB), assemblies are vulnerable to moisture intrusion, air leakage, and reduced energy efficiency. Mortar joints, surface voids, and embedded masonry ties can allow water and air to pass through the wall, leading to condensation, corrosion, and thermal loss over time.

For background on how air leakage affects building comfort and energy performance, see the U.S. Department of Energy’s Energy Saver resource on air sealing.

Why CMU Walls Still Need Membrane Protection

Although CMU performs better than framed construction under wet conditions, moisture can still migrate through block and mortar joints. In humid or cold climates, this movement can cause freeze-thaw damage, deteriorate embedded metals, and reduce the service life of finishes and adjacent materials.

Early damp-proofing systems offered minimal water resistance but did little to control air movement. Without a dedicated air and water-resistive layer, the porous structure of CMU allows air and water to travel through the wall assembly, compromising thermal performance and interior comfort.

The Building Science Corporation’s Understanding Air Barriers (BSD-104) confirms that typical CMU wall construction is highly permeable to air. There a properly detailed and continuous membrane is essential for long-term durability and performance.

The Case for Exterior Liquid-Applied Membranes

For CMU walls with drained exterior cladding systems, the most effective strategy requires applying an air and water-resistive barrier to the exterior face of the CMU wall. This placement intercepts bulk water, reduces air leakage, and simplifies tie-ins with roofing and foundation systems.

Because CMU surfaces are rough and irregular, liquid-applied membranes perform better than sheet-applied systems. Fluid-applied products create seamless coverage, adhere well to textured masonry, and provide reliable continuity around penetrations, control joints, and embedded ties.

When applied to the CMU’s exterior, liquid-applied barriers make the wall assembly more airtight, prevent moisture from penetrating the block, and protect any exterior insulation installed over the membrane. In addition, these systems are tested to meet key performance standards such as ASTM E2178 / E283 for air leakage and ASTM E331 for water intrusion resistance.

Benefits of Fully Adhered Liquid-Applied Systems

Liquid-applied air and water barriers form a seamless membrane that eliminates fasteners and mechanical laps. When used with compatible flashings and sealants, they create continuous protection even at complex details such as window openings, control joints, and transitions.

Fully adhered systems help buildings meet air leakage targets, lower long-term maintenance costs, and extend the life of both the cladding and the structure beneath. They also improve occupant comfort by stabilizing indoor temperatures and reducing drafts. For CMU walls, the flexibility and crack-bridging properties of liquid-applied systems make them particularly effective where movement and minor substrate variations occur.

Building Beyond Code Minimums

While the International Building Code (IBC) does not explicitly require an air or water-resistive barrier for CMU walls with drained cladding, incorporating a liquid-applied membrane delivers measurable benefits that extend beyond what minimum compliance offers.

For example, installing a liquid-applied air and water-resistive barrier layer provides:

• Greater building envelope continuity to protect against uncontrolled air and moisture movement.
• Reduced condensation risk within CMU cavities and insulation layers.
• Improved occupant comfort through better temperature regulation and humidity control.
• Lower maintenance and lifecycle costs by preventing premature material degradation.
• Enhanced energy efficiency that supports compliance with local energy codes and sustainability goals.

For more on air- and water-barrier requirements across the U.S., visit the U.S. Department of Energy’s Energy Code Adoption Map.

Henry® Barritech®: Proven Liquid-Applied Systems for CMU

Henry® offers a family of Barritech® liquid-applied air and water-resistive barriers engineered for the demands of CMU construction. The Barritech line provides monolithic, fully adhered protection that meets ASTM requirements for both air leakage and water resistance.

Barritech® VP is a vapor-permeable, one-component membrane that allows CMU walls to dry while resisting liquid water and air infiltration. It is ideal for above-grade walls where balanced vapor control is desired. For environments requiring higher moisture and vapor resistance, Barritech® NP offers a non-permeable, fire-retardant formulation that serves as an air, water, and vapor barrier in a single product. It is also ideal for projects with wall assemblies requiring NFPA 285 standards.

Both membranes adhere directly to CMU, forming a continuous barrier that ties easily into adjacent components using compatible Henry® products such as Air-Bloc® LF and Blueskin® SA Flashing. When properly installed, these systems provide reliable protection from foundation to roofline.

Air-Bloc®: Alternate Options for CMU Applications

In addition to the Barritech line, Henry® offers Air-Bloc® liquid-applied membranes that deliver comparable performance and flexibility across a range of project conditions. Air-Bloc® 16MR is a water-based, vapor-permeable barrier for vertical wall assemblies, while Air-Bloc® 17MR provides a non-permeable, fire-resistant option for CMU and concrete where vapor control is critical. Air-Bloc® All Weather STPE offers strong adhesion and flexibility for cold-weather or damp applications.

These products share the same installation advantages as Barritech membranes, creating a smooth, monolithic layer that improves constructability and long-term wall performance.

Installation Best Practices for Liquid-Applied Membranes

Proper application is essential to long-term performance. Following these best practices ensures strong adhesion and consistent coverage:

• Prepare the substrate. CMU must be clean, sound, and free of dust, efflorescence, or standing water.
• Apply at correct thickness. Use wet-mil gauges to verify the specified film build for each product.
• Detail transitions carefully. Seal joints, corners, and penetrations with compatible Henry® materials such as Air-Bloc® LF or Blueskin® SA Flashing.
• Tie into adjacent systems. Maintain continuous protection from below grade through roof level.
• Inspect the work. Perform visual and field verification to confirm full coverage with no pinholes or voids.

The Air Barrier Association of America (ABAA) provides guidance on field quality control and air barrier testing to help ensure that installed systems meet specification and performance standards.

Learn More: Specifying Complete Henry® Building Envelope Systems

For deeper insights into selecting and integrating liquid-applied air and water-resistive barriers, explore Henry’s free, AIA-accredited course: Specifying Total Envelope Solutions in a Low-Bid World. The course discusses ways to protect specifications and improve project outcomes across all phases of design and construction.