The incorporation of barriers and flashing to exterior wall assemblies can significantly reduce moisture intrusion.

Beyond apparent aesthetic features, a building’s exterior is primarily a weather barrier, where cladding serves as the first line of defense in protecting occupants from the outdoor elements. Unfortunately, no matter how tightly the cladding is sealed, moisture often finds its way through small crevices and gaps. This moisture can cause a range of damaging problems affecting the durability, indoor air quality and thermal efficiency of a building.

In light-frame construction, wood exterior wall sheathing, typically plywood or oriented strand board, is an integral part of the wood-frame structural system. It provides lateral strength against wind, rain and other forces, and its surface supports the exterior cladding. Moisture trapped between the exterior cladding and sheathing can cause sheathing to deteriorate.

Because of this, building professionals should employ a systematic approach to moisture management to prevent water from reaching the sheathing or framing. Primary water management strategies should include water-deflecting components such as overhangs, eaves and durable exterior claddings. Equally important is the inclusion of an integrated weather resistant barrier and flashing system, which is used to help protect structures from air and moisture intrusion.

The integration of these two measures creates a drainage plane behind the cladding that is crucial to the long-term success of the wall system. When water penetrates the exterior finish, the weather barrier works in conjunction with integrated flashing to guide water back to the exterior. This helps keep framing dry and prevents the build-up of moisture in exterior wall cavities.

Studies show that moisture damage plays a significant role in the premature deterioration of buildings. Fortunately, moisture problems can be significantly reduced through proper design and construction of exterior wall assemblies. This article will review three key elements of such assemblies: weather resistant barriers, air barriers and flashing.


Across the United States, concerns about moisture management have led to increased acceptance of weather resistant barriers in construction. Commonly referred to as housewrap or water-resistive barriers, these thin, spun-bonded fabric materials are a crucial component of the building’s exterior wall system.

Designed to prevent air and water from entering the stud wall cavity from the outside, housewrap is installed over the sheathing and behind the exterior cladding to act as a secondary barrier of defense for a home or building. For example, if wood framing or sheathing gets wet from wind-driven rain and is unable to dry out, it may rot, resulting in structural damage. Extended moisture exposure can also lead to mold growth, which can cause degradation of indoor air quality.

Not to be overlooked, normal daily activities inside a building, such as cooking, washing clothes and dishes, and even occupants’ breathing, produce moisture vapor that can cause problems if trapped inside the wall cavity. In response, many of today’s housewrap products are designed to permit water vapor to pass freely through the wall cavity to the outside of the building.


A number of housewraps today are also classified as air barriers in accordance with ASTM E2178, the standard test method for air permeance of building materials. Housewrap materials that qualify as an air barrier must be installed in strict accordance with the manufacturer’s instructions in order to ensure top performance. A properly installed housewrap system that includes sufficient flashing and sealing around fenestrations will vastly improve the overall thermal efficiency and performance of the building’s wall system.

Using a home as an example, the average 2,500-square-foot house can have more than a half mile of cracks and crevices that are open to wind and wind-driven rain. As an air barrier, housewrap systems reduce wind washing against sheathings and air infiltration into stud wall cavities. As part of a continuous barrier assembly, a housewrap can reduce drafts, increase comfort and reduce energy use by decreasing the amount of non-conditioned air entering and exiting conditioned wall cavities.

The use of housewrap also contributes to the requirements for the Energy Star Qualified New Home program as an important component in the air sealing and insulating category. This program can reduce heating and cooling costs by as much as 20 percent. Various housewrap products can also contribute to the U.S. Green Building Council’s LEED certification points in both LEED – New Construction (commercial) and the pilot program LEED for Homes (residential).


It is extremely important to prevent uncontrolled moisture from entering the building envelope through window and door openings, seams, footings, roofs or other openings. Flashing is a critical component of any construction project, as it directs water away from the opening to the weather resistant barrier, which in turn directs the water to the exit point in the wall.

This is important because head, jambs and sills around doors and windows can be a potential weak point in the wall for air and moisture. Improper installation of flashing, however, can result in water leakage, mold, warping and eventually structural damage, requiring costly repairs.

Compatibility of flashing materials with the rest of the assembly components should also be taken into serious consideration. For example, sealants with high solvent or plasticizer content can damage bitumen flashing products. Also, some rubberized asphalt products are not compatible with single-ply flexible PVC roofing materials. It’s a good idea to first check with the manufacturer to verify that the product is appropriate and compatible with all other materials used in the wall assembly. Another option is to simply work with manufacturers that provide a complete system of compatible materials.


Attention to detail when designing and/or constructing a wall assembly is the ounce of prevention that can help protect against damaging moisture, whether it originates inside or outside of the building. A water-resistive system that includes proper flashing and sealing around fenestrations help improve building durability, indoor air quality and decrease maintenance costs by reducing the risk of moisture intrusion. In addition, by acting as an air barrier that prevents hot and cold air movement through the wall cavity, weather resistant barriers also help reduce utility costs and increase comfort. W&C

Sidebar: How an Integrated Weather Barrier and Flashing System Can Reduce the Risk of Mold

In recent years, mold-related insurance claims in homes, schools and hospitals have heightened public perception about the consequences of mold and other moisture-related problems.

Today, many builders and contractors believe addressing moisture management through product specification and smart building processes can drastically reduce potential moisture problems. With this in mind, the Partnership for Advancing Technology in Housing building and manufacturing partners collaborated with lending and real estate consulting firm, Environmental Assurance Group, to construct the first-ever Mold Safe Model Home. The home was designed to highlight various techniques and products that will help reduce the potential for mold growth in new construction.

The 3,000-square-foot, two-story model home in Chesterfield, N.H, is a showcase for builders, architects, planners, manufacturers and consumers, all with a stake in the prevention of mold growth. It was vital that materials specified for the project be strong, moisture-resistant, energy-efficient products that were able to endure extreme weather conditions and protect the home against moisture intrusion from both inside and outside.

Given that the model home location is lakeside, the structure is subject to heavy amounts of moisture along with a wide seasonal humidity range. Knowing this, choosing the right weather-resistant barrier and flashing system was critical to the effectiveness of the structure’s overall performance.

After careful deliberation, the Typar Weather Protection System was specified for the project. Typar HouseWrap, flashing and construction tape were used to ensure seams, edges and openings were closed, creating a tight building envelope. Before the roofing shingles were installed, Typar RoofWrap, a polymeric roofing underlayment, was used as a secondary barrier to reduce leaks caused by, among other things, storm damage and worn roofing materials.

“We knew a good housewrap system would be a great start to preventing moisture intrusion,” says Kipp Rancourt, partner, Courtlan Construction and general contractor on the Mold Safe Model Home. “Due to New England’s volatile weather, it was essential that we used a strong, resilient barrier to wrap the home.”