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A Definitive Guide to Thru-Wall Flashing


Dry buildings are healthy buildings. The fundamental role of thru-wall flashing is to help keep buildings dry by diverting moisture that has entered the wall to the outside of the structure before it causes damage.

Common materials for flashing include corrosion-resistant metals such as stainless steel, copper, and aluminum, along with durable synthetics, like PVC and rubberized asphalt. Thru-wall flashing must be seamlessly integrated with other building components, including cladding, insulation, and vapor barriers, to create a comprehensive moisture barrier.

This guide explores performance features and various applications of thru-wall flashing that, when installed properly, contribute significantly to a building’s longevity and structural integrity.

Importance of a Well-Protected Envelope

A well-protected building envelope forms a shield against external elements. By effectively blocking water infiltration, it prevents moisture-related damage, including mold growth, rot, and deterioration. A well-sealed building envelope also contributes to indoor air quality, and by extension, can help to reduce health hazards associated with high humidity and mold growth. A robust envelope also minimizes energy loss by preventing air leaks and maintaining temperature control, thus reducing operational costs.

Investing in proper construction techniques, and high-quality thru-wall flashing materials, is essential to creating a waterproof building envelope. This type of proactive approach can minimize or prevent future repairs and sustain the building's value over time. Ultimately, choosing the right thru-wall flashing, and employing installation techniques that help to ensure a waterproof building envelope, will protect the resiliency, structural integrity and sustainability of the building.

Understanding Thru-Wall Flashing

Thru-wall flashing works by creating a barrier that intercepts and redirects water away from vulnerable points in a building's exterior. It utilizes the principle of gravity and water's natural behavior to channel water downward and outward. When water penetrates the building envelope, the strategically placed flashing ensures that it is guided away from openings, joints, roof-to-wall intersections, window openings and foundation walls. This mechanism prevents water from seeping into the building's interior spaces and causing damage.

The effective operation of thru-wall flashing relies on its continuous structure and proper installation, which together create a pathway for water to follow along the flashing's surface. By utilizing the principles of physics and water flow, thru-wall flashing efficiently manages moisture, and prevents water infiltration.

Installed as a continuous barrier within the exterior wall assembly, thru-wall flashing is typically made of corrosion-resistant metal or synthetic materials. Spanning from the innermost face to the exterior face of a wall wythe (a single row of brick or stone), the placement of thru-wall flashing varies across building sections, adhering to industry standards and the building's specific design. Without thru-wall flashing, buildings constructed from absorbent materials, such as brick and stone, can face water accumulation on porous surfaces, and potentially harm walls, if left unaddressed. Proper installation is essential, and it requires attention to detail and skillful integration with other building components, such as cladding and insulation.

Regular maintenance is crucial to ensuring the ongoing effectiveness of thru-wall flashing. While access to original blueprints can expedite the process, consulting a professional, or meticulously surveying the building's exterior, typically results in locating thru-wall flashing.

Benefits of Thru-Wall Flashing

Thru-wall flashing offers many benefits for constructing a well-designed, structurally sound building, such as safeguarding against water infiltration, preventing mold growth, enhancing structural durability, and ensuring long-term building integrity.

Water Diversion

Thru-wall flashing redirects water by forming a protective barrier within the building's exterior wall assembly. Positioned at critical points, including roof-to-wall intersections, window and door openings, and along the foundation, thru-wall flashing intercepts and diverts water before it infiltrates the building envelope. This redirection is vital, as it prevents moisture-related damage, including mold and decay, to help preserve the building's structural integrity and longevity.

Moisture Management

Thru-wall flashing facilitates the proper management of moisture within the wall assembly by allowing trapped moisture to escape. It serves as a drainage path for water vapor, and prevents condensation buildup within the wall assembly, to reduce moisture-related risks. It also enhances the drying process after rain or other exposures to moisture. Thru-wall flashing, in conjunction with proper insulation and vapor barriers, creates a comprehensive moisture management strategy that ensures a healthier and more durable structure.

Enhancing Longevity

Thru-wall flashing plays a pivotal role in extending the lifespan of a building. By strategically redirecting water away from critical points in the building envelope, thru-wall flashing prevents moisture from permeating the structure. This proactive measure addresses the moisture-related issues that erode a building's durability over time, and helps reduce the risk of long-term damage and costly repairs. By preserving the soundness of materials, it prolongs the building's overall lifespan, to help ensure its value and functionality for years to come.

Types of Thru-Wall Flashing

The two most commonly used types of thru-wall flashing are metal flashing and synthetic flashing. Metal flashing, often made of stainless steel, copper, or aluminum, provides durability and corrosion resistance. Synthetic flashing, manufactured from materials like PVC and rubberized asphalt, offers flexibility and water resistance.

Sheet Flashing

Sheet flashing consists of a large, continuous metal or synthetic membrane. It is preferred over other types of flashing, because it has fewer joints and potential points of entry for water. Its seamless nature reduces the risk of leaks and enhances its effectiveness in redirecting water away from vulnerable areas. Its comprehensive coverage and reduced vulnerability to water entry make it an excellent option in construction applications where water diversion is critical.

Weep Hole Systems

A weep hole system is a drainage mechanism used in conjunction with thru-wall flashing to manage water that may accumulate within the wall assembly. Weep holes are small openings strategically placed at the bottom of masonry walls that allow water to escape, to prevent trapped moisture. The weep hole system complements thru-wall flashing by contributing to enhanced moisture control.

Drip Edges and Drip Caps

When combined with thru-wall flashing, drip edges and drip caps play a vital role in building construction. Drip edges, typically installed at the edges of roofs and eaves, guide water away from the building's fascia, to prevent water from seeping under roofing materials and causing damage. Drip caps, positioned above windows and doors, direct water away from these openings, to help reduce the risk of water infiltration.

When integrated with thru-wall flashing, these components create a comprehensive water management system. Thru-wall flashing directs water away from vulnerable areas within the wall assembly, while drip edges and drip caps prevent water from collecting or infiltrating at critical points. This synergistic approach ensures that water is efficiently diverted and channeled away from the building envelope.

Regulatory Standards

In North America, specific regulatory codes that apply to the use and installation of thru-wall flashing include:

  • International Building Code (IBC): This code includes requirements for moisture management, flashing installation, and building envelope integrity.

  • International Residential Code (IRC): Specifically applicable to residential construction, the IRC contains guidelines for building envelope performance, including thru-wall flashing and moisture control.

  • National Building Code of Canada (NBC): In Canada, the NBC guides thru-wall flashing installation with comprehensive guidelines for building design, construction, and safety. Construction professionals should refer to the relevant NBC sections and provincial/territorial codes for proper thru-wall flashing use.

  • American Society for Testing and Materials (ASTM): Thru-wall flashings should meet or exceed ASTM E2178 Standard Test Method for Air Permeance of Building Materials. ASTM E2112 addresses the installation of windows, doors, and curtain walls, which often involves thru-wall flashing.

  • National Fire Protection Association (NFPA): NFPA 5000, the Building Construction and Safety Code, contains provisions related to fire protection and life safety, which can impact thru-wall flashing installations.

  • State and Local Building Codes: Local building codes should be consulted for specific requirements for thru-wall flashing based on regional climate, building practices, and considerations.

An excellent resource for comparing thru-wall flashing materials, testing standards, and chemical compatibility for components is available from the Air Barrier Association of America.

Best Practices for Installation

Best practices to ensure effective moisture management and long-term building integrity include:

  • Precise Placement: Install flashing at critical points, such as roof-to-wall intersections, window openings, and doorways. Proper placement is essential for redirecting water away from vulnerable areas.

  • Continuous Barrier: Create a continuous barrier by using a single sheet of flashing whenever possible to minimize potential entry points for water.

  • Integrated Design: Coordinate flashing installation with other building components, including insulation and vapor barriers, to create a seamless moisture management system.

  • Overlap and Seal: Ensure overlapping joints are adequately sealed to prevent water from infiltrating between flashing sections.

  • Proper Sloping: Flashing should be sloped downward to direct water away from the building. This prevents pooling that can lead to water infiltration.

  • Weep Holes: Integrate weep holes at the base of walls to allow trapped moisture to escape for enhanced drainage.

  • Compliance with Codes: Adhere to relevant building codes and industry standards specific to flashing installation in your region.

  • Regular Inspection and Maintenance: Periodically inspect flashing to identify any damage or deterioration and perform necessary repairs promptly.

The National Concrete Masonry Association offers a technical bulletin with flashing details for concrete and masonry walls at this link. The Copper Development Association also provides thru-wall flashing details, including CAD files at this link.

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