The approach to building envelope design has changed drastically over the last 20 years. Historically, the dynamics of structural loads and bearing have had few adjustments. However, the laws of physics factoring in air/moisture movement and drying through wall assemblies have transformed exterior wall design. As a result, design features in today’s wall system include air barriers, vapor retarders, water resistive barriers and flashing systems to address these performance concerns. 

Penetrations through the wall assembly are of the greatest interest and are mission-critical when it comes to integrating these features into wall design. Fenestrations, or windows, are typically the largest penetration through the building envelope. This opening, both by the component’s design and flashing installation, must combine all the features of protection to prevent air movement, moisture and water intrusion. 

There are four critical elements that are fundamental for a window flashing system to provide the building envelope with energy efficiency, durability and longevity. Subsequent consideration as to specific design including climate, wall structure, fenestration characteristics and exterior cladding should always complement these fundamental elements, which include:

  • Sill protection and drainage
  • Head flashing design
  • Air, moisture and thermal protection
  • Product performance and compatibility

Sill Protection and Drainage

When a sill pan is applied (flexible or rigid) to the rough opening there are three performance characteristics the sill pan must provide:

  • Protection: The sill pan must protect the rough sill and all wall components below the window opening from water intrusion. It is also required that the sill pan should prevent water from penetrating the joint between the rough sill and side jamb. In addition, this protection must include resistance against the wicking of water into the rough side jambs.
  • Water collection: A non-sloped sill pan must collect or prevent water from migrating past the interior window frame or beyond the sill flashing termination. This can be achieved by applying a “back-dam” using sealant, closed cell spray foam or an up-turned leg. The up-turned leg can be formed utilizing flexible flashing tape or a rigid sill pan. A sloped sill pan directs water to the exterior of the rough opening and, by design, will prohibit water from interior migration.
  • Drainage: All sill pans must drain water to either a membrane drainage system or “through wall” to the exterior cladding. When a flanged window is fastened to the opening, the bedding sealant used behind the flange should be applied as a discontinuous bead. This would provide voids to allow water to drain to the membrane drainage system.

Head Flashing Design

Head flashing design can be misunderstood depending on cladding type, window protection and procedure. No matter the cladding type, the gap between the head of the window and exterior cladding should not be sealed. This will allow for water to drain and not collect or load above the window. 

Rigid head flashing should always be applied above windows with multiple or mulled units. Rigid head flashing is not required on single window units only. When rigid head flashing is installed bedding sealant should be applied to the bottom and up-turned leg of the flashing. 

In all cases and methods used in ASTM E-2112-07, the water resistive barrier should overlap the head flashing tape and the up-turned leg of the rigid head flashing. Head flashing tape should always overlap and extend 1-inch past side jamb flashing tape on both sides of the window.

A continuous bead of bedding sealant should be applied behind head and side flanges of window. If the window has field installed or non-integral flanges, sealant should be applied to the joint between window and flange connection.

Air, Moisture and Thermal Protection

Redundancies are built into flashing systems. This means that cladding and window sealant, flashing tape and bedding sealant can provide several layers of defense against air infiltration and water intrusion. The final perimeter gap to be sealed is applied from the interior between the rough opening and the window frame. 

Although it may vary, the recommended gap between the rough opening and window frame is a 1/2-inch. Specific window types will have a larger gap between the window and rough sill. If not sealed properly, this perimeter gap will allow air and moisture to pass between interior and exterior conditions. Additionally, the absence of thermal performance can boost thermal conduction. In both examples this can decrease energy efficiency and promote moisture development.

There are a number of methods used to prevent air and moisture movement and promote thermal performance. The use of window and door spray foam is a common application to consider. For example, closed cell polyurethane window and door foam needs to be low expansion, vapor- and water-resistant and air seal large gaps. It also should provide an insulation R-value of 5 to 6 R per inch. 

A few important points to consider with foam application:

  • Professional foam guns provide improved foam application. Many foam guns have adjustable valves to regulate the flow of foam, which can control excessive use and ensure proper application.
  • A minimum depth of 2 inches of foam is recommended in the perimeter gap.
  • If using foam between the windowsill frame and rough sill, never fill the gap to the exterior window frame. A minimum 2-inch space is necessary between the exterior window frame and foam edge. This will encourage water collection and drainage.
  • It is not necessary to fill the gap from the exterior to interior window frame at the head and side jambs.

The performance and compatibility between sealants, flashing tape and foam used for this application are crucial and play an important role for reliability, longevity and reduced risk. 

Product Performance and Compatibility

The performance and compatibility between sealants, flashing tape and foam used for this application are crucial and play an important role in ensuring reliability, longevity and reduced risk. 

Sealant performance and compatibility with air barriers, water resistive barriers and self-adhered flashings are most important to long-term adhesion. The consequences with no sealant or low performing incompatible sealant are similar when it comes to water intrusion. When selecting sealant, self-adhering flashing and window and door foam, these products should conform to AAMA 711 and 713 specifications and testing.

 The design-build team should require product written specifications that provide all performance testing results to confirm the use of each product for the particular flashing application. Notable and trustworthy sealant manufacturers provide all the tools and products needed for a complete window installation system and complement it with a written install warranty.