Windows are one of the most delicate areas of the building envelope, requiring precise installation to prevent moisture intrusion, air infiltration, and energy loss. Today’s trendy, large windows and wide expanses of glass may not require many more steps than traditional units—but the larger the product, the more sensitive the installation. Mike Westfall, senior field support manager for JELD-WEN, shares nine key areas to consider when installing large windows: 

  1. Building envelope integration. The window must be an integral part of the envelope in order to eliminate or direct air and water from the wall cavity and avoid durability issues. Westfall suggests a complete pre-installation mockup of the window and wall section with input from the full design team.
  2. Continuity with the weather resistive barrier. Continuity between the windows and the barrier prevents air, water, and vapor from penetrating around the window. Along with ensuring proper installation, the mockup allows the WRB representative to verify that the window installation process doesn’t void the warranty.
  3. Proper sealing. Not only does the sealant need to be installed thoroughly and properly, specifiers must consider the joint movement and should verify that the sealant is compatible with products it touches. Proper surface preparation is essential to ensuring the sealant works properly.
  4. Proper shimming. The larger the window, the more critical shimming becomes. If under-shimming occurs, it will cause sagging at the sill or head; over-shimming will lead to bowing of the jambs.
  5. Proper fastening. Fasteners must penetrate the structural members of the wall. Follow the schedule (spacing and number) from the manufacturer, as it is dictated by an independent third party that certified the window’s performance.
  6. Proper flashing. Follow all instructions and sequencing from the flashing manufacturer to ensure proper integration with the drainage plane. When installing the window, tip it in to help prevent tears.
  7. Proper head flashing. With large, mulled windows, drip caps are crucial to prevent water from pooling at the head and from getting behind the unit. The larger the window, the greater the potential to receive water during heavy rain loads.
  8. Proper pan flashing. Two of the most critical points for moisture intrusion are the two bottom corners, so follow all manufacturer instructions and sequencing. Again, tip-in installation will help ensure the integrity isn’t compromised.
  9. Air-tight installation. Proper air sealing provides additional redundancy to control air and water vapor, an essential step for meeting energy codes as well as the demands of homeowners.

Westfall emphasizes the importance of the pre-installation mockup, as well as the convening of the full team, including manufacturers. “There’s a lot of work behind the scenes that goes into installation of windows and doors,” he notes. “If you start out of the gate right, your potential for success is a lot better.”

Learn more about the widest portfolio of big windows in the industry by exploring JELD-WEN’s windows on

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