Large window installation requires precision to protect against air and moisture intrusion and energy loss. 

Addressing the exacting requirements of a sound building envelope has grown even more critical in recent years as larger windows and wider expanses of glass have become more popular. Read nine key installation considerations as offered by Mike Westfall, senior field support manager for JELD-WEN. 

Windows can have a dramatic impact on the look and functionality of a home. Finding the right one means evaluating everything from their design to their engineering. However, we know that little of that matters if one key step is not completed properly — installation. Anything short of precision here can lead to costly moisture intrusion, air infiltration, and energy loss.

Addressing the exacting requirements of a sound building envelope has grown even more critical in recent years as larger windows and wider expanses of glass have become more popular. In other words, as the size of windows has grown so too has the importance of a flawless installation.

To help simplify the process, Mike Westfall, senior field support manager for JELD-WEN, offers nine key things to keep in mind when installing large windows:  

1. Building envelope integration

The window must be an integral part of the building envelope in order to eliminate air and water or direct it away from the wall cavity and avoid durability issues. Westfall suggests a complete preinstallation mockup of the window and wall section with input from the full design team. 

2. The weather-resistant barrier (WRB)

Continuity between the windows and the barrier prevents air, water, and vapor from penetrating around the window. Along with ensuring proper installation, the recommended mockup allows the WRB representative to verify that the window installation process doesn’t void the warranty. 

Here’s an instructive example of proper sealing of a window.

3. Proper sealing

Not only does the sealant need to be installed thoroughly and properly, but specifiers must consider the joint movement and verify that the sealant is compatible with the products it touches. Proper surface preparation is essential to guarantee the sealant works properly. 

Proper shimming prevents sagging and bowing which is especially important with large windows.

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. 

Window fastening should follow installation instructions specific to the size and type of window.

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. Here’s an example of flashing and window installation. 

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. Airtight 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. 

Consider a complete preinstallation mockup of any large window and wall section, with input from the full design team, before undertaking installation.

Westfall emphasizes the importance of the preinstallation 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.” 

Get started with JELD-WEN windows

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