Windows and doors are full penetration items—they go through the entire wall. This means interior pressures are not really well-buffered to exterior environments. These through-wall pressures are more significant than a halfway penetration, such as outlets or water bibbs. The pressure can vary greatly and are related to the height of the structure, exposure to wind, and the size and location of the opening. This is referred to as “design pressure.” Quality fabricated windows are tested and rated for pressure and then assigned a performance class. Selecting the properly rated window is critical, as well as ensuring the transition from the window to the cladding provides a seal against water intrusion. A bad window leaks, a good window does not.
Types of Windows
There are many types of windows and configurations. There are two basic categories: flanged and non-flanged. Flanged windows are the most popular window installed today. They are used in the residential and light commercial markets. Most are made from PVC and installed with the flange being the attachment to the sheathing. The metal non-flanged window is also known as a “storefront” style. Many wood-framed windows are also a non-flanged style. Regardless of the style or type of window used, flashing or sealing the perimeter of the window frame to prevent water intrusion is critical. Success typically relies on following a recognized industry standard for flashing and coordination between the trades. Communication is important, as no one trade is typically responsible for all phases.
There are many standards and methods on flashing windows. While experts have opinions, preferences and stories to relate to, it is best to use a published and recognized flashing practice. Simplicity tends to be a good key to success, as complication tends to create confusion that in turn opens the door to errors that result in leaks. Make sure workers understand the process—that is why it works and is important to make sure they get it right. Keeping water out with a continuous seal is critical. The second item is to allow incidental water to drain and exit the wall assembly. This also the prime tenant of the building code for wall assemblies.
The building code mandates flashing at perimeters of doors, windows and penetrations. The code does not prescribe exactly how this is to be done. For those who believe it does tell them how, note the code also has Section 104: “The provisions of this code are not intended to prevent installation of any material or prohibit any design or method of construction not specifically prescribed by this code.” Chapter 14, the one on exterior walls, covers flashing and simply states that flashing shall be installed in such a manner so as to prevent moisture from entering the wall or redirecting it to the exterior. While the language is clear on the need and intent of flashing a window, it purposely is unclear on the method to be used. This is America and we allow for a variety of methods to accomplish the goal.
Stucco/EIFS and Window Flashing
Stucco and EIFS, properly applied, are very water-resistant. This is good and bad news.
The good news is that both these claddings keep water out better than every other cladding available to the market. Both stucco and EIFS are seamless. While stucco can crack, a hairline crack it is not a source for significant leakage. This can be verified by third-party testing reports. Cement plaster absorbs moisture, so even if incidental water gets in, it will be absorbed by the cement and evaporate outward. Stucco on framed walls was developed on the “concealed barrier” approach, meaning it can handle incidental water, but there is a limit to this moisture management concept. EIFS and rainscreen stucco can handle enhanced drainage more than just incidental moisture. Failure to effectively flash windows and doors is a disaster for stucco or EIFS. This is because unlike lap sidings, both are water-tight claddings, making flashing more critical for stucco and EIFS.
Flashing need not be complicated or expensive to work. In fact, I think the KISS principle, or “Keep It Simple Stupid,” is best. For stucco, the SMA provides some basic and simple flashing methods that are easy to follow. For EIFS installation, you should refer to EIMA or the specific EIFS manufacturer for their approved methods. EIFS manufacturers provide excellent training classes on proper installation.
I would caution any contractor that wants to just do what they have always done in the past and think it is good enough. It may indeed work and be a good method. If that is true, there will be a group that will provide written back-up. Get it, keep it on file and follow it to the letter.