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WINDOW FLASHINGS THAT WORK!

11.20.19   Thomas L. Bane, PE | More by this Author

WINDOW FLASHINGS THAT WORK!

For the purposes of this blog, “window” will refer to a commercial, aluminum-framed, non-finned, storefront-style window in a membrane/drainage type wall.

Windows have always had issues with leaking. Actually, it’s probably more accurate to say that windows have always leaked, and people have issues with leaking windows. Understandably so! But contrary to popular belief, the windows themselves are usually not the source of leakage. The main leakage path is the joint between the window and the surrounding wall. Flashings bridge the gap and protect this joint. Avoiding mistakes that cause frustrating and costly window leaks starts with proper design of flashings.

The ASTM Standard E 2112 provides guidance on how to flash these types windows. When it comes to avoiding common mistakes and minimizing leaks, the most important part of the standard is the part that discusses window sill flashings.

Following the standard’s recommendations will help you avoid these seven mistakes:

MISTAKE 1: OMITTING A SILL PAN UNDER WINDOWS
When a sill pan is omitted, there is nothing to catch the water that enters between the window and the wall. Water that reaches the sill is likely to leak to the interior. Never omit a sill pan.

The flashing around this window did not have a sill pan.


This window leaked without a sill pan.

MISTAKE 2: USING A SILL PAN WITH NO END-DAMS
The sill pan should have three sides: a back-dam and two end-dams. If it doesn’t have end-dams, it doesn’t act like a sill pan, it acts like a gutter. The pan will collect the water and direct it to the jambs. There is a saying among building enclosure consultants: “the only thing worse than a window without a sill pan is a sill pan without end-dams.”

A sill pan without end-dams directs water into the jambs.

MISTAKE 3: APPLYING WINDOW SEALANT THAT PREVENTS DRAINAGE
Traditionally, a sill pan is a soldered and sealed piece of sheet metal that sits under the window. Once installed, the window installer typically applies sealant all around the exterior of the window. Unfortunately, this sealant joint seals the sill pan to the window and prevents the sill pan from draining. A sill pan that doesn’t allow water to drain to the exterior will fill up and leak to the interior. Don’t let this happen to you! To prevent water from leaking to the interior, you need weeps in the sealant joint or provide an alternative drainage path to the exterior.

A sill pan is overflowing to the interior because the pan doesn't drain to the exterior.

MISTAKE 4: THE SILL PAN DOES NOT INTEGRATE INTO THE WALL FLASHINGS
If a sill pan does not integrate with the wall flashings, water will not be directed into the sill pan. Sometimes, a window subcontractor will “value engineer” the sill pans out in favor of a sill can (also known as a starter sill). Sill cans are not designed to integrate with the surrounding flashings. When water enters between the window and jamb, the water is not directed into the sill can. Because the water isn't directed into the sill can, it does not drain to the exterior. This results in leaks.

A sill can was used in lieu of a sill pan. Someone tried to integrate the sill can into the wall flashings with sealant, but this voids the window's warranty.

MISTAKE 5: THE SILL PAN IS NOT PROPERLY SLOPED
Most sill pans have joints on the flat part of the pan. The joints are usually sealed in some fashion. Seals fails much faster when submerged. Therefore, if a sill pan has poor slope, water is likely to linger and begin to erode those seals. Failed seals create leaks.

The sill pan was poorly sloped, so water collected in the sill pan. The water either found a defect in the seal or lingered long enough to erode a seal. The water leaked into the building because the pan did not drain it away.

MISTAKE 6: PENETRATIONS THROUGH THE SILL PAN HAVE NOT BEEN SEALED
Windows need fasteners to hold them in the openings. These fasteners invariably need to penetrate the sill of the rough opening. If the fastener penetrates the sill pan, that hole needs to be sealed to prevent leakage.

The window sill was pre-drilled and a fastener was used to secure the window to the wall. The head of the screw was sealed with sealant, but the hole through the sill pan was not.


The fastener penetrated the window sill and the sill pan, but was only sealed at the window sill. The hole through the sill pan was not accessible, so it wasn’t sealed.


The unsealed fastener hole leaked when tested.

MISTAKE 7: FAILING TO USE AN AIR SEAL
Failing to install an air seal can cause big problems later. The air seal performs three functions. First, it stops interior air from leaking out around the window. When air leaks outside, it leads to energy loss and increased risk for condensation. Second, it prevents wind from pushing water into the building. Third, like a pressure-equalized rain screen, it prevents wind from pushing water into the sill pan. This lets the sill pan drain.

Wind pushed water into the building at the window jamb because there was no air seal.

Flashing windows is tricky. Knowing how to avoid these seven common mistakes can help your window flashings perform at high levels and help prevent frustrating and costly window leaks. Effective designs for sill flashings differs based on the surrounding materials and sequence, but avoiding these mistakes is always a good start.

If you have trouble with window leaks in your commercial building, or want to prevent them in the first place, contact Tom Bane.


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