Chicago Window Expert

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  • By Mark Meshulam

    Bad Window Replacement: Top 10 Installer Screw-ups

    When good windows happen to bad installers
    Part 2, continued from Part 1

    #6. Crappy caulking: Joints too tight!

    Caulk is supposed to be a permanently flexible material that can stretch and contract when the substrates move, such as when an aluminum window frame shrinks in cold weather. But here’s the rub: Caulk can only stretch 30% to 50% of its freely moving thickness. If you install a caulk joint between two surfaces that are 1/4″ from each other, the maximum movement the best silicone caulk will tolerate is +/- 50% of the gap, or 1/8″. If the service conditions cause that caulk to stretch 3/16″, it will fail.

    Just about every door threshold I see beneath swing and sliding doors has a caulk joint size of 0″. No space at all! Even if the threshold was originally bedded in gobs of caulk, that caulk was then squeezed down to being paper thin during installation. The caulk will fail and water will enter the building under the window or door.

    Water damage due to tight, non-working caulk joint

    Caulk joint at the bottom of this sliding door was so tight (left) that it sheared during thermal cycling, allowing water beneath the threshold for years. We removed the limestone sill to view the wood damage beneath (right).


    #7. Not sealing the trim!

    Window installer seals weep hole shut but fails to seal trim

    This installer did not caulk where he should have, where the trim meets the window. But he did caulk over the weep hole which is supposed to remain open for drainage.

    Many window systems consist of the window itself plus a series of trim pieces that attach to the window. These pieces might be called mullion connectors, expanders, trim extenders or simply, trim. If these pieces are exposed to the exterior, they must be sealed to the window.
    Bad window replacement caulking job

    This installer wasn’t thinking or trying. He left trim edges and splices unsealed and even missed a section of caulk.


    Clad wood windows often have an “accessory groove” around their edges that receives some sort of trim or mullion connector. It is a pandemic that these seem to rarely be sealed to the window. The capillary-sized space created between the metal pieces loves to inhale water.

    How can you tell is a trim piece is sealed to the window? You should be able to see some caulk squeezing out of the space where trim and window meet. This exposed caulk is called “squeeze-out.”

    #8. Not sealing mullions!

    When the side of one window stacks onto another with the use of an “H” shaped piece called a mullion, incredibly these are often not sealed. Not incredibly, these mullions then drip water into the wall and damage wood, drywall and other finishes. Why are supposedly professional installers doing this?

    Bad window replacement  with unsealed mullions

    In this forensic disassembly we remove a leaking pair of windows to discover the problem: the installers did not use sealant to seal the h-shaped mullion (blue) to the mating surfaces on the adjacent windows (red dotted lines). This resulted in a puddle of water beneath the mullion on the stud framing (right)


    #9. No second line of defense!

    Here is where the installer and the managers who train and oversee them need to start using their heads. They should always ask: “What will happen when the first line of defense fails?”

    The first line of defense will usually be the exterior caulk and any sealant covering fastener heads in the “wet area”. It doesn’t last forever. Where will the water go when these things fail?

    Quality installers and window companies try to include a second line of defense to their installation design. Often this is an interior caulk joint that will prevent water that bypasses the first caulk joint from entering the walls. It might also be a flashing that has an upturned edge beneath the interior face of the window. If that flashing is end dammed (see #2 above), the system will tolerate failure of the exterior caulk and keep protecting the building from water damage. See below for an example from DuPont Tyvek. The full instructions are here.

    Replacement windows with second line of defense

    Here are some details from DuPont showing how to create a second line of defense with Tyvek, flashings and a line of interior sealant. Window installations with this type of redundancy are the way to go for getting the most out of replacement windows

    #10. Final Quality Check!

    Unsealed window frame corner

    The installer walked away from this window that had an obvious gap in the critical sill corner. Water leaked into the room through that crack. It would have taken less than a minute to seal it.

    A good installer will not just stay on his side of the installer/manufacturer divide. A good installer realizes that a problem with the window product will reflect upon him and his work. A bit of window product QC will go a long way toward increasing customer satisfaction and reducing costly callbacks. Here are the quick checks that an installer should do to make sure that the overall installation, not just his work, is acceptable:

    1. Fit and finish: Does everything look ok?
    2. Operation: Does the window open, close and lock easily?
    3. Tightness: Listen closely when closing the window. Do the street sounds dramatically reduce as they should?
    4. Visible sources of water leaks: Any holes, gaps or unsealed fasteners in the “wet areas”, especially in the sill?


    Mark Meshulam assists clients  to avoid bad window replacement

    Mark Meshulam, Chicago Window Expert ready to help you avoid a bad window replacement


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