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

    WARNING This article contains profanity, which is not gratuitous but rather fundamental to the art, as well as being a true and accurate expression of my frustration with aspects of the window industry. We can put a man on the moon, so why do we sometimes do bad window replacement?!?

    The window industry is so big and non-uniform that you can find window installers of every skill level, from ignorant to brilliant, and anywhere in between.

    Forensic window removal and investigation of bad window replacement

    When a window leaks, sometimes the best thing to do is to remove it and look inside. The window installer performing this forensic investigation of this bad window replacement was actually well trained in window installation, but his safety training is lax. He has no safety ropes or railings on his work platform, but that’s an article for another day.

    This is because of a gaping absence of uniform installer certification and a widespread ignorance of important construction principles, even at the management level. You will be hard pressed to find a window company that provides a uniform training curriculum to their workers.

    I continue to be appalled when I see bad window replacements that involve good windows installed with fundamental errors that result in window water leaks, air leaks, poor operation, caulk failures and even structural failures. What is going on? Did nobody in the company from owner to apprentice figure out the basics that are needed for a successful window installation?

    In the absence of good training, too often the brilliant workers just figure it out for themselves and care enough to do an excellent job, and the ignorant workers do bad window replacements that are doomed to fail without much caring for the result.

    While it may sound like I am beating up on the workers, and to an extent I am, by far I am laying most of the responsibility for bad window replacement at the feet of management and the window industry. Window installer curriculum and training must come from above. First, management and the window industry must determine the concepts that are important, then they must require workers to learn and implement it.

    When the window replacement goes bad, the hapless customer is often faced with a three-way finger pointing exercise where the window supplier, the window manufacturer and the window installer each blame one another for the bad window replacement. The customer spent good money for windows and got bullshit instead.

    Window people! Is this who we want to be? Let’s be better!

    So in the interest in industry improvement and personal catharsis, here is my list of…
    Top 10 Window Installer Screw-ups

    #1. Not tying to the Tyvek!

    In most buildings built in the last 20 years, there is a “membrane” inside the wall that acts as the primary weather resistive barrier (WRB). Often this WRB is DuPont Tyvek, a waxy paper-like sheet that resists air drafts and incidental water but allows water vapor to escape.

    Bad installation of window flashing

    The worker peeled back the siding at the window jamb to find that the window installer left the original flashing (#1) in place, then placed a new flashing over part of it (#2) which was then connected to a piece of wood (X) but not to the new window. This is a lame, lazy job. The WRB must be connected to the window through the use of the peel-n-stick flashing


    A fundamental of WRBs is complete continuity with all elements in the wall including window and doors frames. The Tyvek or other WRB must be sealed to the window frame on all sides or the building envelope will be violated. Yet when clients complain of bad window replacements that leak air and water, too often I find that the connection between WRB and window is missing or incomplete. The incorrect open space between the window and the WRB provides a wide-open passageway for air and water to enter the building.
    Poorly installed head flashing on a bad window replacement

    The picture at left shows a window head that is seemingly well giftwrapped in peel-n-stick flashing. But when we pull back the siding, we see that the flashing is not sealed to the WRB above. If water gets through the siding (and you should always assume that water will get through the siding, or even the brick), it will drip right behind the flashing and into the building.

    Bad window flashing at corner of window

    Corners and tops of windows are particularly vulnerable to water leaks, so care must be taken to neatly “shingle” or overlap upper WRBs over lower flashings in an orderly fashion. Here a lazy installer installed a window with a horror-show hodgepodge of materials that are just stuck to the face of the WRB and to the face of the original flashing, which is a practice that should be avoided. Clean off the old stuff, or fully encapsulate it, then do a neat job! To add to the mess, the original WRB seams are not taped and neither the original or replacement flashings are tied to the window, but rather to a piece of wood next to the window. The original flashing is labeled #1 and the flashing for the bad window replacement is #2.


    Here’s how DuPont says Tyvek should be used.

    #2. No damned end dam!

    Imagine a gutter with the end missing. Water will spill out the end, right? Well, window systems have gutters too, but they are called sill flashings or subsills. Just like gutters, they need to have something at the end that prevents the water from running out and into the wall cavity. The water should drain, or weep to the front only.

    Gutters or flashings or subsills are needed beneath window openings when 1) there are multiple windows in an opening, or 2) when there are trim pieces or receptors that might admit water into the system. Avoid penetrating these with fasteners!

    End dams are needed anywhere there are gutters, flashings or subsills. End dams are also needed to terminate the ends of masonry flashings. So why are these critical features missing or incomplete in so many buildings?

    Bad window end dam being water tested and failing the test

    In this commercial bad window replacement, the installer neglected to fully seal the subsill end dam to the adjacent material, which in this case was a mullion, which is a structural post between windows. The leak necessitated a forensic window removal allowing a firsthand view of the problem. We then surrounded the unsealed end dam with testing putty and poured water into the suspect area. The water freely flowed around the unsealed outer edge of the end dam and into the building

    #3. Shi**y shimming!

    Shims are small pieces of plastic or wood that act as spacers between the window frame and the surrounding structure. Usually there are fasteners, such as screws, that pass through or near the shim to anchor the window in place. Shims are usually evenly spaced at the bottom, sides and top of the window.

    Wood and plastic window shims for residential and commercial window installation

    Window shims are usually wood for residential windows and plastic for commercial windows. Wood shims are (#1) driven into the space between window and stud framing until the desired spacing is achieved, (#2) a fastener is added through the assembly, then (#3) the excess shim is trimmed off. In commercial work, the fastener is installed first, then a stack of U-shaped plastic shims are slid into the space between window and surround right onto the fastener shaft, then the fastener is tightened


    Shims at the bottom of the window also carry the weight of the window. These must be spaced according to manufacturer instructions to distribute the weight so the bottom of the frame is not distorted.
    Insufficient shimming of bad window replacement

    In this bad window replacement project, the installers did not adequately shim the bottom of the window and instead rested the window on the edge of the interior trim. Under the weight of the window, the trim tilted downward at the exterior and upward at the interior, disrupting the interior caulk and throwing the window out of adjustment


    Bad shimming of double hung window causes air leaks

    One of the problems with this bad window replacement was an absence of shimming at the meeting rail of the double hung windows. This caused a gap to develop between the sash and frame that was too big to be spanned by the weatherstrip. Air leaks were the result.


    Shims around the window also create a space where caulk can be installed with a caulk joint sufficiently sized to handle long term movement. Shims at the sides of the window keep the frame square. Without shims at the sides, the rectangular window could become a parallelogram over time, which is very bad for performance. Shims must support much of the front-to-back depth of the frame so the frame does not twist out of position over time.

    Some windows, such as double hung windows, have a special need for shimming at both jambs at the height of the meeting rail, so proper weatherstrip compression is maintained.

    #4. Crappy caulking: Cleaning!

    Caulk is a fundamental part of window installation. It is the stuff that seals between other stuff and maintains water and air tightness. In order to work, it must stick. So why do I see window installers apply caulk to dusty, dirty surfaces, where it will have no chance to achieve a good bond?

    You have to clean the surface, also known as the substrate, before caulking. Using isopropyl alcohol (not denatured) with a wet rag/dry rag method will produce a reasonably clean surface for caulk adhesion.

    Poor cleaning of substrates caulse caulk failure in this bad window replacement project

    This is what happens when the installer doesn’t properly clean the substrate. Adhesion is lost because the caulk is stuck onto unstable dirt and dust. This allows water past the seal and into the building


    #5. Crappy caulking: No tooling!

    Most caulks also need to be tooled in order to stick. Tooling is similar to buttering bread. In fact, good caulkers have a series of metal spatulas of varying width for tooling caulk joints of any size. These spatulas look look a lot like cake decorating tools. Sometimes they are cake decorating tools.

    When a caulk joint is properly tooled, it has a pleasing, continuous concave surface. It shows that a skilled worker applied the caulk. When a caulk joint is not tooled, the joint is bumpy and uneven. Its life expectancy is reduced. It was installed by someone who needs better training and practice.

    Bad and good caulk joints

    Here we see bad (left) and good (right) caulk tooling. The caulk on the left is bulgy because it was just applied with “gun pressure” which is insufficient for good adhesion. The bad caulk joint is an atrocity at the bottom corner where it oozes out, then dissipates to nothing at the critical sill area. This makes it a bad window replacement. The good caulk at the right is nicely tooled and even. It is well adhered and will last a long time.

    Continue to Part 2 of Bad Window Replacement: Top 10 Installer Screw-ups


    Mark Meshulam is a consultant striving to end bad window replacement

    Mark Meshulam can save you from bad window replacement

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At 200x magnification, more activity can be seen in the glass. The inclusion doesn't look spherical at all, but rather like a bird diving into water. The birdlike shape is a combination of the nickel sulfide inclusion, other impurities around it, and vents (small cracks) emanating from it.
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