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  • Window Sealant Failure

    By Mark Meshulam

    Mark Meshulam is an expert witness and consultant for window sealant failures.
    Since sealant (caulk) is usually the primary means of sealing elements of a window system together and to the surrounding condition, sealant failures are often part of a window leakage problem.
    Email me for a great caulking contractor at

    To the uninitiated, sealant is gooey stuff you buy in a tube at Home Depot and smear on cracks. To window-ologists, however, sealant is a science.

    Window leaks related to sealant failures

    1. Perimeter sealant failure
    Windows must be caulked to the surrounding conditions or they will surely leak. I did see a job once where an inventive contractor tried to use the EIFS coating of the exterior wall overlapped onto the edge of the window, instead of using caulk. Of course this brittle material cracked at the first thermal movement. Water leaks in this building filled 5 gallon buckets and the wall became the worst mold colony I ever saw. Lesson: use caulk around your windows, not creativity.

    Caulk joint is not tooled, is used in insufficient amount, has gaps and poor joint design. In short, it sucks! It will leak! This is a classic sealant failure.

    Caulk joint is not tooled, is used in insufficient amount, has gaps and poor joint design. In short, it sucks! It will leak! This is a classic sealant failure

    2. Sealant deterioration or reversion
    In this type of sealant failure, old caulk or a “bad batch” of caulk never fully cures. It might have a gummy quality, or turn into a disgusting liquid. This is a bad problem and will require removal and cleaning or full encapsulation to rectify it.

    Sealant failure can occur when it has exceeded its expiration date

    Do not use sealant which has exceeded its expiration date

    This sealant failure is characterized by releasing from one or both substrates. This is called sealant adhesive failure.

    This sealant failure is characterized by having released from one or both substrates. This is called sealant adhesive failure.

    3. Sealant failure: adhesive
    Adhesive sealant failure occurs when the sealant does not stick to one or both substrates. Ironically, the adhesive bond can release days, weeks or even months after application. It probably will not be known during application. The best way to prevent this type of sealant failure is to perform adhesion tests on substrates before performing all of the caulk work. Perform the tests with and without the recommended primers. This will flesh out potential adhesive incompatibilities. Equally important is the use of proper cleaning techniques immediately prior to sealant application. And always “tool” the joints! Carefully follow manufacturer’s instructions and recommendations for cleaning methods and follow them religiously.

    4. Sealant failure: cohesive
    Cohesive sealant failure occurs when the caulk shows a rupture or tear within its boundaries as opposed to where it sticks to something else. Cohesive sealant failure can occur as a result of sealant deterioration or poor joint design. Caulk joints must be installed according to manufacturer-approved designs in order to function well. Generally, the design will include the use of a foam backer rod, creation of an hourglass shaped sealant cross section, a specified ratio of width to thickness, a minimum amount of area of adhesion at each substrate, and “tooling” of the joints.

    When a rupture occurs within the sealant itself, this is called cohesive sealant failure

    When a rupture occurs within the sealant itself, this is called cohesive sealant failure

    Sealant adhesion and cohesion failures in tension and shear

    Everything you wanted to know about sealant failure, both adhesive and cohesive, in shear and tension, in one richly illustrated graphic. How do we do it?

    5. Sealant failure: discontinuity
    Sealant is only as good as the person who installs it, and the substrate that si being sealed. If the caulker misses a spot, even a small one, the building will leak and possibly quite a bit. Sometimes the window, flashing, or adjacent wall has overlaps or cracks which penetrate the caulk joint. These cracks or overlaps will conduct water into the building even with a world-class caulker on the job.

    6. Sealant failure: immersion
    Unless your building is caulked with aquarium sealer, I would be willing to bet that your warranty will be void, and sealant failure will occur prematurely, if the caulk is subjected to immersion in water. The most classic example of this is at “back-pitched” sills or balconies. If you see your sealant joints immersed in puddles of water, find a way to either correct the pitch (slope) of the sill, or create a drainage path which will carry water away from the caulk.

    7. Sealant failure: incompatibility
    Caulk utilizes fairly complex chemistry which includes the actual solids involved, the solvents, the cure chemistry, the crosslinking of molecules as cure takes place, the release of by-products of cure, the development of adhesion. If the caulk is in the presence of other materials which also have a reactive nature (as opposed to being inert), you might find that these two materials will react to one another, possibly not in a good way. They might degrade one another, or fail to adhere to one another, or prevent the cure of one or both materials. Or, they might simply discolor one another while continuing to otherwise perform.

    Take sealant compatibility very seriously and test all substrates with the caulk you want to use. Also be sure to use the cleaning solvent which is compatible with the sealant. Do not improvise with your solvent selection. If the instructions say to use isopropyl alcohol (IPA) for example, don’t use denatured alcohol, grain alcohol or vodka. Use what the manufacturer specifies. The wrong solvent can be incompatible with and cause seal failure.

    Separating incompatible sealants

    One of the frustrating ironies of waterproofing is that the two best and most frequently used waterproofing materials don’t like each other. I am referring to the silicones, which are generally regarded as the best window and wall sealants, with the bitumastic family, frequently used on roofs and flashings.

    Bitumastic, or bituminous materials are derived from coal or oil. They are almost always black. They might be in a liquid or mastic form, or heated and rolled on, or manufactured into a roll with an adhesive backing. They are great waterproofers because they repel water, but they tend to either remain in a somewhat liquid state or release oil at their surface. Few caulks will stick to them as a result.

    I have spent many sessions with design professionals who grapple with this incompatibility. They generally devise an intermediate substrate that both sealants will adhere to, such as a metal flashing. This approach can yield success, however it can also fail. Failures will typically occur at corners, splices or intersections of the flashing, where maintaining sealant continuity (while also maintaining separation) through that location is nearly impossible.

    Great resource: Dow Corning Contractor’s Handbook. A contractor’s guide to Dow Corning construction products and procedures

    For a good sealant job, involve an expert

    If you have a project which involves either failures involving sealant, or a project which will utilize a significant amount of sealant, make sure you are doing it right. A good sealant job will last 20 years or more. A bad job can fail during the next rainfall. It does not cost much more to do a good job. Involve an expert (call me, for instance) to ensure that your sealant performs and gives you the value and water-tightness you bought. And if you just need a great caulker, let me know.

    Mark Meshulam, Chicago Window Expert inspects sealant from scaffold on highrise building

    Mark Meshulam, Chicago Window Expert inspects sealant from scaffold on highrise building

    Email me for a great caulking contractor at

    Have sealant problems?
    No matter where you are,
    contact me, Mark Meshulam,
    the Chicago Window Expert
    For the expert attention you deserve
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22 Responses to “Window Sealant Failure”

  1. After reading this article, I feel that I really need more info. Can you share some more resources please?

  2. Hi ExBack, good question.
    Here is my favorite resource for learning more about sealants:
    Dow Corning Americas Fenestration Technical Manual


  4. After reading through this article, I feel that I really need more information on the topic. Can you share some more resources please?

  5. I Want know more about you – types of sealants ,

  6. A little more about sealants which might be used in windows…
    Sealants fall in families: silicones, urethanes, latex, terpolymers and butyls.
    The silicone family will generally provide the most long-lasting perimeter joints and glazing seals.
    The urethane family is a lower cost solution for perimeter sealant and does not last near as long, however in my experience, the urethane family still has a large market share in horizontal applications such as deck coatings and self-leveling joint sealers. Latex is for interior use only. Terpolymers are used residentially, especially with vinyl windows. Butyl is a highly water repellant material but never hardens, so it is a good glazing sealant when used in conjunction with another sealant that can encapsulate it. Silicones and butyls are not paintable, the rest can be painted. All taste terrible.

  7. Great post! I’ll subscribe right now wth my feedreader software!

  8. Hi, gr8 post thanks for posting. Information is useful!

  9. Great web site Mark.

  10. Audience, that compliment was from one of the great windowologists roaming the earth today. Ken, when are you going to stop roaming and sit down? It’s easier to run AutoCad that way. Thanks for the compliment!

  11. Your site is worth beeing in the top cause it contains really amazing information.

  12. I should say that has lots of interesting information. Looks like the author did a good job. I will be coming back to for new information. Thank you.

  13. Hi Mark,
    Thank you for your very informative site!

    I’m trying to resolve an issue of interior caulk cracking on vinyl slider windows that were installed in the last few months in my third floor condo home. The cracks are most apparent when sun is hitting the windows (worse vertically than horizontally). The installer has agreed to remove the DAP non-silicone caulk used in installation, replace it with the window manufacturer recommended more flexible OSI QUAD caulk, and add an additional screw to each of the sides of the windows to make the windows more secure and limit the magnitude of the thermal expansion.

    The largest window is 81″ wide by 57″ high. The other windows are 37″ wide by 57″ high.

    Do you have any recommendations?


    P.S. I will e-mail you a image showing the cracking of the DAP non-silicone caulk and the OSI QUAD technical data sheets. I am also going to try to inspect the exterior silicone caulk application for adherence to the vinyl of the windows as I understand from your site how critical that is.

  14. […] line. Types of edge seal failures If you have been diligently reading these articles, especially this one, you will not be surprised to find that failures of insulated glass seals mirror failures that we […]

  15. […] window and see what happens in the next rain. If this is your issue, before you caulk, learn more here. This is an adhesive caulk failure where the caulk remained adhered to the window but disengaged […]

  16. Hi Mark,

    I’m writing from Toronto, ON and tempted to call you to ask for advice. However, I’ll try to describe the problem in writing and see if the advice you offer does not work, I will call you.

    The window in the basement is leaking from the top sill. In two years since owning the house, I have replced the window sill twice. The first time, the window sill was finished with drywall and in no time did it get black and mouldy. So I have the sill replaced with wood. Now, when it rains, I can see 3 spots on the top sill where the water drips. The one place of drip is really strong that I have to put a funnel and bucket to collect the water.

    I called a roofing contractor to see if the roof is the source of the leak. But I was told that although the roof needs some tune-up, it is not the source of the leak. The wall outside the window is brick and the caulking around the window looks fine. She suspects the water comes from the water pipe from the floor above, which happens to be where the kitchen sink is located. I was surprised by her assessment because there is absolutely no visible leak coming from the pipes under the sink.

    Do you think this is a right assessment of the problem? Please tell me what I should be checking myself so I have enough information to discuss with the next contractor I call. Thank you for your help!

  17. Hi AMS,

    The word “sill” usually means the bottom part, but in this case you seem to be referring to a leak at the top of the window, so let’s call that the “head” of the window.

    If it leaks when you run the kitchen sink water, it could be what your roofing contractor says. It leaks when it rains, you have rainwater, and need to focus your attention on the area above the window on the exterior.

    Can you send me one picture of the entire window from the inside, and one showing the entire side of the house where the basement window is, all the way up to the roof, and one more close up at the exterior top of the basement window? Send to and we will get it fixed!

  18. Did you make sure the area was completely dry, brefoe you applied the caulking? I had a similar situation at my apartment years ago. The caulking wasn’t sticking because the area wasn’t completely dry. When I finally realized what I was doing wrong, I fixed it.-Go into the cracks that are leaking, an pat them with paper towels.-Get your blow dryer, and blow hot air all over the problem areas.-Repeat all the other steps you mentioned.-Good Luck!

  19. Gaaaaa! Autoplay!
    Kill it! Kill it with fire!
    I hate, hate, hate it when a website has something (audio in this instance) which auto-plays.
    Just because of that, I’m completely skipping whatever advice this website has. It might have contained something useful but I’ll never see it now.
    Yes, I do realize that I’ve spent more time commenting than it would have taken to scan the article, but that’s simply how much I hate autoplay.

  20. EIFS, not EFIS, and swing stage not scaffold, any thoughts on silicone sealant being buried under tiles at the balcony? Thank you

  21. Hi,
    I take care of a few properties, residential not commercial. I noticed at one of the homes that the caulking around the windows and doors has hair line cracks all over it, basically its starting to break but not at the point of disintegrating.
    I lived in a building where over the years the caulking had cracked in the same manor and moisture (not noticeable) had seeped in through the cracks and walls and eventually caused all the parquet flooring near any external window/door to pop up, this was later confirmed by a building inspector.
    The building was brand new when I moved in , it took 15 years to reach that point but its amazing what internal damage tiny cracks in the caulking can cause, non of the inside damage was noticeable until it was too late.
    So my question is this (like the other poster I too live in Toronto) the owner of the property I manage has asked me if I can do the Caulking on the windows but as handy as I am I really do not want to touch them, this is one area I believe an expert should be involved. Reading your blog it seems you share the same sentiment
    1. Should the old deteriorating caulking be removed and new caulking applied or should new caulking be applied on top of the old.
    2. If it is a case of just applying new caulking on top of the old what are your thoughts on a non expert doing this.
    Thanks for your time, and a reply would be greatly appreciated;)

  22. When replacing caulk, where practical it is better to remove the existing to the greatest extent possible before installing the new. The new caulk should be highest quality silicone, installed per manufacturer recommendations by a professional caulker.

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Workmen fitting the bottom pieces at the corner so they form a miter (a 45 degree cut on each piece that fits together like a picture frame corner).