Aluminum Window Leaks

Mark Meshulam is an expert witness and consultant often involved with aluminum window leaks.

I spend a majority of my consulting time dealing with aluminum windows because they are the product of choice for most larger projects in the great town of Chicago and across the world for that matter. Aluminum has dominated the market for large, custom projects for many reasons:

Aluminum windows can leak through the sill frame extrusion
Aluminum sill frame extrusion – many aluminum window leaks come through the sill
  • Aluminum can be extruded into almost any imaginable cross-section to achieve whatever structural and functional properties are required.
  • Aluminum is non-porous and doesn’t rot under prolonged exposure to water. Even if the paint is scratched to bare metal, the aluminum will endure (it may not be pretty but it won’t “rot through” unless extreme conditions exist). By contrast, wood will turn into black mush unless kept dry.
  • Aluminum can be finished in an infinite variety of colors and paint qualities, some of which are amazingly durable.

Today’s article, however, is not to extol the many virtues of aluminum windows, but rather to discuss how they can fail. Sadly, aluminum windiows can leak. But with knowledge, failure can lead to success.

This list is created from my experience alone. That experience consists of 30 years as a window contractor, overseeing the installation of millions of square feet of windows, witnessing over a hundred field tests and having spent year in testing laboratories. Enough with the credentials, let’s get going.

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Failure of the Subsill or Gutter System

High quality subsill (gutter) sealed to surrounding condition. Windows sit on this gutter.
High quality subsill (gutter) sealed to surrounding condition. Windows sit on this gutter.

Most aluminum window systems have, or should have, some form of gutter system. A gutter system is essential to catch any water which might penetrate the system through frame joints, gasketed engagements such as at receptors or mullions, or even (depending on the design) minor sealant failures.

A gutter system could be a continuous flashing under a “run” of windows, or a subsill which is capable of holding a large volume of water. In both cases, the gutter system must have a provision to allow water to drain to the exterior without also allowing the water to leak to the interior.

There is a bit of physics involved in the subsill design. The height of the subsill (the “water column” height) is related to the wind pressure that the system can endure before overflowing and leaking. For more on this, search “Rain Screen and Pressure Equalization Principle”.

Types of Subsill Gutter System Failures

  • Holes or penetrations in the gutter system. Good rule of thumb: Avoid penetrating the gutter system, for instance, with a fastener in the wet area. Even if you seal the fastener hole, the sealant can later fail and the repair will be difficult because it will probably be inaccessible.
  • Leaks at splices in the gutter system. In long runs of windows, there will inevitably be multiple pieces of “gutter” which must be sealed to one another. These splices must be sealed with proper joint design, allowing all anticipated movements, and must be “married” to the surrounding perimeter caulk.
Window sitting in subsill. Subsill of aluminum windows will leak if not properly terminated
Window sitting in subsill. Aluminum windows will leak if not properly terminated
  • Leaks at gutter terminations Frequently overlooked despite frequent failures, design professionals and window folks often fail to pay adequate attention to what happens at the end of the gutter. This is due to an affliction I call “detailitis.” . Architectural drawings and window shop drawings are loaded with “sectional details”, which are drawings of what you would see if you took a saw, sliced out a hunk of the building or window system, then looked at the end of the hunk. People afflicted with detailitis fail to look beyond the sectional detail.

    The sectional detail, also called an isometric detail
    The sectional detail is a disembodied, two dimensional image which can never fully convey what happens to the pieces shown as they travel through space and terminate against something else. Only a 3-D (we call it an isometric) image can tell that story. 3-D drawings require more skill to draw, and therefore cost more, but a 3-D of a gutter termination is worth its weight in gold. A set of 3-D drawings is the best cure for detailitis.

    Isometric assembly detail showing sealant application. This is the right way to present critical areas and prevent aluminum window leaks
    Isometric assembly detail showing sealant application. This is the right way to present critical areas and prevent aluminum window leaks

    Gutters can terminate against jamb conditions which consist of multiple elements, such as a stud wall with sheathing, vapor barrier and other flashings.

    Architects must decide what part of the wall assembly constitutes the primary sealant plane, and make sure that the adjacent window system, and especially the gutter termination, marries to that plane. If it is not drawn in 3-D and clearly understood by all parties involved in the construction, you may have to get out the umbrellas.

  • Leaks in end dams. Many subsill assemblied are terminated with an “end dam” which is typically a flat piece of metal sealed to the end of the gutter, often in the factory. Done well, a sealed end dam can do a great job of containing water within the system as well as providing a good area for sealing to adjacent substrates.

    The sealant connecting the end dam to the adjacent construction is called the “end wash”. End wash sealant must be tooled so that it will direct extraneous water into the gutter. Done poorly, however, a bad end dam will leak, and that can spoil an otherwise good aluminum window system. This is why we recommend that a percentage of all subsills with end dams be “pond tested” during construction.

  • Water head higher than gutter height. We haven’t yet delved into the mysteries of Rain Screen and Pressure Equalization, so I will just give this one a quick shot. Gutters are designed to accumulate and then “weep” water to the exterior. Depending upon the volume of water coming into the gutter, as well as the pressure difference between exterior and interior, varying levels of water can build up in the gutter.This water height, or water head, can be actually calculated. If, under local weather conditions, wind pressures during a heavy rain cause the water head within the gutter to exceed the gutter’s interior height, get out the galoshes.

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Mark Meshulam, Chicago Window Expert witnesses laboratory test of aluminum window leaks
Mark Meshulam, Chicago Window Expert witnesses laboratory test of aluminum windows hoping to prevent leaks

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11 thoughts on “Aluminum Window Leaks”

  1. I have a project which requires your advise. The aluminum windows on a high rise Building have exterior seals which are degraded, a rep from a local company has recommended that the seals be cut back and apply a silicone sealant under and over the existing seals. Comment please.

  2. Cutting back and replacing “seals” may be your answer, but a round of diagnostic work will be needed in order to develop and validate the appropriate procedure.

    Take a look at the article Aluminum Window Leaks then give me a call at 847-945-9200 ext 229. I would like to see the building firsthand.

  3. hi I have just moved into a townhouse that was gut renovated and it has Crystal Windows installed. I believe they are Aluminum Windoes — double pane. They seem to be effective in general but we have things blowing around inside whenever the wind outside is strong or when a car or van drives by outside the house. We have not made any progress with the contractor. They came and re-caulked the interior frame. It seemed to really help retain the heat that day while the caulk was wet (it may also not have been v windy that day), but subsequently we are back to the same situation. I put cloth streamers on a string across 2 windows and they flutter quite a bit when it is windy outside.
    Our heating bill last month was $900 even though we have the heat turned off in most areas of the house.
    Will really appreciate your thoughts as to what to check and how to argue with the contractor

  4. Hi Manu,
    You appear to be fighting air infiltration on a major scale if your heating bill is $900/mo in a townhouse. Please send me some pictures of the exterior of the house and interior of the windows where the blowing is taking place.

  5. i have standard aluminum windows which are caulked all around on the outside. Vinyl siding was installed with a j channel. All is caulked outside, but one of the windows leaked (water pooled in the corner). The windows were originally installed about four or five years ago when i closed in the porch. They did not have the flanges on them as they were simply installed like replacement windows. I caulked all around them, and they did not leak for about two or three years. After that, I couldn’t get them to stop leaking around the corners. I hired someone to install vinyl siding. They put J Channels next to the windows, installed all siding and caulked around the windows and the J Channel. However, one of the windows continued to leak around one corner. Do you have any ideas?

  6. I recently had Crystal aluminum replacement windows installed in my condo. They are double hung and rated heavy commercial. Despite multiple adjustments, air infiltration at the interlocks is awful. I’m so worn out dealing with my NEW windows that I want to start over. What aluminum replacement windows would you recommend that do not have air infiltration problems? Thank you

  7. Hi Mark

    I’m in Australia and have 40 year old aluminium windows in my house and the bottom rubber seal flap has cracked and partly fallen out. This looks terrible and would let some water into the structure. What are my options seal replacement would require the window to be taken out, can i cut off the old seal and silicone the gap?

    Any suggestion would be appreciated.

    Regards Dave

  8. Mark,
    I am an architect and would like to talk with you about window leaks in an existing condo association. I am a board member working on solving the problem.
    My telephone number is 224 715 4800.

    Thank you,
    Richard Sorock

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