Windows and Doors: Windows

Condensation and Suction Cup Window Marks

This letter is in response to the photo below from Joe, a residential window contractor with the same problem…

window with suction cup marks
Glass suction cup marks


Good afternoon, Joe –

I’ve read your email, and judging from the attached photo, I see the issue as two-fold. First, there is condensation on the exterior of the IGU (Insulated Glass Unit). Second, when that condensation formed, there is the appearance of suction cup marks on the exterior surface of the IGU.


Condensation forms on an object when that objects surface temperature goes below the dew point. The dew point is defined as the temperature where the air is 100% saturated with moisture – or where the air is at 100% relative humidity. This window probably has condensation because the surface of the window is below the dew point.

This is not considered a defect. What is happening on this window with the condensation is that the window is performing exactly as designed. It is blocking heat from one side of the IGU from reaching the other side. Exterior condensation on energy efficient windows is quite common, and it is perfectly normal. It does not affect either the performance or the longevity of the IGU. The limited warranty is still applicable.Steve’s note: On the other hand, any condensation developing on the interior (the inaccessible surfaces) of the IGU would indicate a failure of the IGU and considered a defect.

Whether or not a window develops exterior condensation is actually a rather complex bit of environmental and performance issues. For example, condensation is much less likely to form on a cloudy night. Trees or other obstructions close to the windows, bushes under the windows, and even the length and angle of the soffit or other overhangs can affect the formation of condensation. A tiny change in either temperature or humidity from one room to the next might raise or lower the dew point just a little bit and you might see a whole different level of condensation.


Glass condensation with cup marks
Condensation showing suction marks

The marks visible on the glass surface are referred to as suction cup marks. They may occur on the glass surface as a result of the manufacturing or installation process. During these processes, suction cups are sometimes used to move the glass.

If you examine the glass surface under a microscope, you will see the glass surface has peaks and valleys in it. Very small, minute particles on the suction cup may be deposited on the glass surface and settle into these valleys.

The particles are typically not visible but do adhere to the glass surface enough that it affects how water droplets or ice forms on the glass surface. This explains why suction cup marks are more visible on wet or icy glass than on dry glass.

Over time, with normal exposure to the elements and washing, the suction cup marks will diminish or disappear. In the meantime, they can sometimes be minimized or removed with the use of a glass polishing agent, such as CRL’s Sparkle.


It is my professional opinion that the glass is not defective and is functioning properly. The suction cup marks were caused by suction cups being used to properly manufacture the glass. As mentioned above, cleaning the glass may minimize or eliminate the issue.

UPG is willing to supply the CRL Sparkle cleaning product in an effort to resolve the issue. I do not believe replacing the IGU will result in any different results. I have attached PPG Glass Technical Document 107 for your review in an effort to further explain the phenomena that is occurring.

Please call me with any questions and concerns.


John Vissari
General Manager, United Plate Glass

Help me to help you

Congratulations on taking time to use this free website. I truly hope you find the information here fun and useful? Please send me your comments and feedback. Writing and maintaining this site is a lot of hard work. By sharing these pages with your friends, you help me to continue the effort.  You can easily share these pages to all your favorite social media sites via the shortcut buttons on the sidebar. You can also simply email the page link to your friends. Please share often.

The very best way to help me occurs when you use the embedded links to buy things. Using a link is easy and free, yet it provides a great source of support for this site. The vendors I’ve chosen to link to have proven to be the very best resources available so that is why I recommend them personally.

If you have questions or just want to say hi, please send me a note.

Thanks again!

Latest Posts