Why do I have condensation in the middle of my windows?

A Sure Sign of Collapsed Glass

Collapsed Glass Syndrome

I was recently visiting my brother and he pointed out a strange condensation effect he was having on some double glazed windows. Condensation formed in an oval pattern in the middle of the windows. This is really strange because condensation forms on the coldest parts of windows first. Thermal windows usually insulate best at their centers so condensation starts forming at the edges. But these windows were showing exactly the opposite condensation pattern as shown in the photo above.

Approaching this scientifically, we knew a few things:

  • Condensation forms on colder surfaces first
  • The pattern was so symmetrical that this had to be caused by some aspect of the window and not some strange air flow

This was a fascinating issue, so let’s walk through how we analyzed it.

Note that the condensation was on the glass exposed to the inside of the house, not in-between the glass. Sometimes you get condensation inside of thermal windows when the seals break and moisture gets in-between the panes, but that’s not the case here.

Anderson windows mentioned this exact effect on their website, referring to it as “collapsed glass.” It turns out, when the windows are tightly sealed and the glass is relatively thin, the glass actually warps so much when it gets cold out that the space between the sheets of glass actually disappears! When this happens, the window provides minimal insulation, allowing the center of the window to get very cold.

At the corner, the spacer holds the panes apart

At first, I didn’t believe it, so we did a “flashlight” test. If you shine a pinhole of light on the glass and look at the reflections of the light, you’ll see four reflections in a double glazed window – one reflection from each surface of glass. With a larger light, you may only see two reflections, one from each glass surface, like in the above photo.

Sure enough, at the edge, the two inner-most reflections were widely separated but as I moved the light towards the center, the gap narrowed until the two reflections were touching! It was really dramatic and eliminated any doubt in my mind as to the problem.

Near the middle, the two reflections merge to one

When you see this ‘live’ it becomes really obvious what is happening, putting all controversy to an end. Sometimes these simple tests really work!

Another way you can see this effect is to place a straight edge (like a yardstick) on the window. You’ll see that it sits flat at the edges, and you’ll actually be able to see the window bow away from the straight edge towards the center. In our case, the windows bowed in almost a 1/4″ between the edges and the center! Again, once you see this, you’ll be amazed at how clear it is.

Is it worth fixing?

Short answer: yes!

When the Argon in the windows has escaped and the windows bow in towards one another like this, your thermally insulating windows no longer insulate well at all. They’ll get really cold and will often be covered with condensation or frost. Even worse, if the condensation repeatedly drips down onto a wooden sash, you can end up rotting out the sash – then you’ll have a really expensive repair on your hands!

There’s a great discussion of this phenomenon on the Gardenweb discussion site. As with any discussion, there’s some misinformation in there, so you have to be a little careful in your interpretation of what they say.


A special thanks to my brother Chris for providing these images.


11 thoughts on “Why do I have condensation in the middle of my windows?

  1. At last I found a plausible explanation. Thank you so much. I had asked several window suppliers but no one really had a clue.
    I notice that the moisture “disc” is correlated to the wind. My house on the beach on Georgian Bay faces NNE and the wind combined with the low temperature frequently causes this problem. The window, located in the dining room and open to the kitchen (hence higher humidity) measures 62in.x64in. so I am not surprised that there can be sufficient pressure to flex the outside pane and “collapse” it. Since there is no moisture whatsoever between the panes I feel that the Argon has not been displaced, although being heavier than air, I wouldn’t rule that out.
    My solution has been to use a silent little squirrel-cage fan on the window-sill to circulate the warm air across the pane. Not a perfect solution but cheaper than replacing the whole works.
    I did come across a service, some time ago, that drills a couple of small holes in the glass and reflushes the space with Argon but I’m not willing to risk it.
    The other 4 windows, on the same wall but only partially open to the kitchen, each measure 50x60in. and have never exhibited the problem.

  2. Seems that we suffer collapsed glass in our new picture window. It was replaced last spring and seemed fine until the cold weather began a couple of nights ago.
    You indicated that it is worth repairing but didn’t say how.

    • Sorry, just saw your message.
      Glass repair companies can usually remove the defective glass component and replace it with a new one. This is a specialty skill, so you’ll need to find a reputable local company to do the repairs.

    • This one works. Thanks.
      From the looks of it, maybe you have two different issues. The condensation on the bottom of the pain isn’t too unusual – natural convection tends to make the bottom colder.
      You say that the condensation is internal at the bottom and on the outside for the round areas?
      Is it only that window that shows these condensation patterns? And is it only when the window is open or do you get condensation at other times? I could conjecture but more information would help.

      • thanks….yes…only the windows…I dont consider it a problem…just of interest …trying to use my knowledge of physics to explain the pattern…we leave the window open at night…the other closed ones dont show the outside round pattern..ian

      • Sorry for the typos. What I meant to say is it only the windows that you tilt open that show the central condensation pattern on the outside? What if you tilt open one of the other windows in a similar way?

        Here’s my hunch.

        All objects radiate heat at a rate related to the difference in temperatures between the object and the rest of the “universe” that the object is exposed to. When the window is tilted open like shown in the photo, it is exposed to more sky and less terrestrial objects, so it cools down more rapidly. This is the same effect that lets the top of your automobile get coated with morning frost while the doors remain frost free. Here is a brief description of the effect: http://en.wikipedia.org/wiki/Radiative_cooling

        It could be that this effect is combining with other factors (better or worse window insulation for example) that cause that particular window to get condensation while others do not.

  3. Hi…I’ve always wondered about this…But I’m not convinced about this explanation…what about when our upper small window is opened at night, an oval patch of condensation develops on the outside of the glass opposite the upright window handle!…also our double glazed panels have about 11/2 cm gap…too wide to “close ” the inner and outer panel to touch??

    • I’m sure it varies depending on situation. In the example I showed, we definitively proved the cause.
      In your situation, it sounds like there’s something else going on. One thing that you always know is that the condensation forms first on the coldest section of the window, so the question is why is that section cold? Maybe you could post a photo showing your situation and we could try to figure it out.

    • Hi…as requested a pic of 1. internal condensation along bottom of pane…2. two round areas on the outside of the pane.I think these are different scenarios…hope the pic arrives???


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