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 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.
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.