Causes of shattered thermal pane?

I’m asking for a good customer of mine.

This is a Pella thermal pane transom, probably 20 years old. One of her internal panes shattered today. Windy conditions yesterday, then 24f overnight.

Any suggestions? It’s nothing I did, last time we cleaned it was over the summer.

That’s an impact breakage, right about the middle of the glass

Does she have kids? Maybe they were playing frisbee in the house?

Barring that, it could be from outgassing of the thermal pane. It’s about the right age. But I’ve only ever seen it happen to older Andersen windows, and usually it’s the exterior glass that breaks.

No, nothing hit the glass, she just heard a loud bang and found the window shattered. Someone else in her Facebook thread posted it happened to them as well.

I know the house fairly well, some of the other vertical windows have blown seals. I guess this transom just finally had enough.

Any foundation cracks or settling cracks in the walls?

No, nothing I’m aware of.

I’m sure, but not being your home there could be issues like that you are not aware of. Some extreme temperature changes “might” be the culprit if a flying object is ruled out, otherwise weight shifting may be to blame.

@dcbrock. I had the same thing happen to 2 windows at a customers house. When there are really quick changes in temperature, glass can break from that. I did a lot of research on it. If I had to guess, that is what happened.

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Learn something everyday.

They said it scared the heck out of them when they shattered. They thought someone was breaking in. Primarily happens when the temp drops real fast.

Thermal breakage really only cracks float glass, thermal breakage in toughened can explode but since this is float glass it is 100% an impact that broke it

The only way thermal breakage could result in that is if the window was under quite a bit of pressure when it cracked

I’m 100% sure it wasn’t an interior impact that broke it. She was the only one home at the time.

I agree with the temperature theory. The repair would likely be more than her deductable. I would recomend her filing a claim with her homeowners insurance.

it’s quite difficult to break glass with thermal change UNLESS there is a flaw on the outer edge. you cannot see these because they are covered with the seal material and frame.
to me it looks as though the problem could have started in the top left corner.

I am in favor of Alex Laceys idea. Had a friend here with the same problem. It is called negative deflection from outgassing of argon. You can tell however. Soap the glass and scrape with a six inch. First left to right then up and down. Is the glass curved in. If so it has outgassed and has a negative deflection. When the barometric pressure changes quickly this makes the problem way worse too quickly;…then bang! Loud enough to take the hair off a cat. But it is an implosion not an explosion. Defective windows. Usually argon outgassing. You did nothing. Not to blame. A manufacturing defect. Go on the net and do the research. I even wrote an article on it in the Glass Smart Blog. Got to get over to you this year Alex. Look around for a nice motel for me to set up a seminar. I would love to see the Dad again too.


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I’ve had this happen before on a window just like that. I personally cleaned it moved on to the next one and then POP. It scared me! :slight_smile:

Fortunately I’ve never had a window do that when I was working on it, but I had a customer once that had a skylight explode while they were in the house. It had been tinted and there wasn’t a gap on the edge so heat could escape. Sun hit it, pressure built up and exploded. Tempered of course. If you look at tint jobs you will see they’ll leave a 1/4 inch gap on the edge - supposed to allow heat the escape as I understand. Thought that might have been what happened to the window above but doesn’t look like it was tinted. Still, either someone hit it or thermal changes, unless the house is collapsing …

Not once have I ever seen that in 24 years. Must be a regional thing.

Yeah, looks like 1/8 inch is closer to the size recommended by gila. On sealed units. Never got into tint myself though, just what I’d heard over the years

Seventh question in the faqs section