Sometimes it’s just junk glass from the factory—tiny flaws you can’t spot until it’s too late. Especially with some of the cheaper panes out there now.
Totally agree with this. I mean, you can line up the frame, shim everything just right, and still end up with a crack months later. I had a new window in my mudroom last year—brand name, supposedly decent quality—and sure enough, a hairline showed up right across the middle after our first cold snap. Installer swore the frame was dead-on, and honestly, I believe him.
I’ve noticed the cheaper glass just doesn’t hold up the same. Sometimes you can even feel a weird roughness along the edge if you run your finger along it, like it wasn’t finished right. Not saying frame tension never matters, but sometimes it’s just bad luck with the glass batch. If you’re seeing it on multiple panes from the same order, that’s a red flag for factory defects, not install.
For what it’s worth, I’d rather have a slightly drafty old window than a new one that cracks every winter… at least you know what you’re dealing with.