I've used plexi, film, glass, and polycarb on coldframes. You don't want plastic from the hardware store unless you like doing windows every year. You want a decent angle on the glazing, my standard is 30 degrees, but less is OK if you can get the snow to slump off when sunny. Film might be best, especially film that diffuses light. I suspect the old windows had lost their integrity, moisture got in the seal-pane interface, expanded, and broke the window.

Dan

Were the windows even double pane? Being 'old' they could have been single strength, single pane, glazed windows.

Marla

Yes, they were old single-pane windows. Are double-paned ones better, or not really because of the higher risk of the kind of breakage Dan Staley described?

Definitely better, better insulated and stronger. You can get triple-pane now adays. Plexi works also. I've used plexi for storm windows and they're almost unbreakable.

I used the construction plastic, but after a couple of years, you get plastic confetti. Check the greenhouses in your area. Sometimes you can get the old plastic or leftover pieces. I found some from my local greenhouse that got rusty in one end, but the other end was just fine. We've used it for 3 years now, plus the greenhouse had it for 1-2 years. Plus it was free.

Marla

You want dead air between layers of glazing or film. Double layer achieves that. Old single-pane, slightly twisted, with expanding ice through old caulking will crack no problem. Newer framing material and sealants resist such failure.

And if there are no greenhouses in area, FarmTek has pretty good pricing on film.

Dan

I've been using the same double walled polycarbonate panels in my cold frames since sometime in the 1990's. Each panel is 4x4 and there are two of them per cold frame. These, combine with one automatic opener per cold frame has been very successful all these years.