Modern day window glass the way they manufacture them these days also make them unsuitable for greenhouses.
My good fellow, what makes glass unsuitable for greenhouses? I use old shower doors made of glass as a buffer tween the sun and my spring transplants!
I know a little about greenhouse growing, having worked in a greenhouse complex for the last 18 years.
First off, old fashioned glass CAN be a good greenhouse covering. BUT (and it's a big but) modern window glass is more often than not manufactured as Low-E glass. Regular off the shelf Low-E glass prevents the transmission of solar heat from entering. IF you are looking to use Low-E glass in a greenhouse make sure the Solar Heat Gain Coefficient (SHGC) is at least 0.52 or greater.
I made the mistake of installing low E windows in my home and my southern exposure rooms no longer are passively heated. Here in Colorado, where our days during winter might be chilly outside, are still bright and sunny and a southern window with a higher SHGC can save us alot of the expense of natural gas heating. Had I to do it all over again, i would have put in custom low-e glass with higher SHGC in my southern exposure. the other exposures are fine and do save some energy.
A little trick to see if a used window in a recycled building materials center is suitable for greenhouse use is to face the window towards the sun and place your hand behind it. If you can feel the solar heat radiating through it and warming your hand, then it is suitable for greenhouse use. Chances are good that used shower doors will be high SHGC as low-E shower doors just don't make economic sense