[GpsFrontier]

They come in different ratings, it all depends on the UV rating. They range form no UV inhibitors at all, to a 4 year poly. They rate them by the amount of UV inhibitor in the plastic. The UV inhibitors resist degrading the plastic. I would need to check, but I think the highest rating is like a 6 year, but I'm not sure. The higher the rating, the more expensive the plastic is, and the 4 year is the most common and widely used for greenhouses (even commercial greenhouses).

I could have used the clear 6 mil construction poly (with no UV inhibitors in it). In fact that was my original plan, the 20x100 foot roll of the construction poly costs $94 at Lowe's. That would have been enough to cover the greenhouse twice. And I planed to replace it (with the second half of the same roll) at the 6 month mark. After that I expected to replace it at about the one year mark with a good UV rated poly film I bought with the profits I got from the sold products. But because the sub "T" system/s are smaller than my original plan (even though they are larger than the recommended for a 200 square foot greenhouse), I wanted the UV inhibitors in the plastic to help lower solar gain (heat from solar radiation) during summer. So I splurged.

P.S.
If I were to use the corrugated sheets of poly for the greenhouse glazing, it would cost me about $2,500+ just for the glazing, then I would still need to build the greenhouse framing (that probably would have run another $1,000 to $1,500). But going the rout I am, by the time all is said and done the greenhouse (greenhouse structure only) will run me about $800 to build. That's less than it cost to build and install the sub "T" system. The Sub "T" system cost me about $1000 to install, including equipment rental and labor costs (I pay $10 an hour for labor help).