Rather than retype, please see the link below for my opinion.

Here is a link that might be useful: greenhouse

Great timing as I'm researching different ones and what the warranties are for the long term.
A bit steep in price though.

Julia

i'm not sure about the particular company that you are looking at, but it has always been my understanding that cedar is a wonderful building material for greenhouses. it's extremely rot resistant and will last longer than even treated wood. your other option could be to build your own with cedar boards if you have them available locally. in most cases, it is less expensive to build your own than to buy a kit. then again, a warranty is always a nice thing to have when you are making such a big investment.

Googling I found some on Growers Supply store.

What size did you need?

Click some on the page and you can see some warranty documents. Looks like 5 years.

Peace, Miwa.
The Japanese Gardener

Here is a link that might be useful: Cedar Greenhouse

I agree with ncdaydreams

I find cedar to be well worth the investment. It's naturally resistant to decay, rot, and insect attack. While other synthetic materials may have those same attributes, Cedar is the only natural material that is going to give so much back for your investment.

Good luck!